Oxygen Cost of Breathing in Postoperative Patients: Conclusion

Our results are close to those published by Thung et al. These authors found Vo2resp to be 13 percent of Vo2tot in patients studied after cardiac surgery. Higher values have been reported by Wilson et al in the immediate postoperative period of cardiac surgery. In patients with various degrees of cardiorespiratory diseases, Vo2 resp was found three times higher than that reported in the present study2 (8.7 vs 2.6 ml OJh of ventilation). This is related to the underlying lung disease which is associated with an increased work of breathing and/or a decreased efficiency of the respiratory muscles.

Compared to CPAP ventilation, IPSV was associated with a lower RR and a higher TV. Similar changes have been observed by other investigators. The normal PaCOz values which we found during IPSV confirms that this mode of ventilation allows the patients to match their minute ventilation to their Vco2. During IPSV, the level of pressure support was set at 15 cm H20, a value lower than the 30 to 42 cm HaO peak airway pressures which were observed during CV. This suggests that, under IPSV, an active inspiratory effort was still required by the patients to provide a tidal volume of the same order as the one observed during controlled ventilation. This assumption is also supported by the sustained inspiratory diaphragmatic contraction which was observed in patient 7 during the application of IPSV. However, we found that the amount of oxygen required to supply energy to the respiratory muscles under IPSV was insignificant, since it did not affect whole body oxygen consumption. In the same way, it can be assumed from our data that the work which is necessary at the onset of inspiration to trigger the ventilator is of little magnitude.
In conclusion, the present study was conducted in the weaning period of a major surgical procedure in seven patients with normal preoperative pulmonary function test results. We found Vo2resp to be about 11 percent of Vo2tot. Secondly, under IPSV, Vo2resp was insignificant, which indicates that this mode of ventilation took over the major part of these patients’ work of breathing.