Effect of end-inspiratory pause on airway and physiological dead space in anesthetized horses

Abstract

Objective: To evaluate the impact of a 30% end-inspiratory pause (EIP) on alveolar tidal volume (VTalv), airway (VDaw) and physiological (VDphys) dead spaces in mechanically ventilated horses using volumetric capnography, and to evaluate the effect of EIP on carbon dioxide (CO2) elimination per breath (VCO2br–1), PaCO2, and the ratio of PaO2-to-fractional inspired oxygen (PaO2:FIO2). Study design: Prospective research study. Animals: A group of eight healthy research horses undergoing laparotomy. Methods: Anesthetized horses were mechanically ventilated as follows: 6 breaths minute–1, tidal volume (VT) 13 mL kg–1, inspiratory-to-expiratory time ratio 1:2, positive end-expiratory pressure 5 cmH2O and EIP 0%. VCO2br–1 and expired tidal volume (VTE) of 10 consecutive breaths were recorded 30 minutes after induction, after adding 30% EIP and upon EIP removal to construct volumetric capnograms. A stabilization period of 15 minutes was allowed between phases. Data were analyzed using a mixed-effect linear model. Significance was set at p < 0.05. Results: The EIP decreased VDaw from 6.6 (6.1–6.7) to 5.5 (5.3–6.1) mL kg–1 (p < 0.001) and increased VTalv from 7.7 ± 0.7 to 8.6 ± 0.6 mL kg–1 (p = 0.002) without changing the VTE. The VDphys to VTE ratio decreased from 51.0% to 45.5% (p < 0.001) with EIP. The EIP also increased PaO2:FIO2 from 393.3 ± 160.7 to 450.5 ± 182.5 mmHg (52.5 ± 21.4 to 60.0 ± 24.3 kPa; p < 0.001) and VCO2br–1 from 0.49 (0.45–0.50) to 0.59 (0.45–0.61) mL kg–1 (p = 0.008) without reducing PaCO2. Conclusions and clinical relevance: The EIP improved oxygenation and reduced VDaw and VDphys, without reductions in PaCO2. Future studies should evaluate the impact of different EIP in healthy and pathological equine populations under anesthesia.