Oxygen Transport and Metabolism in the Conscious Lamb: The Effects of Hypoxemia

Abstract

Printedin U.S.A. METHODS Subjects.A total of seven lambs of mixed breed with documented dates of birth was studied. The lambs were surgically prepared and studied within 24 h after surgery. Two of these lambs, although in good health during the study, died after their initial study, one due to complications of an intracardiac catheter and another due to trauma. The lambs were studied multiple times over the next 2 to 3 months following surgery.Under lidocaine local analgesia, a Tygon catheter was placed either in a hindlimb or carotid artery and advanced into the aorta. A Tygon catheter or balloon flow directed catheter was passed through a jugular vein into the pulmonary artery while pressure was monitored to ensure proper placement. All newborn lambs had a second venous catheter inserted through either the 177 ABSTRACT. We studied the response of O 2 consumption, systemic O 2 transport, arterial blood lactate concentration, base deficit, and respiratory exchange ratio (C0 2 production/O, consumption) to graded alveolar hypoxia (fractional concentration of inspired oxygen 0.21, 0.16, 0.12, and 0.08) in seven intact, conscious chronically catheterized lambs at <1 wk after birth, at 25-40 days and at >40 days. To test whether there was an age related difference in the metabolic response to a decline in O 2 transport due to hypoxemia, we related the changes in blood lactate and base deficit to the systemic O2 transport and to the fractional change in O 2 consumption. Systemic O 2 transport and O 2 consumption decreased with severe hypoxemia, while lactate concentration and respiratory exchange ratio increased in all age groups. The changes in base deficit were not significant. When the hypoxemia-induced reduction in O 2 consumption was <15% of the baseline value (at fractional concentration of inspired oxygen 0.21) there was no elevation in lactate concentration; however, when O2 consumption was reduced by more than 15%, lactate concentration consistently increased by more than 1 mmol/ liter. There were no apparent differences amongst the age groups in this response. Therefore, the newborn lamb does show metabolic consequences of a fall in O2 consumption with hypoxemia which is similar to the older sheep, however, there is a "buffer zone" in which O2 consumption may decrease before evidence of tissue hypoxia can be found. The mechanism by which O 2 consumption can decrease without accumulation of lactate remains speculative. (Pediatr Res 22: 177-183, 1987 Supported by grants from the American Heart Association with funds contributed in part by the Connecticut Heart affiliate and from the National Institute of Child Health and Human Development HD-15834. M. M. and G. M. were supported by a National Heart, Lung and Blood Institute Training Grant HL-07272. G. L. is a recipient of an American Heart Established Investigatorship.In anesthetized adult subjects, the systemic O 2 consumption has been found to be independent of the systemic O 2 transport (product of arterial O 2 content and cardiac output) when the transpor...

Systemic and Myocardial Effects of ATP and Adenosine during Hypoxic Pulmonary Hypertension in Lambs

“…This may reflect an attempt to optimize SOT during a period of decreased 0, availability. A similar increase in Hb concentration during hypoxia was previously reported in lambs (5). The Hb concentration gradually returned to baseline as hypoxia was continued.…”

“…Inasmuch as V 0 2 did not decrease, the decrease in temperature is unlikely to be an attempt to sacrifice thermogenesis during decreased 0, supply. Moss et al (5) speculated that hypothermia in newborn lambs during hypoxia may represent greater heat loss at this age because of increased body surface area. The increase in work of breathing during hypoxia may also cause greater evaporative heat loss from the respiratory tract.…”

“…Alveolar hypoxia was then induced by placing a loosely fitting plastic bag over the animal's head and allowing it to breathe a gas mixture of 10% O,, 5% CO,, and 85% N, at a flow rate of 5-8 L/min. Lambs frequently hyperventilate during hypoxia and lower their Paco,, which tends to attenuate the pulmonary vascular response to hypoxia (5). We added 5% CO, to the gas mixture to maintain a stable Paco, at or close to 5.3 kPa (40 mm Hg).…”

Effect of Theophylline on Metabolic and Ventilatory Response to Hypoxemia during Quiet Sleep in Piglets

“…Indeed, the small decrease in oxygen consumption with hypoxemia during the baseline period was still present after theophylline administration, despite the slight overall increase in metabolic rate. A decrease in oxygen consumption upon exposure to hypoxemia has been described in various newborn mammals (8,12,21,22) and in piglets studied in similar conditions and at an age similar to ours (9). It has been shown that this decrease in oxygen consumption is not accompanied by either lactate accumulation (21) or accumulation of an 0, deficit (13,22), both signs of tissue hypoxia.…”

“…A decrease in oxygen consumption upon exposure to hypoxemia has been described in various newborn mammals (8,12,21,22) and in piglets studied in similar conditions and at an age similar to ours (9). It has been shown that this decrease in oxygen consumption is not accompanied by either lactate accumulation (21) or accumulation of an 0, deficit (13,22), both signs of tissue hypoxia. This led to the suggestion that, during periods of 0, limitation, newborns actually decrease some of their oxygen consumption-but without signs of 0, deficit-by decreasing nonessential metabolism (processes requiring 0, that are part of normal metabolism but not essential for survival) (13,22).…”