Computed Tomography Assessment of Tidal Lung Overinflation in Domestic Cats Undergoing Pressure-Controlled Mechanical Ventilation During General Anesthesia

Abstract

ObjectiveThis study aimed to evaluate lung overinflation at different airway inspiratory pressure levels using computed tomography in cats undergoing general anesthesia.Study DesignProspective laboratory study.AnimalsA group of 17 healthy male cats, aged 1.9–4.5 years and weighing 3.5 ± 0.5 kg.MethodsSeventeen adult male cats were ventilated in pressure-controlled mode with airway pressure stepwise increased from 5 to 15 cmH2O in 2 cmH2O steps every 5 min and then stepwise decreased. The respiratory rate was set at 15 movements per min and end-expiratory pressure at zero (ZEEP). After 5 min in each inspiratory pressure step, a 4 s inspiratory pause was performed to obtain a thoracic juxta-diaphragmatic single slice helical CT image and to collect respiratory mechanics data and an arterial blood sample. Lung parenchyma aeration was defined as overinflated, normally-aerated, poorly-aerated, and non-aerated according to the CT attenuation number (−1,000 to −900 HU, −900 to −500 HU, −500 to −100 HU, and −100 to +100 HU, respectively).ResultAt 5 cmH2O airway pressure, tidal volume was 6.7± 2.2 ml kg−1, 2.1% (0.3–6.3%) of the pulmonary parenchyma was overinflated and 84.9% (77.6%−87.6%) was normally inflated. Increases in airway pressure were associated with progressive distention of the lung parenchyma. At 15 cmH2O airway pressure, tidal volume increased to 31.5± 9.9 ml kg−1 (p < 0.001), overinflated pulmonary parenchyma increased to 28.4% (21.2–30.6%) (p < 0.001), while normally inflated parenchyma decreased 57.9% (53.4–62.8%) (p < 0.001). Tidal volume and overinflated lung fraction returned to baseline when airway pressure was decreased. A progressive decrease was observed in arterial carbon dioxide partial pressure (PaCO2) and end-tidal carbon dioxide (ETCO2) when the airway pressures were increased above 9 cmH2O (p < 0.001). The increase in airway pressure promoted an elevation in pH (p < 0.001).Conclusions and Clinical RelevanceVentilation with 5 and 7 cmH2O of airway pressure prevents overinflation in healthy cats with highly compliant chest walls, despite presenting acidemia by respiratory acidosis. This fact can be controlled by increasing or decreasing respiratory rate and inspiratory time.

Mechanical ventilation in dogs and cats with tick paralysis

“…Atelectic, overinflated and normally aerated lung units did not differ between groups at end inspiration, but at end expiration dogs receiving Vte 15 ml/kg with PEEP had significantly more normally aerated lung and significantly less poorly- or non-inflated lung than the other groups ( 74 ). Oxygenation was not different between groups ( 74 ). These findings suggest that the application of PEEP may be more important to prevent lung heterogeneity than Vte specifically, supporting the safety of lower Vte in dogs in which lung protective ventilation is indicated.…”

“…A small experimental study of anesthetized healthy adult cats receiving pressure-controlled mandatory ventilation with inspiratory pressures (Pi) of 5 and 7 cmH 2 O and 0 cmH 2 O PEEP generated a Vte of 6–10 ml/kg with minimal overinflation as assessed by computed tomography (CT) ( 74 ). At 15 cmH 2 O Pi, CT identified a significant increase in overinflated lung occurred ( 74 ). These findings suggest that initial inspiratory pressures of 5–7 cmH 2 O are appropriate in dogs and cats without pulmonary disease.…”

“…A randomized control trial of 40 dogs without pulmonary disease undergoing CT were anesthetized and randomized to four groups: Vte 8 ml/kg with or without 5 cm H 2 0 PEEP and Vte 15 ml/kg with or without 5 cm H 2 O PEEP ( 74 ). Atelectic, overinflated and normally aerated lung units did not differ between groups at end inspiration, but at end expiration dogs receiving Vte 15 ml/kg with PEEP had significantly more normally aerated lung and significantly less poorly- or non-inflated lung than the other groups ( 74 ). Oxygenation was not different between groups ( 74 ).…”

“…Current recommendations in humans with uninjured lungs are that ARMs should be performed with PEEP titration at low tidal volume when O 2 saturation is below 92% ( 85 ). Various methods applied to anesthetized dogs and cats with healthy lungs have shown that ARMs increase pulmonary compliance and may increase or have no effect on oxygenation ( 74 , 83 , 84 , 86 ). Transient alveolar overdistention ( 84 ), increased plateau pressures and a decrease in mean arterial pressure (MAP) have been demonstrated during maneuvers ( 74 , 82 – 84 ) and the risk of volutrauma, barotrauma and hemodynamic compromise should be recognized in these patients.…”

“…Various methods applied to anesthetized dogs and cats with healthy lungs have shown that ARMs increase pulmonary compliance and may increase or have no effect on oxygenation ( 74 , 83 , 84 , 86 ). Transient alveolar overdistention ( 84 ), increased plateau pressures and a decrease in mean arterial pressure (MAP) have been demonstrated during maneuvers ( 74 , 82 – 84 ) and the risk of volutrauma, barotrauma and hemodynamic compromise should be recognized in these patients. A prospective clinical study demonstrated evidence of alveolar overdistension in healthy dogs using a stepwise increasing/decreasing PEEP recruitment maneuver at tidal volumes of just 7 ml/kg ( 84 ).…”