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

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 (Vco2 br-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 cmH2 O and EIP 0%. Vco2 br-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 Vco2 br-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.