Impedance cardiography

Ref2242ed to as electrical impedance plethysmography.

Also known as thoracic electrical bioimpedance.

Converts changes in thoracic impedance to changes in volume over time.

Tracks volumetric changes such as those occurring during the cardiac cycle.

Information gathered noninvasively and continuously.

Based on Ohm’s law, applies a constant, low-amplitude, high-frequency, alternating electrical current to the thorax and measures the corresponding voltage to detect changes in thoracic impedance.

Four dual disposable sensors on the neck and chest are used to transmit and detect electrical and impedance changes in the thorax, which are used to measure and calculate hemodynamic parameters.

The outer sensors transmit the alternating electrical current, and the inner sensors determine the thoracic impedance.

Base impedance, which depends on thoracic blood and plasma volume, chest skeletal muscle, cardiac muscle, lung tissue, chest wall fat, and air.

Dynamic impedance, which is caused by changing blood volume and velocity in the thoracic aorta.

Since blood is the most electrically conductive and arterial blood flow is pulsatile and arterial vessel walls are compliant, pulsatile changes in blood volume occur in the thoracic arterial system, predominantly in the aorta, as a result of ventricular function.

Change in blood volume results in a change in the electrical conductivity and thus the impedance of the thorax to electrical current.

Changes in impedance is measured, beat-to-beat, and applied to an algorithm to calculate stroke volume and cardiac output.

The technique measures the baseline impedance to this current and with each heartbeat, the blood volume and velocity in the aorta change and it measures the corresponding change in impedance.

Impedance cardiography ICG) attributes the changes in impedance to the volumetric expansion of the and uses the baseline and changes in impedance to measure and calculate hemodynamic parameters.

The amount of blood ejected by the heart to maintain tissue perfusion is the cardiac output, and in various disease states hemodynamic imbalances occur and the body is forced to compensate for cardiovascular and systemic vascular dysfunction.

Cardiac output: is affected by preload volume of blood in the ventricle at the end of diastole, by cardiac contractility, the rate of shortening of myocardial muscle fibers, by afterload, the force heart must overcome to expel blood into the vasculature, and the heart rate.

Impedance cardiography can be used to evaluate heart failure, hypertension, pacemakers, and dyspnea.

Accurately reflects cardiac output.

Information improves control of resistant hypertension.

Can predict for worsening heart failure.

Calculates hemodynamic parameters, such as heart rate, cardiac output, cardiac index (cardiac output normalized for body surface area) stroke volume (amount of blood pumped by the left ventricle each heartbeat), stroke index (stroke volume normalized for body surface area, systemic vascular resistance (the resistance to the flow of blood in the vasculature), systemic vascular resistance index (systemic vascular resistance normalized for body surface area), thoracic fluid content (intravascular, intraalveolar, and interstitial fluids in the thorax), acceleration index (peak acceleration of blood flow in the aorta), systolic time ratio (ratio of the electrical and mechanical systole), left cardiac work (reflects amount of work the left ventricle must perform to pump blood each minute), left cardiac work index (left cardiac work normalized for body surface area), systolic time ratio (the ratio of the electrical and mechanical systole), pre-ejection period (the time interval from the beginning of electrical stimulation of the ventricles to the opening of the aortic valve), left ventricular ejection time (time interval from the opening to the closing of the aortic valve).

Cardiac output calculated by the amount of blood the left ventricle ejects into the systemic circulation in one minute, measured in liters per minute (l/min).

Systemic vascular resistance is directly proportional to blood pressure and indirectly proportional to blood flow.

Click to zoom (Enlarge Image) Figure 2.

Changes in aortic volume correspond to changes in dynamic impedance [ CLOSE WINDOW ]

Figure 2. Changes in aortic volume correspond to changes in dynamic impedance

Thoracic fluid content (TFC) is represented as the inverse of baseline impedance. As conductivity in the chest increases (impedance decreases), TFC will increase. Since baseline impedance is dependent on the total thoracic tissue content, TFC is dependent on the total thoracic tissue content. Because fluid (both intra-and extravascular) is the most conductive and variable component of thoracic tissue, changes in TFC primarily occur because of changes in fluid.[12] As fluid increases in the chest, TFC increases. As fluid decreases in the chest, TFC decreases. Therefore, it is clinically important to establish a baseline TFC for each individual patient. Intrapatient directional changes in TFC will be indicative of directional changes in thoracic fluid.[13,14]

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