Changes in myocardial lactate metabolism during ramp exercise in patients with effort angina and microvascular angina

Nocturnal hypoxia and arterial lactate levels in sleep-related breathing disorders

BACKGROUND

Lactate may be useful in pointing out the higher risk subgroups in sleep-related breathing disorders (SRBD) with various patterns of hypoxemia. We aimed to search whether morning and night lactate levels are related to apnea-hypopnea, hypoventilation, and hypoxemia in patients with SRBD and to compare it with patients without SRBD (No-SRBD).

METHODS

Eighty patients with suspected SRBD underwent polysomnography (PSG) testing. SRBD patients had obstructive sleep apnea syndrome with or without sleep-related hypoventilation/hypoxemic conditions. Patients without SRBD were in the control group. Measurements included pulmonary function testing, PSG, analysis of arterial blood gases, and arterial lactate before and after PSG. Arterial lactate was compared with SRBD and No-SRBD patients.

RESULTS

Morning lactate was significantly higher in the SRBD group than the No-SRBD group (1.65 +/- 0.48 and 1.35 +/- 0.57 mmol/L, respectively) (P = 0.003). Lactate levels at night and the change overnight in lactate levels were not significantly different between groups. After an adjustment for age, gender, and body mass index, lactate levels before PSG were related to the apnea-hypopnea index (beta: 0.004, 95% CI: 0.000-0.008) and the rate of sleep-time spent under 90% oxygen saturation (T90%). The following morning lactate level was correlated with the T90% (beta: 0.005, 95% CI: 0.000-0.010). After an adjustment for lactate levels before PSG, lactate in the morning was correlated with T90% (beta: 0.004, 95% CI: 0.000-0.008).

CONCLUSION

As a marker of tissue hypoxia, arterial lactate may be used to assess the severity of SRBD.

Microvascular angina in a patient with aortic stenosis

A 39-year-old woman had exercise-induced ST segment depression associated with chest pain. Cardiac evaluation revealed moderate aortic stenosis (AS), related to the bicuspid valves, with an aortic mean pressure gradient of 22 mmHg, a calculated aortic valve area of 1.3 cm2 and normal left ventricular (LV) peak systolic and end-diastolic pressures, but no LV hypertrophy, resulting in normal LV wall stress. Although the coronary arteries were angiographically normal, rapid atrial pacing and an intracoronary papaverine injection revealed a significantly decreased coronary flow reserve (CFR), which may have played an important role in the pathogenesis of angina pectoris in this patient. Though the CFR is usually decreased in patients with AS, as well as in microvascular angina, in this particular case, it appeared to have decreased as a consequence of microvascular dysfunction rather than of AS-related mechanisms.

Consequences of inspired oxygen fraction manipulation on myocardial oxygen pressure, adenosine and lactate concentrations: a combined myocardial microdialysis and sensitive oxygen electrode study in pigs

Adenosine is a potent vasodilator whose concentration has been shown to increase in cardiac tissue in response to hypoxia. However, the time-dependent relationship between the levels of myocardial interstitial adenosine and tissue oxygenation has not yet been completely established. Therefore, the purpose of this study was to investigate the complex relationship between tissue myocardial oxygen tension (PtiO(2)) and interstitial myocardial adenosine and lactate concentrations by developing a new technique which combines a cardiac microdialysis probe and a Clark-type P O(2)electrode. The combined and the single microdialysis probes were implanted in the left ventricular myocardium of anesthetized pigs. The consequences of the combined use of microdialysis and P O(2)probes on myocardial PtiO(2)and microdialysis performances against glucose were evaluated. A moderate but significant reduction in the relative recovery against glucose of the combined probe was observed when compared to that of the single microdialysis probe (42+/-2 v 32+/-1%, mean+/-S.E. M.n=5 P<0.05), at 2microl/min microdialysis probe perfusion flow. Similarly, myocardial oxygen enrichment, measured by the P O(2)electrode, was negligible when microdialysis probe perfusion flow was 2microl/min. Systemic hypoxia (FiO(2)=0.08) resulted in a significant decrease in PtiO(2)from 30+/-4 to 11+/-2 mmHg, limited increase in coronary blood flow (CBF), and a significant increase in myocardial adenosine and lactate concentrations from 0.34+/-0.05 to 0.98+/-0.06micromol/l and from 0.45+/-0.05 to 0.97+/-0.06 mmol/l respectively (P<0.05). Increasing the FiO(2)to 0.3 restored the PtiO(2)and hemodynamic parameters to baseline values with no changes in interstitial adenosine and lactate concentrations. Nevertheless, myocardial interstitial adenosine remained significantly higher than baseline values. In conclusion, this study demonstrates the ability of a combined probe to measure simultaneously regional myocardial PtiO(2)and metabolite concentration during hypoxia. The hypoxia-induced increase in myocardial adenosine persists after correction of hypoxia. The physiological significance of this observation requires further studies.

Fatty acid oxidation in the reperfused ischemic heart

Myocardial ATP production is dependent chiefly on the oxidative decarboxylation of glucose and fatty acids. The co-utilization of these and other substrates is determined by both the amount of any given substrate supplied to the heart as well as by complex intracellular regulatory mechanisms. This regulated balance is altered during and after ischemia. During aerobic reperfusion of ischemic myocardium, a rapid recovery of energy production is desirable for the complete recovery of muscle contractile function. It is now clear that the type of energy substrate used by the heart during reperfusion will directly influence this contractile recovery. By increasing the relative proportion of glucose oxidized to that of fatty acids, the mechanical function of the reperfused heart can be improved. However, fatty acid oxidation recovers quickly during reperfusion and dominates as a source of oxygen consumption. These high rates of fatty acid oxidation occur at the expense of glucose oxidation, resulting in a decreased recovery of both cardiac function and efficiency during reperfusion. One contributory factor to these high rates of fatty acid oxidation is a decrease in myocardial malonyl-coenzyme A (CoA) levels. Malonyl-CoA, which is synthesized by acetyl-CoA carboxylase, is an essential metabolic intermediary in the regulation of fatty acid oxidation. A decrease in malonyl-CoA level results in an increase of carnitine palmitoyl transferase-1 mediated fatty acid uptake into the mitochondria. This mechanism seems important in the regulation of fatty acid oxidation in the postischemic heart and is discussed in detail in this review, with reference to specific clinical scenarios of ischemia and reperfusion and options for modulating cardiac energy metabolism.

Comparison of coronary hemodynamics during infusions of dobutamine and adenosine in patients with angina pectoris

Pharmacologic stress tests are used to evaluate the significance of coronary arterial stenosis, and adenosine and dobutamine are often used for this purpose. The aim of this study was to examine coronary hemodynamics in 16 patients with stable exertional angina who had organic stenosis (> 90%) in the left anterior descending coronary arteries and in 13 control subjects at baseline and during separate intravenous infusions of dobutamine (20 micrograms/kg per min [corrected]) and adenosine (140 micrograms/kg per min [corrected]). Measurements of great cardiac vein blood flow and lactate and oxygen contents in the blood from the aortic root and great cardiac vein were analyzed in patients with ischemic signs of myocardial ischemia (ST-segment depression in the precordial ECG leads and/or myocardial lactate production) during the infusions (in 11 patients during adenosine infusion and in 12 patients during dobutamine infusion) and in control subjects. Myocardial oxygen consumption during adenosine infusion did not change compared with the baseline in both groups, whereas it increased during dobutamine infusion in both groups (p < 0.01 compared with the baseline in each case). However, great cardiac vein blood flow was significantly increased compared with the baseline during both infusions in both groups (p < 0.01 compared with baseline in each case), and the oxygen contents of great cardiac vein blood, which contains effluent from the ischemic region, showed no decrease but increased significantly during both infusions in the patients in comparison with control subjects. Although the global oxygen supply was excessive, myocardial ischemia occurred during dobutamine and adenosine infusions in the patients. As myocardial oxygen consumption remained unchanged during adenosine infusion, regional myocardial flow reduction, possibly as a result of flow maldistribution, may contribute, at least partly, to adenosine-induced myocardial ischemia. An increase in myocardial oxygen consumption may be the principal cause of myocardial ischemia during dobutamine infusion, but an inappropriate flow increase in normal myocardium may occur during dobutamine-induced ischemia.