Oxygen Cost of Breathing in Postoperative Patients: Discussion

Oxygen Cost of Breathing in Postoperative Patients: DiscussionIn the present study, Vo2resp was estimated as the difference between Vo2tot during spontaneous respiration and that during controlled ventilation. Two points should be emphasized. First, for the estimation of Vo2resp, we used a mass spectrometer system which has the advantage of giving an accurate continuous measurement of Vo2, in contrast with the Fick method, or the Douglas bag method2 which have the limitation of intermittent data availability. Secondly, during CV, particular attention was paid to ensure that ventilation was entirely passive, without any inspiratory, or expiratory effort. To obtain this complete relaxation, it was most often necessary to induce a certain degree of hyperventilation, leading to a decrease in H+ concentration. It has been shown that Vo2 is pH dependent, and that respiratory alkalosis is associated with an increase in Vo2. So, in our study, Vo2 might have been slightly over-estimated during CV, and thus, Vo2 resp might have been slightly underestimated. The hyperventilation associated with CV may also explain the higher value of RQ which was observed during this period. Indeed, 30 minutes after the onset of CV, the C02 washout was probably not completed, leading to an overestimation of Vco2, and thus, of RQ. cipro tablets

The values of Vo2 resp which we measured in our patients during CPAP ventilation were higher than those measured in normal subjects at rest.u Several factors can explain this result. The stress of spontaneous breathing, with an increase in catecholamine release, might be a first contributor. We did not measure plasma catecholamines, but it must be noted that heart rate and systolic blood pressure remained stable when the ventilatory conditions were changed. Probably, a more significant factor is dealing with the work of breathing. In the postoperative period of abdominal surgery, there is a slight shift from predominantly abdominal breathing to rib cage breathing. This change may induce an increase in respiratory work and a change in the mechanical efficiency of the respiratory muscles. Finally, there is additional inspiratory work of breathing during CPAP ventilation to trigger the ventilator and to overcome the resistances due to an eventual imbalance between the patients inspiratory demand and the inspiratory flow delivered by the breathing apparatus.