Preload

Technically the end-diastolic pressure of either the left or right 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.; 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 preload most often results in increased 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....

The blood supply to the ventricle is often referred to as preload. Technically, the definition of preload is the volume or pressure in the ventricle at the end of diastoleThe phase of relaxation during the cardiac cycle; occurs for the atria and the ventricles; blood enters the heart’s chambers and the coronary arteries during diastole; note that diastole is as important as systole – the negative pressure created by.... Note that atrial kickThe contraction of the atria prior to ventricular contraction causes an increased volume and stretch to the ventricles – resulting in increased force of contraction and increased stroke volume (Starling’s Law); this extra stroke volume increases cardiac output by 10-35%.... offers much to preload, especially for those getting on in years (contributing up to 35% of cardiac output). Preload is connected to 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 cardiac output via the Frank-Starling law.

Related to stroke volume is the term ‘ejection fraction’. An ejection fractionThe percentage of blood volume ejected from the ventricle; for example, if blood volume in the left ventricle at the end of diastole is 100 ml., and 80 ml. is the stroke volume ejected, then the ejection fraction is 80%... is determined by an echocardiogram or via a pulmonary arteryMain vessels carrying blood from the heart; the arteries have minimal elasticity and contain approximately 20% of the blood supply. Major Vessels 1. Six Second ECG Guidebook (2012), T Barill, p. 15, 190 catheter. Ejection fraction is the percentage of volume ejected from the left ventricleThe left ventricle ejects blood into the aortic arch to the body. Within the arch, the coronary arteries branch off first followed by three main arteries that branch to the brain (carotids) and the upper thorax (subclavian artery). The chambers.... The left ventricle has about 100 ml of blood just before contraction. Of this 100 ml, about 50-80 ml is normally ejected from the heart with each beat (stroke volume). Therefore, about 50 to 80 percent of blood is ejected. This is a normal ejection fraction.

Most of us have heard of the Frank-Starling phenomenon (often referred to as Starling’s Law – Frank has somehow been left out over the years). Frank and then Starling demonstrated that as cardiac muscle fibers stretch, contraction becomes more forceful. In other words, the more the stretch of the heart’s chambers, the more forceful the contraction (and indeed the greater the stroke volume).

What causes the heart’s chambers to stretch? Blood filling into the chambers increase pressures causing fibers to stretch. Whether you refer to increased pressure or volume in a chamber as the cause of the stretch is probably not important. The key is that either way, you are referring to preload. More preload causes more cardiac fiber stretch and increased 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.

Please refer to Figure 2.3: The Frank-Starling curve. The resting healthy heart depicts the varying contractility of the myocardiumThe muscle layer of the heart; the middle layer that is responsible for contraction of the heart. The muscular myocardium is the thickest layer and the workhorse of the heart. It is composed of specialized muscle and electrical cells that... with respect to changes in ventricular end diastolic pressure (preload).

The slope of each curve is the key to this graph. Compare the healthy resting heart to the curves of both the diseased heart and the heart during strenuous activity. Notice how the effect of sympathetic stimulationIschemia and sympathetic stimulation can enhance a ventricle’s automaticity, stimulating the ventricle to initiate an impulse before a sinus initiated wave reaches the ventricles. This solitary wave doesn’t ride the Autobahn. Rather, this one wave must traverse both ventricles. The... (i.e. norepinephrine) during exercise results in a magnified effect of preload on contractility.

Compare the preload/contractility curve of the healthy heart with that of the diseased heart. While the healthy heart curves peak with a preload of about 12 mm of Hg, the diseased heart requires increased pressures to maximize contractility. The diseased heart depends more on preload than the healthy heart to drive an effectiveSuccessfully producing the intended result, regardless of effort or resources used. Being effective is like hitting a target; whether with a straight shot or a series of steps, the goal is ultimately achieved. contraction.

Note that the higher the preload, the higher the myocardial workload. Therefore, high preload states (i.e. fluid overload) can make matters worse during ischemic episodes. And ischemiaInsufficient supply of oxygen to meet the oxygen demands of tissue. Anaerobic metabolism becomes increasingly important during periods of ischemia. Ischemia results from an inadequate blood flow that fails to meet the oxygen demands (energy demands) of tissues. If tissues... is one precursor to the development of serious dysrhythmiasUsed interchangeably with arrhythmia, refers to any abnormal rhythm – not normal sinus rhythm or sinus tachycardia. 1. Six Second ECG Guidebook (2012), T Barill, p. 196.

Figure 2.3 depicts the relationship between ventricular end diastolic pressure and contractility for a resting healthy heart, a resting diseased heart and a healthy heart during strenuous activity.

Several points are evident here:

1. in general, the force of contraction (contractility) increases as the pressure within the ventricles increase (increases in pressure and volume increase both cardiac fiber stretch and contractility);
2. during strenuous activity, catecholamine release increases the force of contraction;
3. for the diseased heart (i.e. cardiomyopathies), the force of contraction is impaired;
4. increases in chamber pressure do not produce significant changes in contractility for the diseased heart; and
5. 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. 32, 40, 203, 206