Retrospective study of the changes in dynamic compliance and ventilation/perfusion mismatch following salbutamol inhalation in hypoxaemic mechanically ventilated anaesthetized horses.

Objective: To study the changes in dynamic compliance (C), ventilation/perfusion (V˙/ Q˙) mismatch and haemodynamic variables in hypoxaemic anaesthetized horses whose PaO increased following salbutamol inhalation.

Study Design: Retrospective, clinical, cohort study.

Animals: A group of 73 client-owned horses treated with salbutamol when PaO <100 mmHg (13.3 kPa) during anaesthesia.

Methods: Horses were divided into two groups: responders (R), where PaO after salbutamol ≥1.2 PaO before treatment (i.e. ≥20% increase), and non-responders (NR), where PaO after salbutamol <1.2 PaO before treatment. Demographic data and intraoperative variables before treatment were compared between R and NR. C, arterial to end-tidal carbon dioxide difference [P(a-E´)CO], estimated ratio of dead space to tidal volume (est.V/V), estimated shunt fraction (F-shunt), heart rate, systolic, mean and diastolic arterial pressure and dobutamine requirements were compared before and after treatment within R and NR. For each variable, the difference (Δ) between values pre- and posttreatment was calculated and compared between groups R and NR. Numerical data were compared using univariate or bivariate analysis and categorical data were compared using chi-square test; p < 0.05.

Results: Of the 73 horses 50 were classified as R while 23 horses were classified as NR. There was no statistical difference between R and NR for demographic data or initial intraoperative variables except for body weight [R: 531 (170-715) kg, NR: 540 (420-914) kg]. While salbutamol did not alter C in either group, it significantly decreased P(a-E´)CO, est.V/V and F-shunt in R only. ΔP(a-E´)CO, Δest.V/V and ΔF-shunt were significantly greater in R (-17.8%, -19.0% and -24.1%, respectively) than in NR (11.5%, 6.6% and -0.3%, respectively).

Conclusions And Clinical Relevance: In hypoxaemic anaesthetized horses responding to inhaled salbutamol by a ≥1.2 increase in PaO no change in C was detected, but indicators of V˙/ Q˙ mismatch improved.