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Validation of oscillometric blood pressure measurement using a Datex S/5 Compact multiparameter monitor in anaesthetized adult dogs

Published:December 10, 2021DOI:https://doi.org/10.1016/j.vaa.2021.12.001

      Abstract

      Objective

      To compare noninvasive (NIBP) with invasive blood pressure (IBP) measurements from a Datex S/5 Compact monitor in anaesthetized adult dogs, and to evaluate it according to the American College of Veterinary Internal Medicine (ACVIM) and the Association for the Advancement of Medical Instrumentation (AAMI) criteria.

      Study design

      Prospective clinical study.

      Animals

      A group of 34 client-owned adult dogs.

      Methods

      Dogs were anaesthetized for different surgical procedures using different anaesthetic protocols. IBP was measured using a catheter placed in a dorsal pedal artery. A blood pressure cuff was placed over the contralateral dorsal pedal artery for NIBP measurement. Data were recorded using the Datex iCollect program, and paired readings were matched every 3 minutes for 60 minutes. Bland-Altman and error grid analyses were used to estimate the agreement between IBP and NIBP measurements, and its clinical significance, respectively. Data were reported as mean bias [lower, upper limits of agreement (LoA)].

      Results

      The Datex S/5 monitor conformed to most ACVIM criteria. The correlation coefficient was less than 0.9 for systolic, diastolic, and mean arterial pressures (MAP). The best agreement between the noninvasive and invasive methods was observed for MAP, with LoA (–17 to 13 mmHg) and higher percentage of NIBP readings within 5 (55.6%), 10 (81.7%) and 20 (98.6%) mmHg of the IBP values. The Datex S/5 NIBP technology did not meet the AAMI validation criteria and less than 95% of the paired measurements were found within the green zone of the error grid analysis.

      Conclusions and clinical relevance

      The Datex S/5 monitor conformed to most ACVIM criteria but not with the more rigorous AAMI standards. Despite good agreement between IBP and NIBP for MAP measurements, care must be taken when using this device to guide therapeutic interventions of blood pressure in anaesthetized healthy adult dogs.

      Keywords

      Introduction

      The confidential enquiry into perioperative small animal fatalities in the UK reported an anaesthetic mortality of 0.05% and 1.33% in healthy and sick dogs, respectively (
      • Brodbelt D.C.
      • Blissitt K.J.
      • Hammond R.A.
      • et al.
      The risk of death: The confidential enquiry into perioperative small animal fatalities.
      ). One of the most frequent contributors to perioperative death was related to cardiovascular causes (74%), which included cardiac arrest and cardiovascular collapse (
      • Brodbelt D.C.
      • Blissitt K.J.
      • Hammond R.A.
      • et al.
      The risk of death: The confidential enquiry into perioperative small animal fatalities.
      ). However, systemic arterial pressure was only determined in 10% of cases (
      • Brodbelt D.C.
      • Pfeiffer D.U.
      • Young L.E.
      • Wood J.L.
      Results of the confidential enquiry into perioperative small animal fatalities regarding risk factors for anesthetic-related death in dogs.
      ). Hypotension [mean arterial pressure (MAP) < 60 mmHg] (
      • Mazzaferro E.
      • Wagner A.E.
      Hypotension during anesthesia in dogs and cats: recognition, causes, and treatment.
      ) is one of the most common anaesthetic complications in dogs owing to decreased sympathetic tone mediated by anaesthetic agents (
      • Ruffato M.
      • Novello L.
      • Clark L.
      What is the definition of intraoperative hypotension in dogs? Results from a survey of diplomates of the ACVAA and ECVAA.
      ). In human medicine, intraoperative and postoperative complications that are reported as a consequence of anaesthetic-related hypotension include prolonged recovery, organ failure and neurological deficits (
      • Pasch T.
      • Huk W.
      Cerebral complications following induced hypotension.
      ). Intraoperative measurement of arterial blood pressure (ABP) and management of hypotension leads to fewer complications and improved outcomes in dogs (
      • Mazzaferro E.
      • Wagner A.E.
      Hypotension during anesthesia in dogs and cats: recognition, causes, and treatment.
      ) and humans (
      • Monk T.G.
      • Saini V.
      • Weldon B.C.
      • Sigl J.C.
      Anesthetic management and one-year mortality after noncardiac surgery.
      ). For these reasons, measurement of ABP is recommended in anaesthetized humans (
      American Society of Anesthesiologists
      Standards for basic anesthetic monitoring.
      ;
      • Checketts M.R.
      • Alladi R.
      • Ferguson K.
      • et al.
      Recommendations for standards of monitoring during anaesthesia and recovery 2015: Association of Anaesthetists of Great Britain and Ireland.
      ) and animals (
      • Grubb T.
      • Sager J.
      • Gaynor J.S.
      • et al.
      2020 AAHA anesthesia and monitoring guidelines for dogs and cats.
      ). Invasive blood pressure (IBP) measurement is considered the ‘gold standard’ method in anaesthetized dogs (
      • Acierno M.J.
      • Brown S.
      • Coleman A.E.
      • et al.
      ACVIM consensus statement: Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). However, arterial catheterization in small animals can be technically challenging (
      • Mazzaferro E.
      • Wagner A.E.
      Hypotension during anesthesia in dogs and cats: recognition, causes, and treatment.
      ), particularly in hypovolemic dogs (
      • Mishina M.
      • Watanabe T.
      • Fujii K.
      • et al.
      A clinical evaluation of blood pressure through non-invasive measurement using the oscillometric procedure in conscious dogs.
      ). Noninvasive blood pressure (NIBP) measurement may be a suitable alternative to invasive methods for short procedures and in animals where arterial catheterisation is difficult (
      • Ruffato M.
      • Novello L.
      • Clark L.
      What is the definition of intraoperative hypotension in dogs? Results from a survey of diplomates of the ACVAA and ECVAA.
      ) but it is less accurate (
      • Garofalo N.A.
      • Neto F.J.T.
      • Alvaides R.K.
      • et al.
      Agreement between direct, oscillometric and Doppler ultrasound blood pressures using three different cuff positions in anesthetized dogs.
      ). The American College of Veterinary Internal Medicine (ACVIM) consensus statement on hypertension in dogs and cats determined the criteria for validation of blood pressure measurement devices in these species (
      • Brown S.
      • Atkins C.
      • Bagley R.
      • et al.
      Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). These guidelines were based on those previously reported by the Association for the Advancement of Medical Instrumentation (AAMI) (
      • O’Brien E.
      • Atkins N.
      • Stergiou G.
      • et al.
      Working Group on Blood Pressure Monitoring of the European Society of Hypertension. European Society of Hypertension International Protocol revision 2010 for the validation of blood pressure measuring devices in adults.
      ) in humans, but with less stringent criteria.
      To the authors’ knowledge, there is no information describing the accuracy of NIBP measurement using the Datex S/5 technology in anaesthetized adult dogs. Therefore, the aims of this study were to: compare the accuracy, limits of agreement (LoA) and bias of oscillometrically derived ABP values from this multiparameter monitor, with those obtained using IBP measurement. Also, to evaluate the results according to the ACVIM and AAMI validation criteria. Measurements were made in anaesthetized healthy adult dogs. Our hypothesis was that NIBP measurement using Datex oscillometry would be an accurate method compared with IBP measurements and can be used to guide therapeutic interventions.

      Material and methods

      This clinical study was approved by the Ethics Committee of the Animal Health Trust, Newmarket, UK (25-2019E). A total of 34 client-owned adult dogs of various breeds were included in this study. Dogs were considered healthy based on complete physical examination and classified as American Society of Anesthesiologists (ASA) physical status I or II. There were no exclusion criteria based on sex, breed or body weight. All dogs were scheduled for surgical procedures requiring general anaesthesia and where IBP monitoring was clinically justified (e.g. procedures that could lead to excessive haemorrhage). Food, but not water, was withheld for at least 8 hours prior to general anaesthesia.
      Premedication was administered either intravenously (IV) or intramuscularly (IM), and the respective drugs and doses were selected and administered by the designated anaesthetist (Table 1). An 18–22 gauge IV cannula (Biovalve; Vygon, UK) was placed into a cephalic vein if one was not already present. Once appropriate sedation was observed, all dogs were pre-oxygenated with the use of a face mask (Burtons, UK) for at least 3 minutes prior to induction. Anaesthesia was induced with alfaxalone (Alfaxan; Jurox, UK) or propofol (Propoflo plus; Abbot, UK) alone or in combination with other drugs (Table 1). Orotracheal intubation was performed with an appropriately sized endotracheal tube (Portex; Smiths Medical, UK), and attached to an anaesthetic machine and circle breathing system (Datex Aestiva 5; GE Healthcare, UK). Dogs were positioned in sternal, lateral or dorsal recumbency, depending on the surgical procedure. Maintenance of anaesthesia (Table 1) was achieved with an inhalational agent—isoflurane (Isoflo100% w/w inhalation vapour liquid; Abbott) or sevoflurane (Sevoflo 100% w/w inhalation vapour liquid; Abbott) vaporized in oxygen. Mechanical ventilation was started if the end-tidal partial pressure of carbon dioxide (Pe′CO2) was greater than 60 mmHg (8 kPa), or if deemed necessary for surgery or if neuromuscular blocking drugs were used. Hartmann’s solution (Vetivex 11 solution; Dechra Veterinary Products, UK) was administered at a starting dose of 5 mL kg–1 hour–1 in all dogs and adjusted throughout anaesthesia as appropriate. Some dogs were given constant rate infusions (CRIs) which were selected for each individual case (Table 1). If dogs demonstrated signs of nociception during surgery, a fentanyl l–2 μg kg–1 IV bolus (Fentadon; Dechra Veterinary Products), a ketamine 0.2 mg kg–1 IV bolus (Anesketin; Dechra Veterinary Products) or a remifentanil 0.2–0.4 μg kg–1 minute–1 IV CRI (Ultiva 1 mg; GlaxoSmithKline Pharmaceuticals, UK) was administered. Treatment of intraoperative hypotension (MAP < 60 mmHg) was selected and administered by the designated anaesthetist (Table 1).
      Table 1Drugs used for premedication, induction and maintenance of general anaesthesia, and intraoperative constant rate or variable rate infusions in 34 adult dogs.
      Drugs (number of dogs)Doses and route
      Premedication
      Methadone (2)0.3 mg kg–1 IM: 0.5 mg kg–1 IV
      Methadone + acepromazine (10)0.3–0.5 mg kg–1 + 0.01 mg kg–1 IV
      Methadone + medetomidine + acepromazine (10)0.3 mg kg–1 + 2–3 μg kg–1 + 0.01 mg kg–1 IV
      Methadone + medetomidine (12)0.3 mg kg–1 + 2–3 μg kg–1 IM: IV
      Induction
      Alfaxalone + midazolam (1)1.0 mg kg–1 + 0.2 mg kg–1 IV
      Alfaxalone + midazolam + ketamine (2)2 mg kg–1 + 0.2 mg kg–1 + 0.5 mg kg–1 IV
      Propofol (16)1 mg kg–1 IV
      Propofol + midazolam (1)1 mg kg–1 + 0.2 mg kg–1 IV
      Propofol + ketamine (12)1–1.5 mg kg–1 + 0.5–1 mg kg–1 IV
      Fentanyl + propofol + midazolam (2)5 μg kg–1 + 1.5 mg kg–1 + 0.2 mg kg–1 IV
      Maintenance
      Isoflurane in oxygen (28)1.0–1.4% Fe′Iso with FiO2 98–99%
      Sevoflurane in oxygen (6)2.0–2.5% Fe′Sevo with FiO2 98–99%
      Intraoperative drugs
      Fentanyl (3)0.2 μg kg–1 minute–1 IV
      Fentanyl + ketamine (26)0.2 μg kg–1 minute–1 + 5–10 μg kg–1 minute–1 IV
      Fentanyl + lidocaine (3)0.2–0.4 μg kg–1 minute–1 + 40 μg kg–1 minute–1 IV
      Remifentanil + lidocaine (1)0.2 μg kg–1 minute–1 + 40 μg kg–1 minute–1 IV
      Ephedrine (1)0.05 mg kg–1 IV
      Glycopyrrolate (4)0.01 mg kg–1 IV
      Dopamine (3)5–20 μg kg–1 minute–1 IV
      Noradrenaline (2)0.05–0.2 μg kg–1 minute–1 IV
      Calcium gluconate 10% (2)1 mL kg–1 IV
      Morphine 1% (2)0.2 mg kg-1 epidurally
      Medetomidine (2)0.5-2 μg kg-1 hour-1 IV
      Atracurium (1)0.3 mg kg-1 IV
      Neostigmine and glycopyrrolate (1)0.05 mg kg-1 IV
      Fe′Iso, End-tidal isoflurane in %; Fe′Sevo, end-tidal sevoflurane in %; FiO2, inspired oxygen fraction; IM, intramuscular; IV, intravenous.
      Methadone (Comfortan; Dechra, UK), acepromazine (ACP injection; Novartis, UK), medetomidine (Domitor; Vetoquinol, UK), alfaxalone (Alfaxan Multidose; Jurox), propofol (Propoflo Plus; Abbott, UK), ketamine (Anesketin; Dechra), midazolam (Midazolam; Hameln Pharmaceuticals Ltd, UK), fentanyl (Fentadon; Dechra), lidocaine (Lidocaine injection 2%; Hameln Pharmaceuticals Ltd), isoflurane (Isoflo 100% w/w inhalation vapour liquid; Abbott), sevoflurane (Sevoflo 100% w/w inhalation vapour liquid; Abbott), remifentanil (Ultiva 1 mg; GlaxoSmithKline Pharmaceuticals, UK), ephedrine (Ephedrine; Martindale Pharmaceuticals Ltd, UK), glycopyrrolate (Glycopyrrolate; Martindale Pharmaceuticals Ltd), dopamine (Dopamine; Hospira Ltd, UK), noradrenaline (Noradrenaline; Hospira Ltd), calcium gluconate (Calcium Gluconate 10%; Hameln Pharmaceuticals), morphine (Morphine Sulphate; AS Kalceks, Latvia), atracurium (atracurium besilate; Hameln Pharmaceuticals Ltd), glycopyrrolate and neostigmine (Glycopyrrolate and Neostigmine Metilsulfate; Mercury Pharmaceuticals Ltd, UK).
      Muscle relaxation was required in three dogs for thoracic procedures, and this was achieved with IV atracurium 0.3 mg kg–1 (atracurium besilate; Hameln Pharmaceuticals Ltd, UK) and reversed with glycopyrrolate and neostigmine (glycopyrrolate and neostigmine metilsulfate; Mercury Pharmaceuticals Ltd, UK) as required (Table 1).
      A Datex Ohmeda S/5 Compact monitor (Datex Ohmeda Division, Finland) was used to display electrocardiogram, haemoglobin oxygen saturation, Pe′CO2, fraction of inspired oxygen, inspired and end-tidal concentration of anaesthetic vapour, NIBP, IBP and oesophageal temperature. Data were automatically recorded using the Datex iCollect program (GE Healthcare Clinical Systems Ltd., UK) throughout the duration of the procedure.

      IBP measurement

      A 20–22 gauge, 25 mm cannula (Biovalve; Vygon) was placed aseptically by an experienced anaesthetist in the left or right dorsal pedal artery, depending on the animal’s position and surgical procedure. It was connected to an electronic transducer (LogiCal; Smiths Medical) via saline-filled (Aqupharm 1; Animalcare, UK), noncompliant tubing and a three-way stopcock pressurized to 200 mmHg. Prior to anaesthesia of each dog enrolled into the study, both the transducer and monitor were calibrated against a mercury manometer (Accoson Dekamet MK3, UK) up to a pressure of 200 mmHg. The transducer was zeroed to atmospheric pressure and positioned at the level of the right atrium, using the point of the shoulder/manubrium for dogs in dorsal recumbency, and the xiphoid process of the sternum for dogs in lateral recumbency (
      • Deflandre C.J.A.
      • Hellebrekers L.J.
      Clinical evaluation of the Surgivet V60046, a non-invasive blood pressure monitor in anaesthetized dogs.
      ;
      • Garofalo N.A.
      • Neto F.J.T.
      • Alvaides R.K.
      • et al.
      Agreement between direct, oscillometric and Doppler ultrasound blood pressures using three different cuff positions in anesthetized dogs.
      ). The dynamic response of the system was subjectively assessed using a ‘square wave test’ by the same investigator (RF). This was performed by pressing and releasing the fast flush valve of the system and observing the arterial waveform. The presence of at least one but no more than two oscillations after this test indicated a normal dynamic response (
      • Sturgess D.J.
      • Watts R.P.
      Haemodynamic Monitoring.
      ).

      NIBP measurement

      The metatarsal circumference (contralateral to the metatarsus used for IBP measurement) was measured and an NIBP cuff with a width of approximately 40% of this measurement was chosen. The cuff was placed around the metatarsus with the centre of the cuff bladder over the dorsal pedal artery. Systolic (SAP), diastolic (DAP) and mean (MAP) arterial blood pressures were obtained oscillometrically. Measurements of NIBP were taken at 3 minute intervals throughout the surgical procedure. If the oscillometric device failed to read, the monitor was left to measure the blood pressure again at the next scheduled time point. The distance between the cuff and the pressure transducer was measured and a conversion factor of ± 8.0 mmHg for every 10 cm difference in height was applied (
      • Acierno M.J.
      • Brown S.
      • Coleman A.E.
      • et al.
      ACVIM consensus statement: Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). This arithmetic correction factor was applied to the oscillometric results where the NIBP cuff was not level with the pressure transducer.

      IBP and NIBP paired measurements collection

      The Datex iCollect program was set to begin recording data immediately after positioning the dog on the surgical table. Paired readings of IBP and NIBP were then retrospectively time matched every 3 minutes for 60 minutes.

      ACVIM and AAMI validation criteria

      According to the ACVIM guidelines, when validating an NIBP device, the mean difference between paired IBP and NIBP measurements must be ≤ 10 mmHg with a standard deviation (SD) ≤ 15 mmHg (
      • Brown S.
      • Atkins C.
      • Bagley R.
      • et al.
      Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). Additionally, correlation between the paired measurements for SAP and DAP should be ≥ 0.9 across the range of measured values; 50% of all measurements must lie within 10 mmHg of the reference method, whereas 80% of all measurements must lie within 20 mmHg of the reference method (
      • Brown S.
      • Atkins C.
      • Bagley R.
      • et al.
      Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). In this study, the MAP measurements were also included and compared with ACVIM criteria. The AAMI guidelines recommend a mean difference between the ‘gold standard’ and a new device of ± 5 mmHg, with SD of 8 mmHg or less. Additionally, 85% of all measurements must lie within 5 mmHg of the reference method, and 95% of all measurements must lie within 10 mmHg of the reference method (
      • O’Brien E.
      • Atkins N.
      • Stergiou G.
      • et al.
      Working Group on Blood Pressure Monitoring of the European Society of Hypertension. European Society of Hypertension International Protocol revision 2010 for the validation of blood pressure measuring devices in adults.
      ).

      Statistical analysis

      An estimated calculated sample size of 34 dogs was required to detect a 10 mmHg difference between the two measurement methods [based on the ACVIM standards (
      • Brown S.
      • Atkins C.
      • Bagley R.
      • et al.
      Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      )], with a power of 0.8 (1 - β error) and α error of 5%. The mean difference and SD between the paired measurements were calculated and compared with both the ACVIM and the AAMI standards (
      • O’Brien E.
      • Atkins N.
      • Stergiou G.
      • et al.
      Working Group on Blood Pressure Monitoring of the European Society of Hypertension. European Society of Hypertension International Protocol revision 2010 for the validation of blood pressure measuring devices in adults.
      ). The 95% limits LoA (mean differences between methods ± 2 SD) between SAP, MAP and DAP values obtained by IBP and NIBP measurement were assessed using a modified Bland-Altman analysis for repeated measurements (
      • Bland J.M.
      • Altman D.G.
      Agreement between methods of measurement with multiple observations per individual.
      ). This provided a range within which any individual difference between IBP and NIBP was highly likely (95%) to lie. Pearson correlation coefficients were also calculated.
      To assess the clinical significance of the differences between IBP and NIBP measurements, an adapted error grid analysis was performed. For the interpretation of the results from this method, hypotension was considered as MAP < 60 mmHg, and we assumed that any readings below this limit required intervention. Based on this, the grid was divided into three zones (green, orange and red) of varying degrees of accuracy of MAP estimations by the NIBP method. The solid diagonal line represents perfect agreement between the two methods. The green zones represent NIBP values that deviate from the ‘gold standard’ (IBP) method by ≤ 10 mmHg (dark green) and between 10 and 20 mmHg (light green); for the tested device to be considered reliable ≥ 95% of the values should be included within the dark green zone (
      • Mori A.
      • Oda H.
      • Onozawa E.
      • et al.
      Evaluation of newly developed veterinary portable blood glucose meter with hematocrit correction in dogs and cats.
      ). Values falling within this zone are considered clinically accurate and will lead to correct treatment decisions. The light green zone does not extend into the hypotensive region since deviations in the NIBP reading of 10–20 mmHg during periods of hypotension would not be considered acceptable. The orange zone represents NIBP values that deviate from the IBP measurements by > 20 mmHg if blood pressure is > 60 mmHg and > 10 mmHg if blood pressure were < 60 mmHg. Therefore, in the lowermost orange zones, although the magnitude of hypotension detected by the NIBP device may differ by > 10 mmHg, it would still be treated. The upper red zone encompasses IBP readings < 60 mmHg with corresponding NIBP readings > 60 mmHg. Conversely, the lower red zone encompasses IBP readings > 60 mmHg and corresponding NIBP readings < 60 mmHg. Consequently, treatment decisions in the red zones would be inappropriate. To summarize, NIBP values in the green zones are clinically acceptable, whereas those in the orange zone are moderately inaccurate and NIBP values in the red zone may lead to mistreatment (e.g. treating hypotension when it is not present). Statistical analyses were performed using MedCalc Statistical Software (version 19.6 MedCalc Software Ltd, Belgium), Microsoft Excel (v16.0, Microsoft Corporation, CA, USA) and Microsoft PowerPoint (v16.0, Microsoft Corporation).

      Results

      A total of 714 paired measurements were obtained from 34 anaesthetized adult dogs, ranging in body weight from 5.6 to 38 kg [mean ± SD (18.76 ± 9.54 kg)]. The oscillometric method failed to provide blood pressure readings in 71 of the 714 pairs: a failure rate of 9.9%. A total of 25 dogs were positioned in dorsal recumbency, five were positioned in sternal, two were in right lateral and two were in left lateral recumbency. Metatarsal diameter ranged from 6.0 to 13.0 cm, and the mean ratio between the cuff width and metatarsus circumference was 42.7% ± 4.06. The dorsal pedal artery was cannulated in all dogs (19 left dorsal pedal artery and 15 right dorsal pedal artery). Subjective assessment of the system’s dynamic response was acceptable in all cases. Mechanical ventilation was required in 29 dogs, and eight needed drug therapy to maintain MAP greater than 60 mmHg as determined from IBP. The bias, SD, correlation coefficients, and the percentage of oscillometric measurements from the Datex S/5 lying within 5, 10 and 20 mmHg of the invasive method for paired readings of SAP, MAP and DAP are reported in Table 2. In summary, the Datex S/5 conformed to most of the ACVIM criteria except for the correlation coefficients between IBP and NIBP measurements, but not to the AAMI’s more rigorous standards (Table 2). Based on the confidence interval for the mean difference in MAP values between the two techniques, there was no systematic bias, but the Datex S/5 device tended to over-read SAP and MAP (negative bias) and under-read DAP (positive bias). From the LoA, the oscillometric device may over-read by 19.5, 13.1 and 23.0 mmHg, and under-read by 20.6, 16.9 and 13.3 mmHg for SAP, MAP and DAP, respectively, compared with the IBP values. This variability is shown in Fig. 1. The error grid analysis (Fig. 2) for the IBP and NIBP paired readings of MAP revealed that 82% of the values were within the green zone, 10% within the light green zone, 3% within the orange zone and 5% within the red zone.
      Table 2Bland-Altman analysis and correlation coefficients for paired readings (34 dogs, 715 paired readings) of systolic (SAP), mean (MAP), and diastolic (DAP) arterial blood pressure measurements in anaesthetized dogs recorded from an arterial catheter and from the Datex S/5 oscillometric device. Bias (mean difference) with standard deviation (SD), and limits of agreement (LoA) are reported. The percentage of oscillometric measurements from the Datex S/5 lying within 5, 10 and 20 mmHg of the invasive method are also reported. ACVIM, American College of Veterinary Internal Medicine consensus statement on hypertension in dogs and cats; AAMI, Association for the Advancement of Medical Instrumentation guidelines; CI, confidence interval; ND, not described.
      Bias

      (mmHg)

      (95% CI)
      SD

      (mmHg)
      Correlation Coefficient

      (95% CI)
      LoA (mmHg)

      (95% CI)
      ≤ ± 5 mmHg (%)≤ ± 10 mmHg (%)≤ ± 20 mmHg (%)
      Lower limitUpper limit
      SAP–0.52∗† (–0.33 to 1.19)10.17∗0.71 (0.67 to 0.74)–20.58 (–24.45 to –17.69)19.54 (16.65 to 23.41)3966.7∗96∗
      MAP-1.90∗ (–2.48 to –1.35)7.58∗†0.75 (0.72 to 0.78)–16.92 (–20.45 to –14.35)13.10 (10.53 to 16.64)55.681.7∗98.6∗
      DAP4.85∗† (4.15 to 5.51)9.16∗0.68 (0.63 to 0.72)–13.25 (–17.64 to –10.1)22.96 (19.8 to 27.34)38.973.3∗93.1∗
      ACVIM standards± 10∗± 15∗≥0.9NDND≥50∗≥80∗
      AAMI standards± 5± 8NDND≥85≥95ND
      ∗Results which conform to ACVIM criteria. Results which conform to the AAMI criteria, and both depicted in bold type.
      Figure 1
      Figure 1Bland-Altman analysis for repeated measurements plots to demonstrate the variability and limits of agreement (LoA) for (a) systolic arterial pressure (SAP), (b) mean arterial pressure (MAP) and (c) diastolic arterial pressure (DAP) measurements recorded from an arterial catheter placed in the dorsal pedal artery and from the Datex S/5 oscillometric device in 34 anaesthetized adult dogs. The upper and lower lines (dashed lines) represent the LoA between the two different measurement methods [mean difference ± 1.96 standard deviation (SD)]. The solid lines represent the bias (mean difference). IBP, Invasive blood pressure; NIBP, noninvasive blood pressure.
      Figure 2
      Figure 2Error grid analysis for invasive (IBP) and noninvasive (IBP) mean arterial blood pressure measurements. The x-axis represents the IBP values (‘gold standard’ technique) and the y-axis represent the values generated by the NIBP method. The solid diagonal line represents perfect agreement between the two methods, with data points above and below this line representing overestimates and underestimates, respectively. The NIBP values in the green zones (dark and light green zones) are clinically acceptable, whereas those in the orange zone are moderately inaccurate, and the NIBP values in the red zone may lead to mistreatment (e.g. treating hypotension when it is not present), and therefore unacceptable.

      Discussion

      The aim of this study was to compare NIBP values from the Datex S/5 with those obtained by IBP measurement in anaesthetized healthy adult dogs. We compared the results against both the ACVIM and AAMI standards for validation of new NIBP measurement devices. The Datex S/5 conformed to most of the ACVIM standards, but not to the AAMI standards (Table 2).
      ABP measurement is one of the most important variables used in clinical practice to evaluate cardiovascular function in anaesthetized dogs. Anaesthetists often rely on multiparametric monitors for NIBP monitoring in this species. It is therefore important that the noninvasive measurement is sufficiently accurate to guide changes in anaesthetic management and therapeutic decisions. The Datex oscillometric device evaluated in this study conformed to most of the ACVIM standards except for the correlation coefficients between IBP and NIBP measurements (Table 2). However, the value of correlation statistics in agreement studies has been questioned (
      • Bland J.M.
      • Altman D.G.
      Comparison of methods of measuring blood pressure.
      ). We used a modified Bland-Altman method for repeated measurements analysis which considers repeated measurements over time and its variability for each individual (
      • Bland J.M.
      • Altman D.G.
      Agreement between methods of measurement with multiple observations per individual.
      ). The wide LoA from our analysis suggests that the monitor is unreliable in providing accurate NIBP measurements, as it may significantly over- or under-estimate IBP readings. We would have reservations using NIBP measurements recorded from the Datex S/5 to guide any therapeutic interventions. These results are similar to previous work assessing agreement between IBP and NIBP measurements in anaesthetized dogs using other monitors (
      • Deflandre C.J.A.
      • Hellebrekers L.J.
      Clinical evaluation of the Surgivet V60046, a non-invasive blood pressure monitor in anaesthetized dogs.
      ;
      • Wernick M.
      • Doherr M.
      • Howard J.
      • Francey T.
      Evaluation of high-definition and conventional oscillometric blood pressure measurement in anaesthetised dogs using ACVIM guidelines.
      ;
      • Seliškar A.
      • Zrimšek P.
      • Sredenšek J.
      • Petrič A.D.
      Comparison of high definition oscillometric and Doppler ultrasound devices with invasive blood pressure in anaesthetized dogs.
      ;
      • Vachon C.
      • Belanger M.C.
      • Burns P.M.
      Evaluation of oscillometric and doppler ultrasonic devices for blood pressure measurements in anesthetized and conscious dogs.
      ).
      MAP is considered the most important blood pressure measurement during anaesthesia, as it is the driving pressure which determines tissue perfusion (
      • Tibby S.M.
      • Murdoch I.A.
      Measurement of cardiac output and tissue perfusion.
      ;
      • Haskins S.C.
      Monitoring Anesthetized Patients.
      ). Therefore, any therapeutic interventions mainly rely on evaluation of MAP. Datex NIBP showed better agreement and narrower LoA for MAP measurement than for SAP and DAP which may be related to the oscillometric principle of measuring ABP, wherein the point of maximal oscillations that corresponds to the MAP, and SAP and DAP are estimated according to proprietary algorithms (
      • Pickering T.G.
      • Hall J.E.
      • Appel L.J.
      • et al.
      Recommendations for blood pressure measurement in humans and experimental animals part 1: Blood Pressure Measurement in Humans A Statement for Professionals From the Subcommittee of Professional and Public Education of the American Heart Association Coun.
      ). SAP is identified as the cuff pressure at which the pulsations are 25% to 50% of their maximal amplitude and DAP is estimated when the pulse amplitude has declined to a small fraction of its peak value (
      • Schroeder B.
      • Barbeito A.
      • Bar-Yosef S.M.J.
      Cardiovascular Monitoring.
      ). It is possible that adjusting the algorithm to detect pressure endpoints in anaesthetized dogs could reduce the magnitude of the systematic error and improve the LoA between the IBP and NIBP methods (
      • Binns S.H.
      • Sisson D.D.
      • Buoscio D.A.
      • Schaeffer D.J.
      Doppler ultrasonographic, oscillometric sphygmomanometric, and photoplethysmographic techniques for noninvasive blood pressure measurement in anesthetized cats.
      ;
      • da Cunha A.F.
      • Ramos S.J.
      • Domingues M.
      • et al.
      Agreement between two oscillometric blood pressure technologies and invasively measured arterial pressure in the dog.
      ). As Datex oscillometry may over-read MAP by 13 mmHg and under-read by 16 mmHg, caution is advised when using NIBP data from this device to guide treatment.
      Invasive measurement of blood pressure is considered the ‘gold standard’ in dogs and is recommended when validating NIBP devices in this species (
      • Brown S.
      • Atkins C.
      • Bagley R.
      • et al.
      Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
      ). However, when comparing a new, noninvasive device to an invasive method, the readings from both methods will rarely be similar due to beat-by-beat variations (
      • O’Brien E.
      • Atkins N.
      • Stergiou G.
      • et al.
      Working Group on Blood Pressure Monitoring of the European Society of Hypertension. European Society of Hypertension International Protocol revision 2010 for the validation of blood pressure measuring devices in adults.
      ). Furthermore, several validation studies in dogs have compared noninvasive devices with IBP measurement (
      • Seliškar A.
      • Zrimšek P.
      • Sredenšek J.
      • Petrič A.D.
      Comparison of high definition oscillometric and Doppler ultrasound devices with invasive blood pressure in anaesthetized dogs.
      ;
      • Vachon C.
      • Belanger M.C.
      • Burns P.M.
      Evaluation of oscillometric and doppler ultrasonic devices for blood pressure measurements in anesthetized and conscious dogs.
      ;
      • da Cunha A.F.
      • Ramos S.J.
      • Domingues M.
      • et al.
      Agreement between two oscillometric blood pressure technologies and invasively measured arterial pressure in the dog.
      ;
      • Zrimšek P.
      • Sredenšek J.
      • Vengušt M.
      • Seliškar A.
      Evaluation of oscillometric blood pressure monitor BLT M9000 VET in anaesthetised healthy adult dogs.
      ). Despite being considered the ‘gold standard’ for blood pressure measurement in anaesthetized dogs (
      • Haskins S.C.
      Monitoring Anesthetized Patients.
      ), IBP measurement using an intra-arterial catheter may be subject to inaccuracies. The interaction between the damping coefficient and the natural frequency (dynamic response) of the monitoring system will determine how accurately the pressures are transmitted (
      • Gardner R.M.
      Direct blood pressure measurement - Dynamic response requirements.
      ). An underdamped system will resonate and result in falsely elevated SAP and lower DAP; however, MAP is little affected by these system characteristics (
      • Schroeder B.
      • Barbeito A.
      • Bar-Yosef S.M.J.
      Cardiovascular Monitoring.
      ). An objective analysis of this is possible by using the ‘square test’ or fast flush test, which allows the measurement of the frequency and amplitude of the oscillations (
      • Gardner R.M.
      Direct blood pressure measurement - Dynamic response requirements.
      ). In this study, the dynamic response of the system was assessed subjectively using the square wave fast flush test, as per normal clinical practice. This introduces subjective limitations to the methodology of the study and, as MAP measurements are little affected by changes in the system dynamic response, this may explain the narrower LoA compared with SAP and DAP.
      It is recognized that IBP measurement may vary according to body position and anatomic location in dogs (
      • Acierno M.J.
      • Domingues M.E.
      • Ramos S.J.
      • et al.
      Comparison of directly measured arterial blood pressure at various anatomic locations in anesthetized dogs.
      ). The arterial waveform changes from the aorta to the peripheral arteries, owing to arterial wave reflection when the pulse wave enters the different arteries, so called distal pulse amplification (
      • Acierno M.J.
      • Domingues M.E.
      • Ramos S.J.
      • et al.
      Comparison of directly measured arterial blood pressure at various anatomic locations in anesthetized dogs.
      ). The smallest bias and the narrowest LoA between IBP and NIBP were achieved with the NIBP cuff placed on the middle third of the antebrachium and the IBP at the median sacral artery when compared with other peripheral arteries (
      • da Cunha A.F.
      • Ramos S.J.
      • Domingues M.
      • et al.
      Validation of noninvasive blood pressure equipment: which peripheral artery is best for comparison studies in dogs?.
      ). However, that study did not evaluate the agreement between IBP and NIBP measurements using both dorsal pedal arteries. In the present study, we used both dorsal pedal arteries for IBP and NIBP measurements to minimize the influence of the anatomic blood pressure measurement variation. Since this artery is commonly used in clinical practice, our study is also highly representative of the clinical setting (
      • Trim C.M.
      • Hofmeister E.H.
      • Quandt J.E.
      • et al.
      A survey of the use of arterial catheters in anesthetized dogs and cats: 267 cases.
      ). The LoA between the two methods were still relatively wide suggesting that distal pulse amplification is not the main contributing factor.
      To improve accuracy, the NIBP cuff width should be 40% of the limb diameter (
      • Geddes L.A.
      • Combs W.
      • Denton W.
      Indirect mean arterial pressure in the anesthetized dog.
      ). In our study, the mean ratio between metatarsus circumference and the selected cuffs width was 42.7% (range, 35–48%), and therefore should not have affected the accuracy of our NIBP measurements. The positioning of the cuff may also influence NIBP measurements, but there appears to be no consensus within the veterinary literature regarding the best location for NIBP monitoring (
      • Garofalo N.A.
      • Neto F.J.T.
      • Alvaides R.K.
      • et al.
      Agreement between direct, oscillometric and Doppler ultrasound blood pressures using three different cuff positions in anesthetized dogs.
      ;
      • da Cunha A.F.
      • Ramos S.J.
      • Domingues M.
      • et al.
      Validation of noninvasive blood pressure equipment: which peripheral artery is best for comparison studies in dogs?.
      ;
      • Fujiyama M.
      • Sano H.
      • Chambers J.P.
      • Gieseg M.
      Evaluation of an indirect oscillometric blood pressure monitor in anaesthetised dogs at three different anatomical locations.
      ). All dogs in our study had the NIBP cuff placed below the tarsus (over the dorsal pedal artery) regardless of the body position during anaesthesia.
      Our study showed a failure rate of NIBP readings of 9.9% and reasons for this may be multifactorial. Vasoconstriction and bradyarrhythmias may contribute to failure of pulse detection and underestimation of NIBP values in anaesthetized dogs (
      • McMurphy R.M.
      • Stoll M.R.
      • McCubrey R.
      Accuracy of an oscillometric blood pressure monitor during phenylephrine-induced hypertension in dogs.
      ). Likewise, agreement between IBP and NIBP measurements may also be affected by vasoconstriction (
      • Kleinman B.
      Understanding natural frequency and damping and how they relate to the measurement of blood pressure.
      ). Therefore, the administration of α2-adrenoceptor agonists for premedication, and vasopressor drugs for the management of intraoperative hypotension, may have influenced signal quality and detection by the Datex oscillometry in some of the dogs we studied. This may explain the relatively high failure rate when compared with other studies (
      • Deflandre C.J.A.
      • Hellebrekers L.J.
      Clinical evaluation of the Surgivet V60046, a non-invasive blood pressure monitor in anaesthetized dogs.
      ;
      • Shih A.
      • Robertson S.
      • Vigani A.
      • et al.
      Evaluation of an indirect oscillometric blood pressure monitor in normotensive and hypotensive anesthetized dogs.
      ;
      • Yamaoka T.T.
      • Flaherty D.
      • Pawson P.
      • et al.
      Comparison of arterial blood pressure measurements obtained invasively or oscillometrically using a Datex S/5 Compact monitor in anaesthetised adult horses.
      ), and why the AAMI standards were not achieved. However, as this was a clinical study, it was important that the performance of the NIBP device was assessed in real clinical scenarios, where a variety of drugs were administered.
      The use of Bland-Altman and correlation analysis of the paired data may be used to evaluate the accuracy of a monitor compared with a reference method. However, these statistical methods may not be clinically useful (
      • Timothy M.
      • Nikolaus G.
      • Mark R.
      Let’s think clinically instead of mathematically about device accuracy.
      ). Therefore, an error grid analysis was adapted from previous studies, which evaluated newly developed glucose monitoring systems in dogs and cats (
      • Mori A.
      • Oda H.
      • Onozawa E.
      • et al.
      Evaluation of newly developed veterinary portable blood glucose meter with hematocrit correction in dogs and cats.
      ) and from studies that evaluated the haemoglobin concentration in humans (
      • Timothy M.
      • Nikolaus G.
      • Mark R.
      Let’s think clinically instead of mathematically about device accuracy.
      ). The error grid analysis shows the absolute values of the new device, the absolute values of the reference method, and the difference between these values while the clinical significance of this difference may be determined by the clinician (
      • Timothy M.
      • Nikolaus G.
      • Mark R.
      Let’s think clinically instead of mathematically about device accuracy.
      ;
      • Mori A.
      • Oda H.
      • Onozawa E.
      • et al.
      Evaluation of newly developed veterinary portable blood glucose meter with hematocrit correction in dogs and cats.
      ). To the authors’ knowledge, error grid analysis has never previously been described in studies that evaluated NIBP devices. In this study, most of the paired values were within the green zone (82%), 10% were within the light green zone, 3% were within the orange zone and 5% of the values were in the red zone. It was particularly noticeable that some paired values were in the hypotensive region of the red zone (uppermost area), which would lead to severe hypotension being missed by the NIBP device in some dogs.
      There are some limitations in the present study. Only healthy adult dogs classified as ASA I or II were studied; therefore, accuracy and repeatability for animals with higher ASA scores were not tested. Other individual patient factors that could potentially affect the LoA between IBP and NIBP measurements, such as hair length, coat thickness, body condition score and leg shape (
      • Gains M.J.
      • Grodecki K.M.
      • Jacobs R.M.
      • et al.
      Comparison of direct and indirect blood pressure measurements in anesthetized dogs.
      ) were not considered in this study. We also used a variety of anaesthetic protocols, which may have resulted in distinct haemodynamic changes in the dogs studied and thus affected IBP and NIBP agreement, as suggested by
      • Shih A.
      • Robertson S.
      • Vigani A.
      • et al.
      Evaluation of an indirect oscillometric blood pressure monitor in normotensive and hypotensive anesthetized dogs.
      . Although this might have influenced our results, the use of different anaesthetic protocols illustrates variations in clinical practice, and the drugs used mirrored the choices of the individual anaesthetists. Furthermore, for an NIBP monitor to be evaluated, its reliability must be demonstrated across the range of anaesthetic protocols commonly used in clinical practice. Despite the overall wide LoA, some individual dogs had a very good agreement between NIBP and IBP measurements, but it was not possible to predict this based upon the anaesthetic protocol or supportive drugs used.
      In conclusion, Datex S/5 oscillometry technology for NIBP measurement did not meet all the ACVIM criteria for validation. MAP measurements demonstrated the best agreement when compared with invasive measurement. Our results suggest that caution must be exercised when using this device to guide therapeutic interventions for blood pressure management in anaesthetized healthy adult dogs.

      Acknowledgements

      The authors thank Tyfane Yamaoka for the provision of training for data collection and to all the Southern Counties Veterinary Specialists involved in the instrumentation of the selected animals and support throughout the data collection.

      Authors’ contributions

      RF: study design, data collection, statistical analysis and interpretation, manuscript preparation. AGG: study design, data collection, and critical review of manuscript. VC: data collection, critical review of manuscript. DF and AA: study design, statistical analysis, data interpretation, critical review of manuscript.

      Conflict of interest statement

      The authors declare no conflict of interest.

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