Dr Monika Sanghavi Resident Anesthetist,
Jaslok Hospital and Research Centre
In anesthesia and critical care, monitoring hemodynamics with effective tissue oxygenation is on the top. Initial 50 years of anesthesia depended on clinical monitoring and subsequent 50 years on pulse, BP, and ECG monitoring. Bioscience surfaced as a boon in the mid-20th century. Hemodynamic monitors have strengthened anesthesia and critical care by revealing essential physiology. In the early 21st century, I had the privilege to be a part of a global trial of a minimally invasive monitor to measure cardiac output and stroke volume variation continuously. Recently available minimally invasive monitors are based on various principles of measuring cardiac output which include pulse pressure analysis (arterial waveform analysis), echocardiogram and esophageal Doppler, bioimpedance/bioreactance, and Applied Fick’s principle.
Arterial waveform analysis reveals various aspects of cardiac output with applied values, advising volume replacement. A prerequisite for cardiac output measurement by this principle is an optimal and stable arterial waveform signal without any damping. It is affected by interaction between stroke volume and individual vascular compliance, aortic impedance, and peripheral arterial resistance. Depending on arterial waveform analysis it can be classified as calibrated (PiCCO plus, LiDCO plus, and EV1000/volume view) and non-calibrated (Flotrac/Vigileo, LiDCO rapid, PRAM, and Nexfin).
PiCCO (pulse contour cardiac output) is a unique combination of two techniques, transpulmonary thermodilution and pulse contour analysis. A cold saline bolus is injected through a central line and downstream temperature changes are measured at the femoral arterial line with a thermistor, after it passes through multiple mixing chambers of the heart and lungs. Cardiac output is measured by analysis of the area under the thermodilution curve using the modified Stewart Hamilton Algorithm. It measures various parameters giving information regarding cardiac output, volumetric preload, contractility, afterload, volume responsiveness, and bedside pulmonary edema assessment.
The LiDCO Plus (lithium dilution cardiac output) principle is the same as PiCCO but instead of a cold bolus, a lithium chloride bolus is injected into circulation through a central line or peripheral venous access and decay in lithium concentration over time is detected by a lithium sensitive electrode in arterial catheter. It analyses change in pulse power and cardiac output calculated by integrating a concentration over time graph. However, the problem of indicator recirculation, arterial blood sampling, and background tracer build up is unresolved.
EV1000/volume view gives guidance on volume administration and works with a volume view set containing a sensor, femoral catheter, and thermistor which give continuous hemodynamic information and volumetric parameters which help to keep the patient in an optimal volume range.
Flotrac/Vigileo uses dynamic tone technology and measures the same arterial curve 2000 times over 20 s. It reflects real time changes in vascular tone by a standard arterial line connected to the Flotrac pressure transducer which is connected to a Vigileo device which analyses cardiac output. It integrates multiple characteristics of arterial pressure waveform with patient specific demographic data. No external co-calibration or central line is required.
LiDCO rapid uses the pulse cardiac output algorithm to derive stroke volume and heart rate from a blood pressure waveform which is converted to volume to account for aortic compliance and capacitance.
LiDCO unity is a single platform combining LiDCO plus and LiDCO rapid functions. It is a three-in-one platform for monitoring, the third one being the BIS.
PRAM (pressure recording analytical method) is a new less invasive technique allowing beat-to-beat stroke volume monitoring from pressure signals recorded in femoral and radial arteries.
Nexfin HD monitor measures cardiac output continuously by an inflatable finger cuff, by combining continuous BP monitoring and a novel pulse contour method based on systolic pressure area and a physiological three-elements Windkessel model individualized for each patient. It measures continuous BP, heart rate, continuous cardiac output, stroke volume, systemic vascular resistance, and index of left ventricular contractility.
