The effect of moderate hypoxaemia on cardiac performance in the newborn during anaesthesia and surgery - experimental studies in newborn and two-week-old piglets.

Abstract

Circulatory adaptation to hypoxaemia was studied under anaesthetic and surgical conditions in 10 newborn (age 8-42 h, weight 850-1800 g) and 8 two-week-old piglets (12-16 days, 1400-3600 g). Arterial PO2 was lowered from 60-100 mmHg to 30-40 mmHg by reducing FiO2 (concentration of oxygen during inspiration) at otherwise constant conditions; control studies were performed under the same standard conditions maintaining normoxaemia during the whole experiment in 8 newborn and 8 two-week-old piglets of comparable weight. In both hypoxic groups a prompt compensatory, statistically significant increase of cardiac performance defined by aortic blood pressure, cardiac output, peak aortic flow, stroke volume and cardiac power appeared within a few minutes of hypoxaemia, reaching a maximum half an hour later. Thereafter cardiac performance decreased steadily leading to cardiac failure. Newborns survived 197 +/- 96 min and two-week-old animals 172 +/- 128 min. Peripheral vascular resistance decreased significantly at the time of compensatory increase of cardiac performance, but increased subsequently to a maximum shortly before cardiac failure. Compensatory increase of cardiac performance was more distinct in newborn than in two-week-old piglets. Heart rate increased steadily during exposure to hypoxaemia in both groups reaching a maximum 110 min later and falling back to pre-experimental levels at the end of study. The newborn normoxic controls tolerated the experimental model significantly longer (461 +/- 167 min; P less than 0.001) than the newborn hypoxic piglets. Their response pattern, however, was basically similar. Increase of cardiac performance reached its maximum not before 130 min after the end of the pre-experimental period with subsequent fatal decrease of cardiac performance leading to cardiac failure. Two-week-old normoxic control animals responded qualitatively identically compared with the two-week-old hypoxic animals. Despite normoxaemia their average survival time did not differ significantly (209 +/- 86 min). In all four groups severe metabolic acidosis appeared in most instances during the study, irrespective of hypoxaemia or normoxaemia. Failure of stress tolerance depended on the decrease of pH per hour. This relation was more distinct in normoxic animals. In both hypoxic groups survival time correlated well with the increase of heart rate during hypoxaemia (R = 0.71 in newborn, P less than 0.05; R = 0.86 in two-week-old piglets, P less than 0.01, respectively). This correlation could not be found in the normoxic piglets.