cardiac output

Examples of cardiac output in the following topics:

• Cardiac Output

  • Cardiac output (Q or CO) is the volume of blood pumped by the heart, in particular by the left or right ventricle, in one minute.
  • Cardiac output (CO) is a measure of the heart's performance.
  • Sympathetic nervous system activation will stimulate the SA and AV nodes to increase the heart rate, which will increase cardiac output.
  • Parasympathetic nervous system activation will conversely act on the SA and AV nodes to decrease the heart rate, which will decrease cardiac output.
  • Cardiac output is an indicator of mean arterial blood pressure (MAP), the average measure of blood pressure within the body.

• Role of the Cardiovascular Center

  • The cardiovascular center forms part of the autonomic nervous system and is responsible for regulation of cardiac output.
  • The cardioaccelerator center stimulates cardiac function by regulating heart rate and stroke volume via sympathetic stimulation from the cardiac accelerator nerve.
  • The cardioinhibitor center slows cardiac function by decreasing heart rate and stroke volume via parasympathetic stimulation from the vagus nerve.
  • Changes in diameter affect peripheral resistance, pressure, and flow, which in turn affect cardiac output.

• Cardiac Cycle

  • Heart rate is a term used to describe the frequency of the cardiac cycle.
  • The body can increase the heart rate in response to a wide variety of conditions in order to increase the cardiac output, the blood ejected by the heart, which improves oxygen supply to the tissues.
  • The pulse is the most straightforward way of measuring the heart rate, but it can be a crude and inaccurate measurement when cardiac output is low.
  • Complete cardiac diastole occurs after systole.
  • Blood pressure is a regulated variable that is directly related to blood volume, based on cardiac output during the cardiac cycle.

• Short-Term Neural Control

  • The primary regulatory sites include the cardiovascular centers in the brain that control both cardiac and vascular functions.
  • As a result, cardiac output falls.
  • This triggers an increase in sympathetic stimulation of the heart, causing cardiac output to increase.
  • If blood is returning to the right atrium more rapidly than it is being ejected from the left ventricle, the atrial receptors will stimulate the cardiovascular centers to increase sympathetic firing and cardiac output until homeostasis is achieved.

• Congestive Heart Failure

  • Congestive cardiac failure is generally defined as the inability of the heart to supply sufficient blood flow to meet the body's needs.
  • Heart failure is a global term for the physiological state in which cardiac output is insufficient in meeting the needs of the body and lungs.
  • Often termed "congestive heart failure" or CHF, this is most commonly caused when cardiac output is low and the body becomes congested with fluid.
  • Increased heart rate in CHF is stimulated by increased sympathetic activity in order to maintain cardiac output.
  • Binding to beta-1 receptors in the myocardium increases the heart rate and make contractions more forceful, in an attempt to increase cardiac output.

• Effects of Exercise on the Heart

  • A sedentary and inactive lifestyle is associated with greater risk for hypertension, cardiac hypertrophy, atherosclerosis, and myocardial infarctions, due to the metabolic changes that accompany a sedentary lifestyle.
  • A stronger heart does not pump blood faster but does pump it more forcefully with a greater ejection fraction, suggesting more efficient cardiac output.
  • The heart rates at rest of trained endurance athletes are also significantly lower than those of sedentary individuals, because fewer heartbeats are required to produce the same cardiac output at rest in those with higher stroke volumes.
  • For example, some athletes may be at risk for cardiac hypertrophy from too much exercise over long periods of time and sudden cardiac death from exercising to the point that the heart's metabolic demands become too high, causing an arrhythmia.

• Aging and the Cardiovascular System

  • The heart muscle becomes less efficient with age, with a decrease in both maximum cardiac output and heart rate.
  • The phrase heart attack sometimes refers to heart problems other than MI, such as unstable angina pectoris and sudden cardiac death.
  • Some are life-threatening medical emergencies that can result in cardiac arrest.
  • In fact, cardiac arrythmias are one of the most common causes of death en route to hospitals.
  • High magnification micrograph of senile cardiac amyloidosis (brown) and fibrosis (yellow).

• Vagus (X) Nerve

  • The vagus nerve (cranial nerve X) is responsible for parasympathetic output to the heart and visceral organs.
  • Besides output to the various organs in the body, the vagus nerve conveys sensory information about the state of the body's organs to the central nervous system. 
  • The dorsal nucleus of vagus nerve: Sends parasympathetic output to the viscera, especially the intestines.
  • The nucleus ambiguus: Sends parasympathetic output to the heart (slowing it down).
  • Excessive activation of the vagal nerve during emotional stress can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion.

• Pericarditis

  • Other physical signs include a patient in distress, positional chest pain, diaphoresis (excessive sweating), and possibility of heart failure in form of precardial tamponade causing pulsus paradoxus, and the Beck's triad of hypotension (due to decreased cardiac output), distant (muffled) heart sounds, and JVD (jugular vein distention).
  • Pericarditis can progress to pericardial effusion and eventually cardiac tamponade.
  • This can be seen in patients who are experiencing the classic signs of pericarditis but then show signs of relief, and progress to show signs of cardiac tamponade which include decreased alertness and lethargy, pulsus paradoxus (decrease of at least 10 mmHg of the systolic blood pressure upon inspiration), hypotension (due to decreased cardiac index), JVD (jugular vein distention from right sided heart failure and fluid overload), distant heart sounds on auscultation, and equilibration of all the diastolic blood pressures on cardiac catheterization due to the constriction of the pericardium by the fluid.
  • In such cases of cardiac tamponade, EKG or Holter monitor will then depict electrical alterans indicating wobbling of the heart in the fluid filled pericardium, and the capillary refill might decrease, as well as severe vascular collapse and altered mental status due to hypoperfusion of body organs by a heart that cannot pump out blood effectively.
  • The diagnostic test for cardiac tamponade, is trans-esophageal echocardiography (TEE) although trans-thoracic echocardiography (TTE) can also be utilized in cases where there is a high suspicion of aortic dissection and high blood pressure, or in patients where esophageal probing is not feasible.

• Heart Failure

  • The term heart failure is sometimes incorrectly used to describe other cardiac-related illnesses, such as myocardial infarction (heart attack) or cardiac arrest.
  • Heart failure may also occur when the body's requirements for oxygen and nutrients are increased and the demand outstrips what the heart can provide, (termed high-output cardiac failure).
  • Additional signs indicating left ventricular failure include a laterally displaced apex beat (which occurs if the heart is enlarged) and a gallop rhythm (additional heart sounds) may be heard as a marker of increased blood flow, or increased intra-cardiac pressure.
  • Echocardiography may also aid in deciding what treatments will help the patient, such as medication, insertion of an implantable cardioverter-defibrillator, or cardiac resynchronization therapy.