Spontaneous breathing and total body oxygen consumption in children recovering from open-heart surgery.

Abstract

The effects of withdrawal of ventilatory support on cardiopulmonary function, oxygen consumption and carbon dioxide production were assessed in 25 infants and children within seven days (2.9 +/- 2.5 days; mean +/- SD) of an open heart operation, during weaning from ventilatory support. The average age of the patients was 3.4 +/- 3.5 years and weight 12.4 +/- 8.3 kg. Heart rate, blood pressure, arterial and central venous blood gas values, and oxyhemoglobin saturations were measured during controlled mechanical ventilation and during spontaneous breathing with continuous positive airway pressure. Simultaneously, VO2 and VCO2 were measured using indirect calorimetry. Withdrawal of ventilatory support effected an expected, significant decrease in arterial pH (7.42 +/- 0.10 to 7.37 +/- 0.06; p less than 0.001) and an increase in PaCO2 (34 +/- 6 to 40 +/- 5 mm Hg; p less than 0.0001), while arterial blood oxyhemoglobin saturation, heart rate, and blood pressure remained unchanged. A significant increase in central venous oxyhemoglobin saturation (67.9 +/- 11.9 to 74.8 +/- 8.3 percent; p less than 0.001) indicated improvement in systemic blood flow during spontaneous breathing. Average VO2 and VCO2 did not change significantly. A decrease in VO2 by more than 5 percent was seen in seven patients, an increase by more than 5 percent in nine, and a change within +/- 5 percent in nine patients. The change in VO2 was inversely related to the difference between measured and expected VO2 during mechanical ventilation (r = -0.73) and to body temperature (r = -0.69). The results indicate that factors other than the oxygen uptake by the respiratory muscles may have significant effects on total body VO2 in infants and children after open-heart surgery. Therefore, monitoring of VO2 during withdrawal of ventilatory support may not be an accurate indicator of respiratory work and oxygen cost of breathing in these patients.