Oxygen Cost of Breathing in Postoperative Patients

Oxygen Cost of Breathing in Postoperative PatientsPressure Support Ventilation vs Continuous Positive Airway Pressure
During weaning from artificial ventilation, changing from a controlled to a spontaneous mode of respiration is associated with an increase in whole-body oxygen consumption (Vo2tot). This increase, called oxygen cost of breathing (Vo2resp), is less than 5 percent ofVo2tot in normal subjects breathing quietly. In critically ill or postoperative patients, whose tra-cheas were intubated, Vo2resp has been found to be in the order of 20 percent of Vo2tot. Recently, inspiratory pressure support ventilation (IPSV) has been proposed to decrease the work of breathing in spontaneously breathing patients. We hypothesized that IPSV can reduce significantly Vo2 resp in the weaning period of artificial ventilation. We therefore investigated Vo2resp in postoperative patients breathing alternatively with IPSV and with continuous positive pressure ventilation (CPAP). in detail

Materials and Methods
We studied seven tracheally intubated male patients who required a two to three days of respiratory support after a major surgical procedure. They had normal results preoperative pulmonary function tests. Ventilation was provided with a ventilator (Siemens Servo C). Clinical data are given in Table 1. All patients were studied on the morning preceding extubation. At this time, they could alternately be breathing spontaneously, or be on controlled ventilation. They were normovolemic and had normal cardiovascular functions. The nutritional intake was kept constant throughout the study.
Protocol
The protocol was approved by the ethical committee of our institution, and informed consent was obtained from the patients’ nearest relative. Three consecutive 60-minute study periods were achieved, according to a randomized order: controlled ventilation (CV), CPAP, and IPSV. For the three ventilatory modes, the same low level of PEEP (5 cm H20) was applied to the patients in order to diminish the incidence of postoperative respiratory complications. During CV, the ventilator frequency and minute volume were adjusted so that the patients appeared relaxed and comfortable and did not trigger inflation or resist gas flow. During IPSV, pressure support was set at 15 cm H20, and the trigger level at – 2 cm H20.
During the second half of each period, Vo2 tot, carbon dioxide production (VcoJ, respiratory quotient (RQ), tidal volume (Vt), respiratory rate (RR), and minute volume (Ve) were continuously measured and recorded at three-minute intervals. At the conclusion of each period, an arterial blood sample was taken for the measurements of Pa02, PaC02, and H+ concentration. In one patient who had no esophageal surgery (patient 7), simultaneous recordings of respiratory flow, airway pressure, esophageal pressure, and gastric pressure were achieved during the application of the three ventilatory modes.

Table 1—Patients Characteristics Upon Entry Into the Study

Patient No. Age Diagnosis and Surgical Procedure Peak Paw,* cm H20 Flo, Internal diameter of endotracheal tube (mm)
1 59 Esophagus carcinoma : cervicotomy + thoracotomy + laparotomy 38 0.35 8.5
2 58 Cardia carcinoma : thoracotomy + laparotomy 32 0.34 8
3 49 Esophagus carcinoma : thoracotomy + laparotomy 30 0.33 7.5
4 57 Esophagus carcinoma : thoracotomy + laparotomy 32 0.36 8.5
5 53 Esophagus carcinoma : thoracotomy + laparotomy + cervicotomy 42 0.37 8
6 55 Esophagus carcinoma : cervicotomy + laparotomy 38 0.39 8
7 42 Peritonitis : laparotomy 36 0.31 8