Air-test for assessing gas exchange and the potential benefit of alveolar recruitment manoeuvre in anaesthetized dogs

Abstract

Objective: To determine the use of Air-Test in ventilated, anaesthetized dogs for evaluating oxygen uptake and to determine its potential utility in guiding the decision to perform an alveolar recruitment manoeuvre (ARM).

Study design: Retrospective cohort study.

Animals: A total of 25 client-owned dogs undergoing general anaesthesia.

Methods: Dogs were included if volume-controlled ventilation was performed, the dogs underwent an Air-Test and reliable haemoglobin oxygen saturation (SpO₂) readings ≥ 99% with an inspired oxygen fraction (FiO₂) 0.5 were available. Exclusion criteria included laparoscopic surgery, patient-ventilator asynchrony or haemodynamic instability. The Air-Test consisted of a reduction in FiO₂ to 0.21 for 4 minutes while monitoring SpO₂. A positive Air-Test result was defined as SpO₂ < 96% during this period. Positive cases underwent sustained inflation ARMs with adjustments to positive end-expiratory pressure. Haemodynamic variables, gas exchange and static compliance (C_st) were recorded pre-Air-Test, PaO₂/FiO₂ ratio was calculated offline. Data from dogs with positive and negative Air-Test results were compared and they are shown as mean ± standard deviation; p < 0.05.

Results: Of 835 records, 25 met the inclusion criteria. Positive Air-Tests occurred in 10 dogs (40%) and negative in 15 (60%). The baseline PaO₂/FiO₂ ratio was 364 ± 39.6 mmHg (48.4 ± 5.1 kPa) and 510 ± 65 mmHg (67.8 ± 8.6 kPa) in the Air-Test-positive and -negative animals, respectively (p < 0.001). The C_st and PaO₂/FiO₂ increased significantly after ARM in Air-Test-positive dogs (p < 0.001 and p = 0.02, respectively). No differences were observed in the remaining variables.

Conclusions and clinical relevance: A positive Air-Test was associated with a lower PaO₂/FiO₂ ratio and in these dogs, an ARM was associated with increased C_st and PaO₂/FiO₂ ratio, indicating the potential of this noninvasive technique for assessing oxygenation and monitoring changes in response to ARM in mechanically ventilated dogs.

Keywords: Air-Test; alveolar recruitment manoeuvres; anaesthesia; dogs.