Coronary autoregulation and optimal myocardial oxygen utilization

Quantitative Assessment of Myocardial Blood Flow with SPECT

The quantitative assessment of myocardial blood flow (MBF) and coronary flow reserve (CFR) may be useful for the functional evaluation of coronary artery disease, allowing judgment of its severity, tracking of disease progression, and evaluation of the anti-ischemic efficacy of therapeutic strategies. Quantitative estimates of myocardial perfusion and CFR can be derived from single-photon emission computed tomography (SPECT) myocardial perfusion images by use of equipment, tracers, and techniques that are available in most nuclear cardiology laboratories. However, this method underestimates CFR, particularly at high flow rates. The recent introduction of cardiac-dedicated gamma cameras with solid-state detectors provides very fast perfusion imaging with improved resolution, allowing fast acquisition of serial dynamic images during the first pass of a flow agent. This new technology holds great promise for MBF and CFR quantification with dynamic SPECT. Future studies will clarify the effectiveness of dynamic SPECT flow imaging.

Evaluation of the fetal coronary circulation

Prenatal ultrasound today allows the detailed study of small caliber vascular beds including the fetal coronary arteries and the coronary sinus. The coronary circulation is unique because of its critical role in myocardial metabolism and function and its ability to adapt in many fetal conditions. The ultrasound examination techniques for the evaluation of the fetal coronary circulation are presented. Evaluation of the coronary arteries is primarily achieved by color flow imaging and pulsed wave Doppler. Conditions such as fetal growth restriction, anemia, ductus arteriosus constriction and bradycardia are associated with evidence of enhanced coronary blood flow. These findings suggest that short-term autoregulation and long-term alterations in myocardial flow reserve are present in the human fetus. At present, examination of coronary sinus blood flow has limited clinical utility, while increases of the coronary sinus diameter or attenuation of coronary sinus dynamics may provide useful markers of abnormalities of central venous drainage. Abnormal vascular connections between the coronary arteries and the ventricular cavities may be observed in outflow tract obstructive cardiac lesions. In these conditions prenatal detection of coronary fistulae may have a potential impact on outcome and therefore counseling and perinatal management.

The application of individualised fetal growth curves

Individually adjusted or 'customised' growth charts aim to optimise the assessment of fetal growth by taking individual variation into account, and by projecting an optimal curve which delineates the potential weight gain in each pregnancy. This results in an increased detection rate of true growth restriction and a reduction in false positive diagnoses for IUGR. An adjustable standard can apply across geographical boundaries, as individual variation exceeds that between different maternity populations.

A nonlinear model for myogenic regulation of blood flow to bone: equilibrium states and stability characteristics

A simple compartmental model for myogenic regulation of interstitial pressure in bone is developed, and the interaction between changes in interstitial pressure and changes in arterial and venous resistance is studied. The arterial resistance is modeled by a myogenic model that depends on transmural pressure, and the venous resistance is modeled by using a vascular waterfall. Two series capacitances model blood storage in the vascular system and interstitial fluid storage in the extravascular space. The static results mimic the observed effect that vasodilators work less well in bone than do vasoconstrictors. The static results also show that the model gives constant flow rates over a limited range of arterial pressure. The dynamic model shows unstable behavior at small values of bony capacitance and at high enough myogenic gain. At low myogenic gain, only a single equilibrium state is present, but a high enough myogenic gain, two new equilibrium states appear. At additional increases in gain, one of the two new states merges with and then separates from the original state, and the original state becomes a saddle point. The appearance of the new states and the transition of the original state to a saddle point do not depend on the bony capacitance, and these results are relevant to general fluid compartments. Numerical integration of the rate equations confirms the stability calculations and shows limit cycling behavior in several situations. The relevance of this model to circulation in bone and to other compartments is discussed.

Balancing the circulation: theoretic optimization of pulmonary/systemic flow ratio in hypoplastic left heart syndrome

OBJECTIVES

This study examined the effects of the pulmonary (QP)/systemic (QS) blood flow ratio (QP/QS) on systemic oxygen availability in neonates with hypoplastic left heart syndrome.

BACKGROUND

The management of neonates with hypoplastic left heart syndrome is complex and controversial. Both before and after surgical palliation and before heart transplantation, a univentricle with parallel pulmonary and systemic circulations exists. It is generally assumed that balancing pulmonary and systemic blood flow is best to stabilize the circulation.

METHODS

We developed a mathematical model that was based on the simple flow of oxygen uptake in the lungs and whole-body oxygen consumption to study the effect of varying the QP/QS ratio. An equation was derived that related the key variables of cardiac output, pulmonary venous oxygen saturation and the QP/QS ratio to systemic oxygen availability.

RESULTS

The key findings are 1) as the QP/QS ratio increases, systemic oxygen availability increases initially, reaches a maximum and then decreases; 2) for maximal systemic oxygen availability, the optimal QP/QS ratio is < or = 1; 3) the optimal QP/QS ratio decreases as cardiac output or percent pulmonary venous oxygen saturation, or both, increase; 4) the critical range of QP/QS, where oxygen supply exceeds basal oxygen consumption, decreases as cardiac output and percent pulmonary venous oxygen saturation decrease; 5) the relation between oxygen availability and QP/QS is very steep when QP/QS approaches this critical value; and 6) the percent oxygen saturation of systemic venous blood is very low outside the critical range of QP/QS and high within the critical range.

CONCLUSIONS

This analysis provides a theoretic basis for balancing both the pulmonary and systemic circulation and suggests that evaluating both systemic arterial and venous oxygen saturation may be a useful way to determine the relative pulmonary and systemic flows. When high systemic arterial and low systemic venous oxygen saturation are present, pulmonary blood flow should be decreased; conversely, when both low systemic arterial and venous oxygen saturation are present, more flow should be directed to the pulmonary circulation.

Autoregulation in the stenosed coronary circulation

Coronary vessel stenosis increases vascular resistance and limits the dynamic range of autoregulation. In this study, the limitation imposed by stenosed vessels on oxygen delivery to the myocardium was investigated using a theoretical model. For different degrees of stenosis and for different levels of arteriovenous oxygen content difference, the model predicted the limits of the contractility range for which ventricular oxygen balance is positive. The model also predicted the existence of an optimal contractility level which minimizes the cost of arterial pressure generation and provides the largest coronary oxygen reserve. With severe stenosis, myocardial oxygen balance is extremely sensitive to changes in the level of stenosis. The range of contractility in which the coronary circulation can meet the myocardial oxygen needs is dramatically reduced by small increases in stenosis severity or small decreases in arteriovenous oxygen difference. When the optimal contractility level is maintained, the heart can tolerate these detrimental changes to a greater extent.