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Functional Properties of Muscle
Published in Nassir H. Sabah, Neuromuscular Fundamentals, 2020
An important intrinsic property of cardiac muscle is that the force of ventricular contraction increases with the end-diastolic volume, that is, the volume of blood in the ventricles just before the beginning of contraction (Figure 10.19), an effect referred to as Starling’s law of the heart, or the Frank–Starling relation. The larger the end-diastolic volume, the larger is the initial stretch, or preload, of the ventricles, and the larger is the stroke volume, or the volume of blood ejected by a ventricle because of the increased force of contraction. This effect regulates the input-output of the heart, beat-by-beat, without the involvement of control mechanisms from outside the heart and is of physiological importance. For example, the two halves of the heart beat together, like two pumps in parallel. Yet, blood flows in series in the two circuits: the pulmonary circuit, in which blood is pumped through the lungs by the right half of the heart and the systemic circuit, in which blood is pumped through the rest of the body by the left half of the heart. The flow in the two circuits must be equalized to prevent accumulation of blood in the heart. If, for example, the peripheral resistance increases due to vasoconstriction in the systemic circuit, the reduced outflow from the left ventricle will cause accumulation of blood in this ventricle. However, the resulting increase in end-diastolic volume will automatically increase the force of contraction, so as to pump more blood from the left ventricle and equalize the flow in the two circuits.
Elements of Continuum Mechanics
Published in Clement Kleinstreuer, Biofluid Dynamics, 2016
where the stroke volume, ∀st, is measured in [ltrs/beat], the heart rate N in [beats/min], and the cardiac output, Q, in [1tr/min]. The contractility of the heart muscle and high ventricular filling pressure (i.e., preload) both increase ∀st, while elevated arterial pressure tends to decrease ∀st by increasing the afterload on cardiac muscle fibers. The “preload” effect, i.e., the phenomenon that the heart contracts more forcefully when it is filled to a greater degree during diastole, is called the Frank-Starling law of the heart.
A novel treadmill protocol for uphill running assessment: the incline incremental running test (IIRT)
Published in Research in Sports Medicine, 2021
Ricardo Dantas De Lucas, Bruna Karam De Mattos, Alexandre Da Cunha Tremel, Luana Pianezzer, Kristopher Mendes De Souza, Luiz Guilherme Antonacci Guglielmo, Benedito Sérgio Denadai
Interestingly, we observed higher HRpeak during speed-based test compared to IIRT. This result was confirmed by the agreement analysis, which produced bias ± 95%LOA values of 5 ± 6 bpm, although they were highly correlated (Figure 1, D and E). Of note, the subjects in this study were highly motivated during both tests, and it was felt that the subjects performed to the best of their ability. Partially, it was confirmed by VO2max verification test results. Once again, the protocol characteristics could explain this noteworthy finding. We speculate that an enhanced calf muscle pump could trigger a higher central venous pressure (Notarius & Magder, 1996), which could influence the HR response. Thus, the greater calf muscle activation would enhance venous return via peripheral muscle pump (Smith et al., 1976). Based on Frank-Starling law (Stein et al., 1980), the greater the ventricular diastolic volume, the more the myocardial fibres are stretched during diastole, triggering a greater ventricle force of contraction. Thus, the ventricular output increases as the preload (end-diastolic pressure) increases. In this sense, the augmented venous return probably triggered a greater cardiac preload, thus inducing a greater stroke volume (while reducing HR for the same cardiac output) during the IIRT.
Approaches to improving exercise capacity in patients with left ventricular assist devices: an area requiring further investigation
Published in Expert Review of Medical Devices, 2019
Richard Severin, Ahmad Sabbahi, Cemal Ozemek, Shane Phillips, Ross Arena
LV preload is dependent on a multitude of factors including volume status, filling pressures, and right ventricular (RV) function. Right-sided HF was found to be strongly associated with increased morbidity and mortality post-LVAD implant [14]. Adverse reactions of the RV due to interventricular ischemia and altered septal geometry in response to LVAD support have been shown [38,46,47]. Poor RV function and decreased transpulmonary blood flow relative to LVAD flow will lead to decreased preload. Since LVADs have reduced sensitivity to changes in preload, LVAD pump speed may not adjust to the drop in filling pressure and result in LV emptying and suctioning and therefore reduced pump flows and hypotensive events. The propensity of suction events increases as pump speed increases, especially in patients with RV dysfunction. Previous work has demonstrated improved RV function post-LVAD implant, mainly due to reduced RV afterload [14]. Whether RV function poses primary limitations to exercise performance in patients with LVAD support was investigated by Jaski et al., who showed that HF patients with pulsatile flow-LVAD support had decreases in RV dimensions, increases RV filling pressure and RV stroke volume in addition to increases in LV dimensions with exercise suggesting that RV function is not limiting exercise performance [48]. Other studies tend to suggest similar conclusions [29]; however, further investigation into the role of the RV on exercise performance is required.
Physiological principles of Starling-like control of rotary ventricular assist devices
Published in Expert Review of Medical Devices, 2020
Andrew F. Stephens, Shaun D. Gregory, Aidan J.C. Burrell, Silvana Marasco, Dion Stub, Robert F. Salamonsen
Both of the VADs have a different sensitivity to afterload and preload than the native heart. Additionally, since changes in preload are the main variable controlling output of the natural heart, when the preload falls to zero mmHg, the ventricular output also falls to zero litres/minute (l/min), with small lateral shifts of the preload-flow intercept occurring from patient to patient. This mechanism, plus the passive nature of ventricular filling, saves the natural heart from developing ventricular suction states. By contrast, the flow rates of both the HVAD and HeartMate 3 remain significantly elevated even at low preloads except in the presence of unusually high afterloads.