Noninvasive Hemodynamics

Impedance Cardiography opens up to a more effective and comprehensive management

By Michelle Ciriacruz

The chaos theory states, strangely enough, that with repeated guesswork, a more accurate representation of a phenomenon and its patterns could be obtained. The probability of truth increases if there is a series of estimates to draw on, from which, perhaps, a trend could be discerned.

    Translate this to a determination of myocardial function and you get snapshot-like approximations of cardiac output (CO), heart rate, stroke volume (SV), pumping capacity, systemic vascular resistance (SVR), among other clinical parameters, that will guide the management of patients with ischemic, structural, or inflammatory cardiovascular conditions.

    As cardiovascular disease cases increased worldwide, clinicians came to realize that clinical signs and symptoms alone are insufficient to provide them with an accurate trajectory of the diseases' severity and progression. The considerable time lag between the onset of cellular damage, functional impairment, and manifestation of symptoms forbids-or at least lessens the chances of-a timely and effective intervention. And although research is attuned to this realization, clinical decision making is still mostly reliant on clinically evident changes-despite the availability of techniques that can provide critical data on hemodynamics.

    Evidence is quite firm on the importance of knowing the parameters of hemodynamic function. If the heart fails or any of its vascular components is impaired, blood circulation, and consequently, oxygen delivery to the different tissues and organs are interrupted. And while there are a number of methods to monitor hemodynamic performance, their invasiveness or complexity keeps them from being maximized in patient management.

    Cardiac and pulmonary artery catheterization (PAC) is really only resorted to in acute care settings. Although it does provide the objective basis so necessary in this kind of care-providing reliable and reproducible measurements, it is not appropriate for repeated evaluation since its potential for complications, especially infection, and system malfunction increases over time. So is the direct needle aspiration of arterial and venous blood for measurement of oxygen content too time-consuming and dependent upon a steady CO to be practical for repeated use.

    Of course, there are noninvasive methods, such as the echocardiography and Doppler assessment. The ultrasound signals of the echocardiography, however, rely on unobstructed transmission through the heart and great blood vessels to yield technically accurate data. So far it is not routinely applied clinically since its operation also demands a more meticulous and specialized handling than most medical facilities could provide. Getting the Doppler pulse wave to come up with accurate CO measurements is also conditional upon certain ideals of size, shape of blood vessels and velocity of blood. Since these ideals cannot be assumed each time, the derived measurements of this technique are used mostly in correlation with the calculations from other CO evaluating methods.

    Moreover, we should note that chronic conditions like heart failure and hypertension require long-term outpatient management. An index of myocardial function is really an essential ingredient in optimized patient management. It is clear that there is need for a noninvasive technique that demonstrates the same accurate readings as the standard methods, but without the complexity or the uncertainties on patient safety.

Impedance Cardiography

    In the late 1960s, the National Aeronautics and Space Administration thought of using Impedance Cardiography (ICG) to monitor their astronauts in outer space. It worked well enough on healthy subjects but once it was tried on critically ill patients, the inadequacies of its digital signal processing and algorithms constantly resulted in erroneous readings. Thus, it could not be used clinically.

    ICG is based on the thoracic electrical bioimpedance technology (TEB), by which a low voltage high amplitude, alternating current is introduced to the thorax. The thorax is a good conductor of this energy because blood offers little resistance. Sensor pads placed at the root of neck and thorax measure the blood's resistance or impedance to the current, which changes with each increase and decrease in blood volume.

    Aside from the inferiority of the computer technology, the inconsistencies in the early experiments probably also had to do with the lack of scientific knowledge on the effects of the body's compensatory mechanisms (postural change, exercise, neurohormonal activation, critical illness) on the accuracy of the ICG's readings. The 1990s saw sufficient advancements in these fields, however, to reopen the medical community's interest on ICG as a clinical tool.

    Extensive research since then has shown that the ICG's measurements stand up well in relation to the other established techniques. ICG produces serial measures that are relatively accurate, precise, and reproducible. The ICG data correspond to the data obtained with a PAC. This technique is also noninvasive, so obtaining serial measures over time is much more cost effective and safe.

    Dr. Milagros L. Estrada-Yamamoto, head of the intensive and coronary care units at the University of Santo Tomas Hospital (USTH), agrees. She also points out that, with a noninvasive technique, "the potential for infection is less, and yet we get the same thing that we would like to get [from an invasive technique]. Costwise, it's more expensive really to put in a catheter."

    The ICG is able to determine 12 hemodynamic parameters, including CO, SV, SVR, contractility, and thoracic fluid content. It also displays the electrical activity of the heart's muscle tissues. Through four dual sensors placed at the neck and thorax, hemodynamic performance is monitored and recorded real time. Given the hemodynamic variability to which ICG is responsive, individualized patient therapy for a host of cardiovascular disorders is made more feasible. Clinically relevant changes are identified and trended earlier for a more effective intervention.

    Dr. Estrada-Yamamoto also highlights the role of the ICG in patient management. Because it offers real time view of the hemodynamic status, the clinician is immediately able to note the response of the patient to drug treatments. "There are so many medication we want to give," she says. Clinicians have urgent questions on the effectiveness of medication: "Is this the right one, are we doing the right procedure for the patient, does the patient need fluid to increase the blood pressure?"

    With the ICG, they now have a safe and reliable guide on how to approach a patient's treatment. It is very portable, so even one unit is sufficient to service a number of patients at a given time. The machine is very easy to operate as well, so no need for specially trained personnel.

    USTH is the first hospital in the Philippines to make use of the ICG. It acquired a unit last December. Since then, more than 200 patients benefited from the technology. Dr. Estrada-Yamamoto reveals the hospital's plans of acquiring several more units, and perhaps expanding the use of the ICG beyond its present critical care applications to outpatient care of chronic cardiovascular conditions.