Ischemia

Insufficient supply of oxygen to meet the oxygen demands of tissue.

Anaerobic metabolismEnergy production without oxygen with only 2 ATP produced as well as lactic and pyruvic acid. When energy demands surpass the supply of vital energy precursors such as oxygen, cells are left with the much less efficient anaerobic energy production... becomes increasingly important during periods of ischemia. Ischemia results from an inadequate blood flowThe purpose of the cardiac cycle is to effectively pump blood. The purpose of the cardiac cycle is to effectively pump blood. The right heart delivers deoxygenated blood to the lungs. Here oxygen is picked up and carbon dioxide is... that fails to meet the oxygen demands (energy demands) of tissues. If tissues are subject to ischemia, they try to compensate by extracting more oxygen from the blood. Tissue groups such as muscle or the intestines typically use only a third of the oxygen available to them.

The heart is the exception, extracting about 3/4 of the oxygen available to it through the coronary arteriesThe coronary arteries provide blood to the heart tissues, carrying blood first across the epicardium, then the myocardium and finally terminating in the endocardium. The epicardium forms the outer layer of the heart. The myocardium forms the middle layer and.... Because the heart does not have an abundanceA plentiful supply of resources or opportunities, offering flexibility and options. Abundance is like a stocked pantry; it provides a buffer and choice for when needs arise. of extra oxygen available, it is extremely dependent on blood flow for sufficient oxygenation. With increased oxygen demand, the coronary arteries must dilate to increase this blood flow.

Low 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... can cause cardiac ischemiaThe ability to identify cardiac ischemia, injury and infarction is vital in the management of the majority of cardiac emergencies. Most sudden cardiac deaths are associated with an ischemic episode. Patient deaths due to an acute myocardial infarction (MI) typically... – perhaps more so for the heart than other organs because of the heart’s already high rateHeart Rate; calculated by counting the number of QRS complexes in six seconds and multiplying by 10; rate is also determined by measuring the number of large squares between two R waves; i.e. –1 large square = heart rate of 300/minute. of oxygen extraction (see Table 2.1). A vicious cycle ensues. Cardiac ischemia forces a shift towards anaerobic metabolismParamount in importance is the continuous flow of oxygenated blood. This flow is central to metabolism, the production of energy and other materials necessary for life. Energy production is synonymous with life. No energy...no life. Energy production through oxygenation (with... (2 ATP) from the much more efficientAchieving the desired outcome with minimal waste of time, energy, or resources. Efficiency is like taking the fastest route to a destination, conserving energy for what matters most. aerobic metabolismMetabolism that utilizes oxygen is called aerobic metabolism. Energy production through oxygenation (with oxygen) yielding 32 ATP molecules from the interaction of one glucose and one oxygen molecule. "Paramount in importance, though, is the continuous flow of oxygenated blood. This... (36 ATP). With less energy available and increased intercellular acidity, the force of contraction weakens, causing a further reduction in 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.

The bottom line is that cardiac output is intimately coupled with energy production. For the heart, low cardiac output may in turn cause ischemia. Cardiac ischemia weakens 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, further impacting cardiac output. When caring for patients with cardiac ischemia, assess for signs and symptoms of poor cardiac output (shockSigns and symptoms of shock include shortness of breath, chest pain, hypotension, and an altered level of consciousness (due to hemodynamic compromise). As a general rule, a patient with a heart rate that is too fast (>150/minute - not enough...).

For patients experiencing shock states, look also for cardiac ischemia. Cardiac ischemia and poor cardiac output states often occur simultaneously. These conditions can cascade further by causing various 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. Poor cardiac output tends to cause an increase in catecholaminesThe chemicals of the sympathetic nervous system – epinephrine, norepinephrine, and dopamine. 1. Six Second ECG Guidebook (2012), T Barill, p. 193 (i.e. norepinephrine), which, combined with cardiac ischemia, can trigger serious dysrhythmias such as ventricular tachycardiaIs a cardiac rhythm characterized by rapidly occurring wide QRS complexes with absent or infrequent P waves evident; the absence of atrial kick, the lack of filling time, and the tendency for ventricular tachycardia to change to ventricular fibrillation makes... and ventricular fibrillationA chaotic unorganized cardiac rhythm with several firing regions within the ventricles; the result is uncoordinated ventricular activity and NO cardiac output – a lethal dysrhythmia; fine ventricular fibrillation defines an amplitude less than 3 mm; coarse ventricular fibrillation has....

Note that the heart extracts most of the available oxygen from the blood even during periods when the body is at rest. The heart, then, has very little physiological reserve to respond to episodes of high energy demand. Rather, the heart depends almost entirely on increased coronary blood flow to satisfy high energy demand.

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. 27-28, 109, 122, 145, 198