How to Determine Cardiac Output

Cardiac output is the amount of blood the heart circulates through the body in one minute. It is expressed in liters per minute. Healthy hearts pump 4 to 6.5 liters of blood in a minute. Cardiac output monitoring is used in critically ill patients at the emergency department in cardiopulmonary distress and for surgical patients with severe cardiopulmonary disorders. Heart rate multiplied by the stroke volume determines cardiac output.

Stroke volume is the amount of blood ejected during one beat of the heart. The stroke volume measure is needed to calculate cardiac output, but it can only be determined by monitoring techniques in a hospital setting.

Instructions

How to determine cardiac output?

• 1

  The American Society of Anesthesiologists in 2003 recommended clinical guidelines for pulmonary artery catheterization (PAC). All surgical patients at high-risk who are undergoing cardiac surgery, or patients with advanced cardiopulmonary disorders, should have a PAC to monitor cardiac output. A catheter is threaded through a large blood vessel into the right atrium through the right ventricle into the pulmonary artery, usually under ultrasound or fluoroscope guidance.

• 2

  Suprasternal Doppler technique uses a probe called a transducer with a gel applied to the lower neck area above the clavicle to measure cardiac output. It measures blood flow velocity in a vessel called the distal aortic arch above the clavicle. It is limited because it does not provide continuous information, only intermittent information and can not be used in the most critically ill medical and surgical patients for this reason.

• 3

  Another technique is esophageal Doppler cardiac output monitoring. The patient is sedated in this procedure and a Doppler probe called a transducer is inserted into the esophagus and lowered until it reaches the descending aorta. The transducer measures blood flow velocity at the descending aorta and produces cardiac output readings via an attached computer monitor.

• 4

  Bioimpedence cardiac monitoring uses disposable electrodes, applied to the skin surface along the sides of the neck and to the side of the lower chest area. A continuous small electrical current is applied across the chest. Cardiac output is computed for each cardiac cycle and continuously displayed on the monitor as an average value over several heartbeats. This method is limited to healthy patients.

• 5

  Partial carbon dioxide rebreathing cardiac output monitoring is performed on tracheal intubated (tube in the airway) patients and measures carbon dioxide (Co2) as it is exhaled. Changes in the exhaled Co2 are used to calculate cardiac output by applying a special equation, known as the Fick equation. This technique is used clinically on short-term intraoperative patients or mechanically ventilated (respirator) postoperative patients.

• 6

  With pulse contour cardiac monitoring, a pad is placed on the finger and measurements are displayed on a computer monitor. This method allows for continuous beat-to-beat monitoring and is especially useful in mechanically ventilated (respirator) patients. This monitor can also be connected to a catheter placed in an artery.

• 7

  Cardiac catheterization can also measure cardiac output in patients with known or suspected heart disease. A catheter is placed into a large blood vessel and threaded into the coronary circulation to measure cardiac output during coronary artery circulation assessment.

• 8

  With central venous pressure monitoring, a catheter is placed into the superior vena cava, above the right atrium via large blood vessels, to monitor cardiac output during cardiopulmonary resuscitation.