Because of the invasiveness of the PA catheter, people have looked to develop other ways of measuring cardiac output in the clinical setting. Whilst the PA catheter is considered the "gold standard", advances in computer technology, sensor design and refined interpretation of data have allowed the development of useful methods of measuring cardiac output without invasive vascular access. Transoesophageal echocardiography probes can estimate stroke volume. Doppler imaging can measure the velocity of blood in the ascending aorta. The length of a column of blood passing through the aorta in unit time can be measured. This is multiplied by the cross-sectional area of the aorta to give stroke volume. However, this is not feasible for continuous monitoring in awake patients. Thermodilution and pulse wave analysis techniques are also used. These more invasive techniques include the PiCCO (transpulmonary thermodilution and arterial pulse contour analysis), LIDCO (lithium dilution and arterial waveform analysis) and the Transonic (ultrasound indicator saline dilution) systems.
The PiCCO system requires a central venous cannula and a thermistor-tipped femoral arterial line. The femoral arterial line allows pulse contour analysis and from this cardiac output is derived. A known volume of ice cold saline is rapidly injected into the central venous cannula. The injectate disperses into the lungs and all four cardiac chambers. A temperature difference reaches the femoral thermistor and a dissipation curve is generated. The global end-diastolic volume can be estimated from this and this reflects changes in volume status.
The NiCO system uses the indirect Fick principle using carbon dioxide rebreathing but requires intubation rather than vascular access.
Many of the advances in non-invasive cardiac output monitoring include the ability to monitor parameters continuously and to follow trends in therapy and interventions. The most impressive advance in non-invasive cardiac output monitoring has been the development of reliable techniques using advanced forms of thoracic bio-impedance measurements. The EIC (electrical impedance cardiography) and CBII (chest baseline impedance independent) systems are able to provide continuous trend monitoring of heart rate and stroke volume giving derived cardiac output and index parameters using stroke waveform morphology analysis.
The real advantage of these truly non-invasive techniques is the opportunity to begin monitoring these parameters in the Emergency Department, ward, or Coronary Care Unit before admission to the ICU becomes necessary or possible.
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