Impedance cardiography

Impedance cardiography (ICG) is a non-invasive technology measuring total electrical conductivity of the thorax and its changes in time to process continuously a number of cardiodynamic parameters, such as stroke volume (SV), heart rate (HR), cardiac output (CO), ventricular ejection time (VET), pre-ejection period and used to detect the impedance changes caused by a high-frequency, low magnitude current flowing through the thorax between additional two pairs of electrodes located outside of the measured segment.

With ICG, the placement of 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 cardiodynamic parameters.

Digestive disorders, male impotence, tiredness, sleepwalking, environmental temperature intolerance, are classic examples of a low-flow-state, resulting in reduced blood flow.

The level of preload is currently assessed either from the PAOP (pulmonary artery occluded pressure) in a catheterized patient, or from EDI (end-diastolic index) by use of ultrasound.

With the exception of volume expansion, which can be accomplished only by physical means (intravenous or oral intake of fluids), all other hemodynamic modulating tools are pharmacological, cardioactive or vasoactive agents.

In addition, this technique is costly (several hundred dollars per procedure) and requires a skilled physician and a sterile environment for catheter insertion.

As a result, it has been used only in very narrow strata (less than 2%) of critically ill and high-risk patients in whom the knowledge of blood flow and oxygen transport outweighed the risks of the method.

Because of its safety and low cost, the applicability of vital hemodynamic measurements could be extended to significantly more patients, including outpatients with chronic diseases.

[8] Heart failure, hypertension, pacemaker, and dyspnea patients are four conditions in which outpatient noninvasive hemodynamic monitoring can play an important role in the assessment, diagnosis, prognosis, and treatment.

[14] The electrical and impedance signals are processed to determine fiducial points, which are then utilized to measure and calculate hemodynamic parameters, such as cardiac output, stroke volume, systemic vascular resistance, thoracic fluid content, acceleration index, and systolic time ratio.

Fig.1: Aortic blood pressure and aortic blood flow over one heartbeat interval: S = Systolic blood pressure; D = Diastolic blood pressure; MAP = Mean Arterial Pressure; SV = Stroke Volume; DN = dicrotic notch (aortic valve closure)
Fig.5: The Frank-Starling Law and Inotropy: Three Frank-Starling curves shown for normoinotropy, hyperinotropy and hypoinotropy. A patient, who is normovolemic and normoinotropic, exhibits normal level of Ejection Phase Contractility (EPC). However, a patient who is hypovolemic can exhibit the same normal level of EPC if given positive inotropes, and a patient who is volume overloaded (hypervolemic) can also have normal level of EPC if given negative inotropes
Fig.6: Timing considerations of working effects of preload, contractility (pharmacological = inotropes, and mechanical = Frank-Starling mechanism, i.e., effects of intravascular volume) and afterload in respect to Systolic and Diastolic Time Intervals: Diastole => Starts at S2-time, ends at Q-time. Systole => Isovolumic phase starts at Q-time, ends at AVO-time; Ejection phase starts at AVO-time, ends at S2-time. (S2 = 2nd heart sound = aortic valve closure; AVO = aortic valve opening)