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TRANSTHORACIC BIOELECTRICAL IMPEDANCE
The literature listed under general references provides a broad overview of transthoracic bioelectrical impedance. These articles include reviews, theoretical and historical perspectives and the most pertinent articles related to clinical care. These articles, for the most part, are representative of issues and applications currently utilized in nursing, surgery, biomedical engineering, critical care, cardiology and anesthesiology.
Shock, be it due to sepsis, trauma, cardiac arrest, or other causes, continues to have a high mortality rate in spite of very labor intensive and extensive treatment. Irrespective of the initiating events of shock syndromes, concomitant interacting circulatory changes occur in blood pressure, flow, and volume and in oxygen transport patterns; these changes lead to local tissue hypoxia, organ dysfunction, multiple organ failure, and death. The IQ® System is uniquely positioned to simultaneously measure pressure, flow, volume and resistance to aid in patient management.
THORACIC FLUID VOLUME / “CONTINUOUS NONINVASIVE X-RAY”
Thoracic impedance changes correlate closely with alterations in central blood volume. Relative changes in electrical impedance appear to be as sensitive as central venous pressure in the detection of intrathoracic volume changes. The use of an orthostatic challenge (head-up tilt test) for the bedridden, critically ill patient can be used to estimate vascular blood volume and venous compliance.
CONGESTIVE HEART FAILURE / HEART DISEASE
Effective treatment of patients with acute myocardial infarction and acute congestive failure requires quick triage, accurate identification, and appropriate pharmacological management. Bioimpedance monitoring functions similar to a Swan-Ganz catheter without the inherent risks associated with an invasive procedure. Accurate information pertaining to central and peripheral hemodynamics, thoracic fluid volume and myocardial contractility can be continuously attained.
CARDIAC COMPARISON STUDIES
Impedance cardiography is currently the only noninvasive technology available to continuously measure cardiac output, myocardial contractility, total peripheral resistance and thoracic fluid volume in real time with no known risk to the patient. This technology has been repeatedly demonstrated to provide accurate measurements of blood flow that are clinically acceptable for management of the majority of critically ill and non-critically ill patients. It should be appreciated, however, that where discrepancies have been observed, differences in instrumentation, patient populations and technical errors can account for a large portion of the observed differences.
PEDIATRICS AND NEWBORNS
Impedance cardiography has been successfully employed in healthy preterm and term neonates as well as with the pediatric population. It is not feasible to use a Swan-Ganz catheter on very small patients, yet, knowledge of blood flow and cardiac function are critical to successful patient care. Caution should be used, however, in interpretation of results in the presence of intracardiac shunts.
Hemodynamic measurements obtained with impedance cardiography can facilitate optimal programming of pacemaker variables. Relatively small variations in atrioventricular timing can result in significantly altered hemodynamics in some pacemaker patients. The precision of impedance cardiography has been demonstrated to be similar or superior to other frequently used techniques, and the data obtained is valuable for both the clinical and investigational settings.
Management of the critically-ill, hemodynamically challenged patient often necessitates the use of a wide array of pharmaceutical interventions. Monitoring the patient on a continuous basis offers the ability to titrate the drug dosage to optimize the hemodynamic parameters under consideration. The immediate effects of potent vasodilators, vasoconstrictors, positive or negative inotropic drugs, diuretics, and chronotropic alterations can be assessed continuously.
SYSTOLIC TIME INTERVALS / CONTRACTILITY
Systolic time intervals can be used to characterize left ventricular function. Left ventricular dysfunction is usually characterized by lengthening of the pre-ejection period (PEP) and a shortening of left ventricular ejection time (LVET) with little change in the QS2 (PEP + LVET). Diminished left ventricular performance results in an increase in the PEP/LVET. Such indices are well suited for studying the effect of pharmacologic agents upon the heart due to sensitivity, ease of acquisition and continuous nature.
PULMONARY CAPILLARY WEDGE PRESSURE
PWCP obtained with the Swan-Ganz catheter is typically used to guide fluid therapy. It should be appreciated, however, that at times this measurement correlates poorly with intravascular blood volume, particularly in those patients with cardiovascular disease. Compliance of the ventricle itself can significantly alter the normal passive pressure-volume relationship. Decreased compliance (increased “stiffness”) of the myocardium increases left ventricular end-diastolic pressure at any end-diastolic volume. As such, pressure may not be a good indicator of ventricular volume status.
An estimate of left ventricular ejection fraction (LVEF) can be calculated, with knowledge of the systolic time intervals derived from the impedance cardiogram. Left ventricular end-diastolic volume (LVEDV, preload) can be estimated by dividing stroke volume (SV) by EF, SV/EF = LVEDV. Knowledge of LVEDV provides accurate evaluation of the intravascular volume status and hydration status of the patient.
EXERCISE / CENTRAL AND PERIPHERAL HEMODYNAMICS
Cardiac output, or more precisely stroke volume, measured by impedance during exercise is usually within 15% of the more standard invasive techniques, which in themselves are accurate within 15%. The combination of waveform analysis with stroke volume and total thoracic impedance (ZO) measurements represents a simply noninvasive technique to profile cardiac function and evaluate cardiac reserve capabilities