Established about a century passed by a physiologist Dr. Starling and two colleagues, the law states that the more the cardiac fibres stretch (within limits – with increased preloadTechnically the end-diastolic pressure of either the left or right ventricles; simplified as the blood volume supplied to the left or right ventricles; note that the more volume or preload, the greater the myocardial stretch and forceful the contraction; increased...), the more forceful the contraction – and the greater the stroke volumeThe amount of blood ejected by either the right or left ventricle with one beat (contraction). While heart rate is an undisputed contributor to cardiac output, stroke volume is the other major player. As heart rates vary to changes in... and the cardiac outputWhat is it? Why is it Vital? The amount of blood pumped out of the ventricle in a minute (most often refers to the blood pumped by the left ventricle) What is cardiac output? Simply, cardiac output is the amount....
A number of factors govern cardiac output. The more the heart’s muscle fibers stretch, the more forceful the contraction (more blood = more stretch = more pumped out with each beat). This is called Frank-Starling’s Law. Catecholamine stimulation (sympathetic nervous systemThe involuntary nervous system that provides alpha, Beta 1 and Beta 2 responses as stimulated by catecholamines – epinephrine, norepinephrine and dopamine; combined responses include increased heart rate, more forceful contraction, peripheral vasoconstriction and bronchial dilation. 1. Six Second ECG Guidebook (2012), T Barill, p. 206 and the adrenals) increases both stroke volume and heart rateThe number of QRS complexes per minute; note that HR may not equal perfused pulse rate. See also: - Step 1 Heart Rate - Rate - Rule of 300s (Triplicate Method) - Six Second Count - The Caliper Method 1.... to increase cardiac output.
Figure 2.3 depicts the relationship between ventricular end diastolic pressure and contractilityA muscle cell’s ability to shorten or contract through the action of actin and myosin - mediated by the calcium ion; the faster the influx of calcium, the more forceful the contraction. 1. Six Second ECG Guidebook (2012), T Barill, p. 194 for a resting healthy heart, a resting diseased heart and a healthy heart during strenuous activity.
Several points are evident here:
- in general, the force of contraction (contractility) increases as the pressure within the ventriclesThe larger chambers of the heart (3 times the volume and muscle thickness than the atria), responsible for the pumping of blood to the lungs and the rest of the body. increase (increases in pressure and volume increase both cardiac fiber stretch and contractility);
- during strenuous activity, catecholamine release increases the force of contraction;
- for the diseased heart (i.e. cardiomyopathies), the force of contraction is impaired;
- increases in chamber pressure do not produce significant changes in contractility for the diseased heart; and
- there is a limit to the affect of ventricular end-diastolic pressures (VEDP) on contractility. With high VEDP, contractility begins to fall. In other words, with high VEDP, contractility and stroke volumes tend to decrease.
1. Six Second ECG GuidebookA Practice Guide to Basic and 12 Lead ECG Interpretation, written by Tracy Barill, 2012 Introduction The ability to correctly interpret an electrocardiogram (ECG), be it a simple six second strip or a 12 lead ECG, is a vital skill... (2012), T Barill, p. 33, 40, 206