Tracheal gas insufflation during CPAP reduces volume and breathing effort in an animal model of acute injury

Aim of this study was to investigate the effects of tracheal gas insufflation (TGI) during CPAP on gas exchange, ventilation and effort of breathing in sheep with lung injury following injection of oleic acid (OA).

Methods

Ten young female sheep (27.1 ± 4.2 kg) were anesthetized with ketamine, trachotomized and intubated with a 9 mm ID jet Ventilation ETT (Mallinkrodt), with its tip placed 2 cm proximal to the carina. Sheep were ventilated with pressure support ventilation of 5 cm H₂O. PEEP of 5 cmH₂O and FiO₂ of 1.0 (S900C, Siemens). An arterial catheter and a pulmonary artery catheter were percutaneously inserted into the femoral artery and the external jugular vein, respectively. An esophageal balloon catheter (Bicore, Smartcath) was inserted into the left pleural cavity through an intercostal incision. Airway pressure, flow and pleural pressure signals were obtained using a CP100 Pulmonary Monitor (Bicore). Carinal pressure (Pcar) was measured with a transducer connected to the distal side port of the ETT. OA was injected into the right atrium in increments of 0.2 ml every 10 min until a total dose of 0.06 ml/kg was delivered. PaO₂, PaCO₂, and lung compliance (CI) were measured before and 2 h following the OA injection. The sheep were then connected to a CPAP circuit. CPAP, and CPAP combined with TGI (CPAP-TGI) were randomly applied for periods of 40 min each. During CPAP-TGI, a humidified (Conchatherm, Hudson Respiratory Care) gas mixture containing 60% O₂ was delivered at a flow of 10 1/min through a reverse thrust catheter (RTC) having its tip positioned 1 cm proximal to the tip of the ETT. Throughout both steps the FiO₂ was 0.6, the bias flow was 19 1/min and the PEEP was adjusted in order to achieve an average Pcar of 5 cmH₂O. After each step, hemodynamic, blood gas and mixed expired CO₂ fraction measurements were taken while signals from the CP 100 monitor were recorded by a personal computer. During analysis of stored tracings, tidal volumes (Vt), respiratory rates (RR), minute volumes (Vc) and the pleural pressure swings (Dpl) (ie an index of breathing effort) were computed. Also, the Vd/Vt ratio was calculated.

Results

The injection of OA resulted in a decrease in PaO₂ [431.0 ± 67.1–104.6 ± 40.1 (P < 0.01)], and CI [39.3 ± 15.6–10.4 ± 3.17 (P < 0.01)] while the PaCO₂ increased [46.6 ± 5.4–60.4 ± 11.9 (P < 0.01)]. Data obtained during CPAP and CPAP-TGI are shown in the Table.

Conclusions

The injection of OA in spontaneously breathing sheep resulted in lung injury characterized by marked hypoxia, low compliance and hypercapnia. TGI administered during CPAP reduced dead space, allowing reductions in PaCO₂, Vt and Vc, and led to a decrease in breathing effort. In patients with acute respiratory failure, adding TGI to CPAP might increase tolerance to spontaneous breathing.

Table