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You are watching: When the myocardium requires more oxygen

StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.


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Introduction

The cardiovascular system is an intricately draft vascular network that gives blood and oxygen to the entire body. Myocardial oxygen demand is the quantity of oxygen that the heart calls for to maintain optimal function, and also myocardial oxygen supply is the amount of oxygen detailed to the heart by the blood i m sorry is managed by the coronary arteries. As soon as the body is operating at the optimal physiologic condition, myocardial oxygen it is provided does no exceed myocardial oxygen demand <1>.


Function

Since the heart operates solely under aerobic metabolism, myocardial mitochondria have to maintain an abundance of oxygen to proceed oxidative phosphorylation. Heart rate, contractility, and also ventricular-wall tension room the three components that determine myocardial oxygen demand. An increase in any kind of of these variables needs the human body to adapt to sustain enough oxygen it is provided to the heart.

Heart rate is believed to be the most vital factor affecting myocardial oxygen demand. Through an enhanced heart rate, the myocardium have to work more tough to finish the cardiac cycle an ext efficiently. With a shortened cardiac cycle, the time spent in diastole decreases. Due to the fact that diastole ends prematurely, the quantity of blood that generally fills the ventricles decreases, and also oxygen-saturated hemoglobin is not permitted to reach the subendocardium. Under optimal conditions, myocardial oxygen demand will equal myocardial oxygen supply; however, when there is structural damages from a plaque that impedes flow, there can be a mismatch in between supply and demand that results in ischemia.

Contractility or inotropism is the rate of boost in the intraventricular pressure during contraction in ~ a offered muscle fiber length.  Interestingly, myocytes have the innate ability to exert a convulsion at any kind of muscle length. This pressure is measured after the closure the the mitral valve and also before the opening of the aortic valve during which time the intraventricular volume remains constant.

Contractility is influenced by a selection of intrinsic and extrinsic forces including calcium concentration. Calcium is exit by the sarcoplasmic illusion of cardiac muscle and binds come troponin C. This bond stimulates a conformational readjust of tropomyosin the releases that from the binding web page on the actin filament. With the relax of tropomyosin, the active sites ~ above actin are totally free to tie myosin which reasons adenosine tree phosphate (ATP) to be broken down into adenosine diphosphate (ADP) and inorganic phosphate (P). Subsequently, there is a strength stroke that the myosin head which pressures the actin filament centrally towards the sarcomere, and ADP and P detach native the myosin head resulting in contraction. Once ATP re-attaches come myosin, actin is freed. This cycle proceeds until intracellular calcium offers are depleted, there is no longer binding in between troponin C and calcium, and tropomyosin deserve to revert come its initial confirmation to sheathe the binding sites on actin. Therefore, the intracellular calcium concentration is directly proportional come the contraction force <2>.

Ventricular wall surface tension is based upon the thickness the the ventricular myocardium. The legislation of Laplace claims that ventricular wall surface tension is proportional to ventricular radius and also intraventricular pressure. Patients through coronary artery an illness will have reduced blood and oxygen supply to the myocardium. In this case, contractility will be diminished. Since the ventricle have the right to no longer attain adequate contraction, the end-systolic volume will certainly increase. The overabundance blood continuing to be in the ventricle will cause expansion that the ventricular radius and intraventricular pressure resulting in higher wall tension.


Mechanism

The oxygen-carrying capacity of the blood and flow through the coronary arteries regulate myocardial oxygen supply. Oxygen-carrying capacity have the right to be lessened in several conditions that include either decreased red blood cabinet concentration or diminished oxygen saturation the hemoglobin. Because that example, anemia is a lower than average number of healthy red blood cells. Even though the red blood cells may be totally saturated through oxygen, there space not enough of castle to it is provided an adequate amount of oxygen come the muscle. Usually, that is caused by a nutritional deficiency in iron, vitamin B12, or folate which is easily correctable. However, there have the right to be more devastating reasons such together thalassemia, sickle cabinet anemia, and also various inherent enzyme deficiencies that are more challenging to treat <1><3>.

Hemoglobin is the oxygen-carrying portion of a red blood cell, and it is completely saturated when it binds four molecules the oxygen. V every molecule of oxygen the hemoglobin releases to the tissues, it binding the remaining oxygen molecules with more affinity. Usually, hemoglobin only disperses one molecule the oxygen before it return to the lung to become totally saturated once again. Carbon monoxide binding to hemoglobin with a much higher affinity 보다 oxygen. When hemoglobin returns to the lungs to be re-oxygenated, it usually has actually usually one or two out of the full four binding spaces available <1><3>.

Since carbon monoxide binds to hemoglobin through a higher affinity 보다 oxygen, the secures the empty binding spots of hemoglobin more quickly than incoming oxygen. At this point, hemoglobin is fifty to seventy-five percent saturated through oxygen and twenty-five to fifty percent saturated through carbon monoxide. Also though hemoglobin has actually oxygen molecule attached, it will certainly not release them right into the tissues. This occurs since hemoglobin’s affinity for oxygen is inversely proportional come its oxygen saturation. Therefore, the result hypoxia is not because of a absence of oxygen, however rather hemoglobin’s greater affinity because that oxygen once only partly saturated through it <1><3>.

Even though oxygen-carrying capacity can impact myocardial oxygen supply, coronary blood circulation is the major determinant of supply. Coronary artery blood flow is a role of pressure split by resistance. Myocardial oxygen usage is same to coronary blood circulation multiplied through the arterial-venous oxygen difference. Throughout diastole, the ventricles space receiving blood prior to systolic contraction. This filling phase of the cardiac cycle permits the coronary arteries to provide maximum blood circulation to the heart. Additionally, this is the only phase the the cardiac cycle that enables blood to arrive at the subendocardium which is the most distal section <1><3>.


Pathophysiology

A climb in myocardial oxygen need can end up being clinically far-ranging if the exceeds myocardial oxygen supply. This have the right to occur during the later on stages of coronary artery disease (CAD). From years of poorly regulated hyperlipidemia, a patient can construct atherosclerotic plaques in the significant arteries the supply blood to the heart. Once the truth of the vasculature has actually been compromised, plaques deserve to develop and also begin come shorten the diameter the the coronary arteries <1>.


Clinical Significance

Once the vessel has much more than a seventy percent occlusion, the patient will usually begin to suffer symptoms. Usually, these symptoms such as chest pain, dyspnea ~ above exertion, and diaphoresis, current during activity or stress when the love requires more oxygen. This is categorized as stable angina. However, once symptoms start presenting after much less physical task or at rest, the an illness has progressed to one eighty-percent occlusion, and the diagnosis of unstable angina deserve to be made <4><5>.

A human being who presents to the emergency department with angina should be evaluated for a mismatch in myocardial oxygen supply and also demand. The very first test to determine this is a 12-lead electrocardiogram (ECG) which steps the electrical activity of the heart. The ST segment is representative of the time in between ventricular depolarization and ventricular repolarization. If the ST segment is elevated top top arrival, it have the right to be indicative of acute myocardial infarction; however, if there is a depressed ST segment, it deserve to be representative that acute ischemia <2><4>.

If the ECG is unremarkable, cardiologists may select to perform an exercise anxiety test. In a controlled environment, cardiologists have the right to monitor the patient’s blood pressure, oxygen saturation, and electrical task of the heart. By performing an exercise, the patient is leading to the love to rise its rate and contractility for this reason elevating the myocardial oxygen demand. If the vessels space atherosclerotic, the heart will certainly not have the ability to adapt to the transforms in demand thus there will be a mismatch between supply and also demand which will be represented as necessary on the ECG <6><4>.

Additionally, if patients have actually a contraindication come an exercise tension ECG, they have the right to undergo myocardial perfusion imaging (MPI). Myocardial perfusion imaging have the right to be supplied to examine the myocardium for deficits in myocardial oxygen supply because of atherosclerotic plaques from coronary artery disease. First, the patients are offered an adenosene analog and also a atom isotope which space both injected via one intravenous catheter. The adenosene analog emphasize the heart and also increases myocardial oxygen demand. The patience is monitored for alters in oxygen saturation, blood pressure, heart rate, and rhythm <4>.

Next, the second section of the nuclear tension test deserve to begin. The accumulation of the nuclear isotope in the vessels permits clear images of the coronary arteries and surrounding vasculature to it is in obtained. Currently, the standard model has gathering images from seventeen segments: 16 short-axis views and the long-axis check out of the apical segment. Cardiologists grade each view making use of a scale variety of zero come four; 0 = normal, 1 = mildly abnormal, 2 = center abnormal, 3 = severely abnormal, 4 = absent. The synthetic perfusion score (SPS) is the complete score including pictures from rest and stress, the summed resting score (SRS) to represent the severity of myocardial infarction, and the summed stress score (SSS) reflects the extent of myocardial ischemia and infarction. To determine the severity the ischemia alone, the summed distinction score (SDS) should be calculate by individually the SRS indigenous the SSS <6><4>.

A mismatch in between myocardial oxygen supply and demand can an outcome in myocardial ischemia or infarct. Unfortunately, infarct outcomes in irreversible damage to the myocardium. However, ischemia and also potential reasons of mismatch deserve to be managed through a selection of medicine agents that encompass nitrates, beta-adrenergic-receptor blockers, and also calcium-channel blockers. Each of this medication classes increases myocardial oxygen supply and also decrease myocardial oxygen demand to varying levels <1><7>.

Nitrates cause a be safe in the vascular smooth muscle which reasons dilatation the the coronary arteries and also systemic venous circulation. Once the vessels transporting de-oxygenated blood ago to the heart space dilated causing the venous volume to increase, over there are corresponding decreases in cardiac preload, ventricular volume, and ventricular wall surface tension throughout systole. Due to the fact that of the downward transition in workload forced by the heart, myocardial oxygen demand is reduced.  Additionally, with the dilatation that coronary arteries, nitrates additionally increase myocardial oxygen supply. The raised radius the the vasculature offering the heart enables blood to bypass potential plaques to carry out a consistent supply the blood <8><9>.

Beta-adrenergic receptor blockers inhibit the effects of catecholamines in ~ the beta receptors. Usually, the sympathetic nervous system stimulates the relax of catecholamines to bind to beta receptor causing an increase in love rate, contractility, and also blood press through vasoconstriction. Because beta-blockers avoid catecholamines from communicating with the beta-adrenergic receptors, over there is a decrease in heart rate and contractility for this reason diminishing myocardial oxygen demand. Additionally, the slower heart enables for a lengthened diastolic section of the cardiac cycle. Because of this, more blood is allowed to it is provided to the heart during the filling phase <1>.

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Calcium-channel blockers prevent the enntrance gate of calcium through voltage-gated channels. Because contractility is reliant ~ above calcium relax from the sarcoplasmic reticulum, a reduced supply that calcium will certainly inherently reason a drop in contractility. This drop-in contractility causes a to decrease in myocardial oxygen demand. Additionally, the vasodilatory effects of calcium-channel blockers on vascular smooth muscle resulting in an increase in myocardial oxygen supply <10>.