- Frank-Starling Mechanism
- Venous Return and Stroke Volume
- Assessing fluid responsiveness in the spontaneous breathing patient
- The Frank–Starling mechanism and thermal stress: fundamentals applied!
- Frank-Starling Law — Preload and Afterload
- Pulse pressure variation: beyond the fluid management of patients with shock
The Frank—Starling mechanism, by which changes in end-diastolic volume alter stroke volume, is one of the more fundamental concepts of human physiology. In humans, this concept is represented by a hyperbolic pulmonary capillary wedge pressure index of left-ventricular filling pressure to stroke volume curve.
In a recent issue of the Journal of Physiology , Wilson et al. Thermal stress brings about significant cardiovascular adjustments that ultimately affect cardiac filling pressure and therefore the Frank—Starling mechanism.
During passive heat stress, increases in core and skin temperatures require a redistribution of blood flow from non-cutaneous vascular beds and an increase in skin blood flow so that heat can be transferred from the body core to the skin. In such situations, central venous pressure and central blood volume decrease Crandall et al. Despite these reductions in cardiac filling pressure, stoke volume is well maintained due to increased cardiac contractility Brothers et al. Although mean arterial pressure is well maintained in the supine position, a competition develops between the maintenance of blood pressure and the need to maintain adequate skin perfusion for heat exchange when gravitational stress e.
As such, many studies have shown that orthostatic tolerance is greatly reduced in the heat stressed human.
Venous Return and Stroke Volume
On the other hand, during passive cold stress, peripheral vasoconstriction attenuates the rate of heat dissipation from the body by reducing the temperature gradient between the skin and the environment. Contrary to heat stress, decreases in skin blood flow parallel increases in total peripheral resistance and mean arterial pressure Cui et al. As such, cold stress greatly improves orthostatic tolerance not only relative to hyperthermia but also to normothermia.
While studies have established clear differences in orthostatic tolerance between heat and cold stresses, the underlying mechanisms for these differences have not been entirely elucidated. Cold stress has been shown to maintain greater levels of central blood volume, central venous pressure and consequently stroke volume for a given level of LBNP compared to both normothermia and hyperthermia Cui et al. The maintained stroke volume during cold stress suggests a plasticity in the operating point of the Frank—Starling mechanism according to an individual's thermal status, thereby eliciting a different change in stroke volume for a given decrease in left-ventricular filling pressure.
To test the hypothesis that such a mechanism might contribute to the observed differences in orthostatic tolerances between heat and cold stresses, Wilson et al.
Assessing fluid responsiveness in the spontaneous breathing patient
Capillary wedge pressure, an index of left-ventricular filling pressure, was determined by balloon inflation of a pulmonary artery catheter. Cardiac output was determined by thermodilution, and subsequently used to calculate stroke volume. Participants were fitted with a water-perfused suit and were placed in the supine position in a LBNP chamber.
The two different levels of LBNP were used to induce changes in pulmonary capillary wedge pressure, to which changes in stroke volume were subsequently related.
The results revealed that the thermal status of the individual clearly influenced the operating point of the Frank—Starling curve.
Specifically, the pre-LBNP values for the operating point in normothermia were located midway between the flatter and steeper portion of the curve, whereas cold stress shifted the operating point to the right i.
The Frank–Starling mechanism and thermal stress: fundamentals applied!
In contrast, heat stress shifted the operating point to the left i. As such, large reductions in stroke volume during heat stress were brought about by relatively small changes in pulmonary capillary wedge pressure, while stroke volume remained on or near the flat portion of the curve even at the highest level of LBNP during skin surface cooling.
Although orthostatic tolerance was not directly assessed in the study of Wilson et al. In contrast, all participants completed the 30 mmHg LBNP without any signs of syncope during the normothermic and cold stress conditions. Schematic illustration of the Frank—Starling relationship and how the operating point i.
Therefore, when cardiac filling decreases e. LBNP , stroke volume will reach the steep portion of the curve relatively faster during heat stress compared to both normothermia and cold stress.
While multi-mechanistic in nature, orthostatic intolerance during heat stress will ultimately develop due to substantial reductions in cardiac output which cannot be compensated by increases in systemic vascular resistance.
Frank-Starling Law — Preload and Afterload
As evidenced by the data of Wilson et al. In fact, skin surface cooling in the study of Wilson et al.
Thus, attenuated decreases in cardiac output due to skin surface cooling are a likely mechanism by which orthostatic tolerance is preserved. The data of Wilson et al. National Center for Biotechnology Information , U. Journal List J Physiol v. J Physiol. Daniel Gagnon.
Pulse pressure variation: beyond the fluid management of patients with shock
Author information Copyright and License information Disclaimer. Email: ac. Open in a separate window. Figure 1.
Acknowledgments I would like to thank Dr Glen P. Kenny for his assistance in preparing this article.
Cardiac systolic and diastolic function during whole body heat stress. Effects of passive heating on central blood volume and ventricular dimensions in humans.
Effect of skin cooling on central venous pressure during orthostatic challenge. Human cardiovascular adjustments to exercise and thermal stress.
Physiol Rev. Effect of thermal stress on Frank—Starling relations in humans.