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Haemodynamic Monitoring Principles

22 September, 2010 | Haemodynamic Monitoring

Conventional assessment during surgical procedures or in critical care comprises monitoring of heart rate, arterial blood pressure and urinary output, with or without catheter-base measurement of Central Venous Pressure (CVP). The decision to use cardiac output monitoring in addition (or occasionally as substitute) to conventional assessment is a clinical judgment, usually taken by an anesthetist or intensivist.

However, it is currently accepted that management of cardiac output fluid balance and haemodynamic status are the key factors in improving outcomes for high- risk surgery and the care of critically ill patients.

The fundamental technical requirement of haemodynamic monitoring is accurate cardiac output determination.

A range of devices using different techniques are available for cardiac output monitoring. There is no defining method for assessing cardiac output and no method can be regarded as a gold standard. However, the determination of cardiac output by the pulmonary artery cardiac catheter is usually considered as the most reliable.

For a system to be clinically useful it must be able to maintain accuracy of cardiac output determination in a variety of situations, for example in situations where cardiac output varies overtime. An ideal system will respond to dynamic changes quickly and accurately.

A Short Description of Some Systems for Monitoring Cardiac Output

  1. Pulmonary arterial catheterization: the tip of the catheter which includesa temperature sensor, is positioned in a branch of the pulmonary artery. Cold saline is administered in discrete pulses through the catheter, and the resulting temperature changes are recorded. The cardiac output is calculated from the analysis of the temperature changes. By a different approach, a thermal element on the tip emits heat in an on-off sequence – and the resulting pulmonary artery temperature changes are measured. The cardiac output is calculated by using the relation of the output signal to the input signal.
  2. Pulse control analysis: the fluctuations in arterial pressure are used to assess vascular flow. A variety of models exist to predict cardiac output based on measured pulse pressure waveforms.
  3. Flick’s principle: based on the measurement of arterial and venous carbon dioxide concentration. Using this principle, cardiac output can be easily derived for intubated patients.
  4. Electrical bioempedance and bioreactance: when the electrical current paths through the thorax (or the whole body) electric bioimpedance or bioreactance can be measured. Vascular blood flow affects the electrical parametes of the body. Based on such variations, the cardiac output can be calculated.
  5. Thoracic Doppler: if an ultrasound, continuous wave Doppler transducer is positioned on the outside of the chest wall. The function of the heart valves can be monitored. By using the analysis of the acquired information, it is possible to derive the cardiac output.
  6. Esophageal Doppler: ultrasound Doppler flow measurement can be performed in the descending aorta, using a probe positioned in the patient’s esophagus. Cardiac output is derived from measured aortic blood flow velocity. Aortic cross – sectional area is estimated and cardiac output is calculated by assuming a constant partition between descending and ascending blood supply.

Equipment in Clinical Use

All types of cardiac output monitors are in clinical use. The transesophageal aortic monitor is currently used more, as it features good stability and reliable monitoring capability. It is considered to be a minimally-invasive device and is well tolerated by adult patients. The bioimpedance/ bioreactance monitors are being seen more frequently in hospitals. They are non- invasive and simple to use.