To describe some cardiorespiratory effects of an inspiratory-to-expiratory (IE) ratio
of 1:1 compared with 1:3 in ventilated horses in dorsal recumbency.
Randomized crossover experimental study.
A total of eight anesthetized horses, with 444 (330–485) kg body weight [median (range)].
Horses were ventilated in dorsal recumbency with a tidal volume of 15 mL kg–1 and a respiratory rate of 8 breaths minute–1, and IE ratios of 1:1 (IE1:1) and 1:3 (IE1:3) in random order, each for 25 minutes
after applying a recruitment maneuver. Spirometry, arterial blood gases and dobutamine
requirements were recorded in all horses during each treatment. Electrical impedance
tomography (EIT) data were recorded in four horses and used to generate functional
EIT variables including regional ventilation delay index (RVD), a measure of speed
of lung inflation, and end-expiratory lung impedance (EELI), an indicator of functional
residual capacity (FRC). Results were assessed with linear and generalized linear
Compared with treatment IE1:3, horses ventilated with treatment IE1:1 had higher mean
airway pressures and respiratory system compliance (p < 0.014), while peak, end-inspiratory and driving airway pressures were lower (p < 0.001). No differences in arterial oxygenation or dobutamine requirements were
observed. PaCO2 was lower in treatment IE1:1 (p = 0.039). Treatment IE1:1 resulted in lower RVD (p < 0.002) and higher EELI (p = 0.023) than treatment IE1:3.
Conclusions and clinical relevance
These results suggest that IE1:1 improved respiratory system mechanics and alveolar
ventilation compared with IE1:3, whereas oxygenation and dobutamine requirements were
unchanged, although differences were small. In the four horses where EIT was evaluated,
IE1:1 led to a faster inflation rate of the lung, possibly the result of increased
FRC. The clinical relevance of these findings needs to be further investigated.