Michiel Dalinghaus

MYOCARDIAL CARBOHYDRATE, KETONE, AND FATTY-ACID UPTAKE IN CONSCIOUS LAMBS WITH AORTOPULMONARY SHUNTS

ABSTRACT: A left to right shunt increases myocardial work and is often accompanied by increased catecholamine levels. Because both increased myocardial work and increased catecholamine levels may induce increased fatty acid utilization, which could increase resting myocardial oxygen consumption and therefore unfavorably affect coronary reserve, we studied myocardial uptake of glucose, pyruvate, lactate, β-OH-butyrate, acetoacetate, FFA, and triglycerides in 12 7-wk-old lambs with aortopulmonary left to right shunts (58 ± 2% of left ventricular output, mean ± SEM) and in 10 control lambs 2 wk after surgery. Despite the shunt, systemic blood flow in the shunt lambs was maintained at the same level as in the control lambs. This was accomplished by an increased heart rate and stroke volume. Furthermore, the shunt was accompanied by an increased myocardial oxygen consumption in the shunt lambs (834 ± 70 versus 528 ± 43 μmol O2·min−1·100 g−1; p < 0.05). There were no significant differences in arterial substrate concentrations between the two groups. The same was true for arteriovenous differences across the myocardium, with the exception of lactate, which was substantially higher in shunt than in control lambs (72 ± 25 versus 18 ± 23 μmol/L; p < 0.05). As a consequence, myocardial lactate uptake in the shunt lambs was increased 15-fold (18 ± 6 versus 1 ± 2 μmol·min−1·100 g−1; p < 0.02), whereas uptake of the other substrates merely paralleled the increased myocardial blood flow. Our data demonstrate that myocardial substrate uptake is not substantially different between shunt and control lambs, with the exception of lactate, of which the extraction is 10-fold higher than in control lambs. We speculate that the increased myocardial lactate utilization may reflect an increase of lactate and pyruvate dehydrogenase activities.

In Utero ventilation augments the left ventricular response to isoproterenol and volume loading in fetal sheep

ABSTRACT: In its normal circulatory environment, the fetal left ventricle can maximally increase output less than 2-fold, in contrast to the nearly 3-fold increase that occurs at birth. Several studies have attributed this finding to fetal myocardial “immaturity,‘’ and speculated that there is a rapid maturation of the myocardium in the perinatal period. We investigated the importance of the circulatory environment itself, rather than myocardial immaturity, by measuring left ventricular output (LVO) during in utero oxygen ventilation and isoproterenol infusion. We studied seven near-term fetal sheep ≥2 d after placement of intravascular catheters, an endotracheal tube, and an electromagnetic flow transducer around the ascending aorta. We measured hemodynamic variables in the presence and absence of all combinations of oxygen ventilation, isoproterenol infusion, and volume infusion. Baseline LVO was normal (133 ± 27 mL·kg-1 min-1). Individually, oxygen ventilation (136 ± 11 mL·kg-1·min-1, p < 0.001) and isoproterenol (48 ± 11 mL·kg-1·min-1, p < 0.05) increased LVO significantly; volume infusion did not. Their cumulative effect increased LVO nearly 3-fold (to 387 ± 98 mL·kg-1·min-1), similar to levels seen in the newborn lamb. Mean left atrial pressure increased above right during oxygen ventilation (from 0.05 ± 0.54 kPa to 0.82 ± 0.39 kPa, p ≤ 0.0001). We conclude that the previously observed limitation in maximal LVO in the near-term fetus is primarily caused by its circulatory environment rather than relative myocardial immaturity, and speculate that a prominent Starling response is uncovered by decreases in left ventricular after-load and right ventricular constraint.

COMPARATIVE CIRCULATORY EFFECTS OF ISOPROTERENOL, DOPAMINE, AND DOBUTAMINE IN CONSCIOUS LAMBS WITH AND WITHOUT AORTOPULMONARY LEFT-TO-RIGHT SHUNTS

We studied the effect on the circulation of the catecholamines isoproterenol, dopamine, and dobutamine in chronically instrumented lambs with aortopulmonary left-to-right shunts (ages 11 to 87 days) and without shunts (ages 8 to 97 days). Infusion of 0.1 microgram/kg/min isoproterenol or 10 micrograms/kg/min dobutamine markedly increased heart rate and systemic and pulmonary blood flows, while stroke volume and the left-to-right shunt flow did not change. Since pulmonary blood flow increased and the left-to-right shunt flow did not change, the left-to-right shunt fraction decreased with the infusions of isoproterenol and dobutamine. The hemodynamic changes during the infusion of isoproterenol and dobutamine occurred immediately after the start of infusion and stabilized within a few minutes. The pattern of hemodynamic changes was not influenced by the presence of an aortopulmonary left-to-right shunt or by age. Infusion of 10 micrograms/kg/min dopamine caused only small hemodynamic changes. This study shows that heart rate and systemic blood flow in the lamb are closely related. Furthermore, it demonstrates that despite an increased systemic blood flow, left-to-right shunt flow does not change after infusion of isoproterenol and dobutamine. Any decision as to which positive inotropic agent might be preferred at an early age should await experimental work concerning the effect of these agents on the myocardial oxygen demand and on the distribution of the systemic blood flow.

Comparative effects of isoproterenol and dopamine on myocardial oxygen consumption, blood flow distribution and total body oxygen consumption in conscious lambs with and without an aortopulmonary left to right shunt

OBJECTIVESnWe sought to study the effects of catecholamines on myocardial oxygen consumption (VO2), regional blood flows and total body VO2 in lambs with circulatory congestion.nnnBACKGROUNDnCatecholamines are often used to support cardiovascular function in children with circulatory congestion because they increase contractility as well as heart rate. However, these changes increase myocardial oxygen demand and thus can lead to a mismatch between myocardial oxygen supply and demand. Catecholamines can also change regional blood flows and VO2 unfavorably.nnnMETHODSnWe infused isoproterenol (0.1 microg/kg body weight per min) and dopamine (10 microg/kg per min) and measured myocardial and total body VO2 and regional blood flows in chronically instrumented 7-week old lambs with and without a left to right shunt.nnnRESULTSnIsoproterenol increased myocardial VO2, parallel to the increase in heart rate. However, myocardial blood flow and, consequently, oxygen supply also increased. This increase outweighed the increase in myocardial VO2, so that myocardial oxygen extraction decreased. Isoproterenol did not change blood flow distribution. Isoproterenol increased total body VO2; however, systemic oxygen supply increased even more, so that oxygen extraction decreased and mixed venous oxygen saturation increased. In contrast, dopamine had no or little effect on myocardial VO2 or blood flow distribution.nnnCONCLUSIONSnWe conclude that the catecholamines isoproterenol and dopamine do not lead to a mismatch between myocardial oxygen supply and demand, nor do they change blood flow distribution unfavorably in 7-week old lambs with a left to right shunt. We demonstrated that isoproterenol is superior to dopamine, because it shifts the balance between oxygen supply and consumption toward supply so that systemic oxygen extraction reserve increases.

Effects of isoproterenol infusion on the myocardial uptake of fatty acids and other substrates in lambs with an aortopulmonary shunt

Isoproterenol, used in the management of infants with left-to-right shunts and circulatory congestion, increases myocardial work load and oxygen consumption. In addition, it may selectively enhance myocardial fatty acid utilization. The less efficient oxidation of FFA could induce an oxygen wasting effect and thus further increase myocardial oxygen consumption. The combination of such an oxygen wasting effect and the chrono- and inotropic effects of isoproterenol could induce an imbalance between myocardial oxygen supply and demand in hearts of which resting oxygen consumption is already elevated. We studied myocardial substrate uptake (FFA, triglycerides, glucose, lactate, pyruvate, β-OH-butyrate, and acetoacetate) in 10 7-wk-old lambs with an aortopulmonary left-to-right shunt (57 ± 4% of left ventricular output, mean ± SEM) and 9 control lambs during isoproterenol infusion(0.1 μmol·min-1·kg-1). Myocardial blood flow and oxygen consumption increased in both groups but less in shunt than in control lambs because of the smaller rise in heart rate in the shunt lambs. The arterial FFA concentration increased 3-fold in both groups and was not different between the two groups. The FFA arteriocoronary sinus difference, however, was not affected by the isoproterenol infusion. The myocardial FFA uptake thus followed the changes in myocardial blood flow and did not increase more in shunt than in control lambs. Isoproterenol infusion does, in spite of a 3-fold increase in arterial FFA concentration, not induce a shift toward a greater percentage uptake of fatty acids compared with other substrates in lambs with aortopulmonary left-to-right shunt, so that the possibility of an oxygen wasting effect can be ruled out as an unwanted side effect.

LEFT-VENTRICULAR OXYGEN AND SUBSTRATE UPTAKE IN CHRONICALLY HYPOXEMIC LAMBS

ABSTRACT: Myocardial oxygen demand may be increased in chronically hypoxemic lambs because of their increased heart rate. Therefore, we determined whether left ventricular (LV) oxygen supply, oxygen uptake and oxygen demand were proportionally increased in 6-wk-old lambs, after 4 wk of hypoxemia (n = 15), as compared with control lambs (n = 14). In addition, we determined whether LV glucose, pyruvate, lactate, FFA and ketoacids uptake were altered in hypoxemic lambs, because of alterations in arterial glucose, pyruvate and lactate concentrations, that may occur in hypoxemia. Hypoxemia was induced by the combination of an atrial septal defect and pulmonary stenosis. Arterial oxygen saturation was decreased in hypoxemic lambs (67 ± 8 versus 91 ± 3%, p < 0.001), Hb concentration was increased, so that arterial oxygen concentration was similar in both groups of lambs. Myocardial mass (61 ± 13 versus 44 ± 9 g, p < 0.001) and total myocardial blood flow (117 ± 36 versus 62 ± 27 mL.min-1, p < 0.001) were increased, mainly due to right ventricular hypertrophy. LV oxygen demand, estimated by the rate pressure product (2072 ± 465 versus 1467 ± 358 kPa · beat · Min-1, p < 0.001), and oxygen uptake (723 ± 223 versus 556 ± 184 μmol · min-1 · 100 g-1, p < 0.05) were proportionally increased in hypoxemic lambs. LV oxygen supply increased linearly with oxygen uptake (r = 0.96) in all lambs, by adjustments in LV blood flow, which was increased in hypoxemic lambs (168 ± 41 versus 134 ± 45 mL·min-1 · 100 g-1, p < 0.05). The increase in LV oxygen uptake in hypoxemic lambs was proportional to the increase in heart rate (166 ± 33 versus 118 ± 25 beats-min-1, p < 0.001). Arterial lactate, pyruvate and β-hydrox-ybutyrate concentrations were slightly increased in hypoxemic lambs, but LV substrate uptake was practically unaltered as compared with control lambs. FFA and β-hy-droxybutyrate contributed most to LV substrate uptake, whereas the contribution of glucose, pyruvate and lactate was negligible. The total oxygen extraction ratios (0.45 ± 0.43 versus 0.51 ± 0.50) indicate that approximately 50% of the fuels for the LV were identified. We conclude that LV oxygen supply is matched to increased oxygen demand in chronically hypoxemic lambs, by the increase in LV blood flow. LV substrate uptake is unaltered in hypoxemic lambs; glucose, pyruvate and lactate uptake is negligible, despite an increased arterial pyruvate and lactate concentration. FFA and ketoacid uptake are insufficient to fuel LV oxidative metabolism.

Effects of nitroprusside on myocardial blood flow and oxygen consumption in conscious lambs with an aortopulmonary left-to-right shunt.

We studied the effect of vasodilation on regional myocardial blood flow and oxygen consumption of the left ventricular free wall by infusing 10 micrograms/kg/min sodium nitroprusside into 7-week-old conscious lambs with and without aortopulmonary left-to-right shunts. Resting myocardial oxygen consumption in the 13 shunt lambs was significantly higher than in the nine control lambs (989 +/- 104 [+/- SEM] vs. 432 +/- 41 mumol/min/100 g left ventricle). This was achieved by a significantly higher left ventricular myocardial blood flow (294 +/- 33 vs. 143 +/- 16 ml/min/100 g left ventricle) because the arteriovenous oxygen concentration difference across the left ventricular free wall was similar in shunt and control lambs. Infusion of nitroprusside did not significantly change myocardial oxygen consumption and regional myocardial blood flows at 10 and 50 minutes after the onset of the infusion. This occurred despite a substantial drop in aortic and left atrial pressures and stroke volume, which decreases wall stress as well as external work of the left ventricle. Heart rate, however, increased significantly. We postulate that, during infusion of nitroprusside, the potential decrease in myocardial oxygen consumption due to a decrease in wall stress and external work of the left ventricle is neutralized by the consequences of the increased heart rate. In view of this and because of its hemodynamic effects, we do not consider sodium nitroprusside useful in the treatment of circulatory congestion in patients with left-to-right shunts, normal arterial pressures, and normal systemic blood flows.

MYOCARDIAL OXYGEN-CONSUMPTION (MVO2) AND SUBSTRATE UPTAKE IN FETAL AND 8-WEEK-OLD LAMBS

We measured myocardial blood flow to the left ventricular free wall and blood glucose, lactate, pyruvate, fatty acid, triglyceride, β-OH-butyrate, acetoacetate and oxygen concentrations in aorta (Ao) and coronary sinus (CS) of 6 fetal and 12 8-week-old chronically instrumented lambs at rest. We calculated MVO2 and substrate uptake when Ao and CS concentrations differed significantly. Despite significantly lower fetal Ao oxygen concentrations (3378±737 (SD) vs 5300±589 umol-1-1), MVO2 was not significantly different between the two groups of lambs (454±135 vs 514±194 μmol·min−1.100 g−1). Glucose (30.9±20.3 vs 36.5±52 μmol·min−1.100 g−1) and triglyceride (7.9±5.6 vs 6.6±10.4 μmol·min−1. 100 g−1) uptakes were also similar, while the fetal Ao concentrations were significantly lower (920±171 vs 3850±485 and 123±29 vs 319±114 μmol·1−1, resp.). Besides glucose and triglyceride uptake by the myocardium, the fetuses also showed lactate (108±69 μmol·min−1·100 g−1) and pyruvate uptake (19.8±22.8 μmol·min−·100 g−1), while the 8-week-old lambs showed only β-OH-butyrate uptake (22.0 ± 20.4 μmol·min−1·100 g−1). In both groups of lambs there was no uptake of free fatty acids. Assuming all substrates are metabolized aerobically they could account for 200% of oxygen uptake by the heart in fetal and 8-week-old lambs. We assume that during these studies the surplus of substrate uptake is stored by the myocardium.

Chronic hypoxemia in the 5-week-old lamb

The mechanisms involved in the adaptation to chronic hypoxemia, so as to maintain normal systemic oxygen delivery (SOD), are not quite clear. Therefore we studied these mechanisms by inducing chronic hypoxemia in 6 newborn lambs. We placed an inflatable balloon around the pulmonary artery after atrial septostomy, and inserted catheters. On days 3-4 after surgery the balloon was gradually inflated. In another 6 lambs only catheters were placed. At 5 weeks of age the lambs were studied. The hypoxemic lambs had a lower aortic and mixed venous oxygen saturation (60±10 (SD) vs 93±2, and 34±7 vs 58±2%, resp., p<0.001), and higher hemoglobin concentration (132±15 vs 101±10 g·1−1, p<0.002). Their P50 as well as their systemic blood flow were not significantly different. The heart rate of the hypoxemic lambs, however, was higher (205±22 vs 153±42, p<0.05). Although SOD in the hypoxemic lambs was lower than in the control lambs, this difference was not statistically significant (17.0±3.4 vs 21.2±5.9 ml.min−1kg−1). The mean weight gain of the hypoxemic lambs was lower (90±39 vs 153±34 g·day−1, p<0.02), while their oxygen consumption was the same as in the control lambs (7.0±2.2 vs 7.2±1.5 ml·min−1kg−1).These data show that to adapt to chronic hypoxemia the lamb increases its hemoglobin concentration to improve arterial oxygen content, and its heart rate to maintain cardiac output. We speculate that the costs of chronic hypoxemia, such as increased cardiac work, are probably effected at the expense of growth.

MYOCARDIAL BLOOD FLOW (q) AND OXYGEN CONSUMPTION (V O2 ) IN CHRONICALLY INSTRUMENTED LAMBS WITH AN AROTOPULMONARY SHUNT (S)

We showed that heart rate (HR) in lambs with a S is higher than in those without a S. This increase will affect myocardial VO2. We studied myocardial q and left ventricular (LV) VO2 in 7 lambs with a S (aged 7±1 wks, mean±SEM). 4-14 days prior to the studies we inserted catheters into aorta (Ao), coronary sinus(CS) and left atrium, and a Goretex ® graft between descending Ao and pulmonary artery (PA). Electromagnetic flow probes (EFP) were placed around Ao and PA to measure pulmonary (qp) and systemic (qs) blood flows, respectively. As controls another 8 lambs(aged 7±1 wks) were instrumented in the same way except for the Goretex ® graft and PA EFP. Myocardial q of LV was calculated by the radioactive microsphere method. LV VO2 was calculated from myocardial q of LV and the difference in O2 content between Ao and CS blood. The qp:qs ratio of the S lambs was 2.3. The S lambs had a higher HR (151±6 vs 110±7, p<0.001), myocardial q of LV (241±26 vs 136±17 ml/min/100 gLV, p<0.005), LV VO2 (757±74 vs 405±35 μmol/min/100 gLV, p<0.001), and a higher total heart weight (108.2±12.0 vs 77.3±6.9 gr, p<0.05) than the control lambs. Peak Ao pressure and qs were not significantly different in the two groups. These findings suggest that the increase in HR in the S lambs may be the main reason for the increase in LV VO2.