National Physical Laboratory

Confidence in cardiac output measurements

The measurement of cardiac output (the volumetric flow rate of blood being pumped by the heart) is an important component of the management of critically-ill haemodynamically-unstable patients. Historically, cardiac output measurements have been derived from methods that employ pulmonary arterial catheterisation, but the trend is to use less invasive techniques that are capable of providing a measurement of the continuous cardiac output (CCO). The derivation of the cardiac output from the analysis of arterial pressure waveforms using a model of the cardiovascular system is one such approach.

Manufacturers of CCO instruments develop their own proprietary algorithms based on these models. However, a potential problem arises in the application of these algorithms to patients, particularly those who are critically ill, as the clinical data used to establish parameter values for such models tend to be derived from, and validated against, only relatively well patients, such as those recovering from cardiac surgery. When used to monitor patients in more critical conditions, significant variations in the cardiac output value displayed by different manufacturers' instruments have been observed.

To help determine the clinical conditions under which different CCO monitors give different outputs, NPL has developed a pressure waveform generator (PWG) that is capable of reproducing, in a water-filled chamber, digitised recordings of arterial pressure waveforms. The operation of this instrument is detailed in NPL Report ENG 44.

NPL's pressure waveform generator
NPL's pressure waveform generator


Arterial pressure waveforms have been recorded from patients in the Intensive Care Unit at Guy's and St Thomas' NHS Foundation Trust, and these waveforms are reproduced by the PWG. The pressure in the water-filled chamber is controlled by the force exerted on a piston by the electro-dynamic shaker. The shaker is controlled by the output voltage from the computer, which is based on the digitised waveform. In addition, a PID loop, based on the pressure measured by transducer R, is used to optimise the fidelity of the generated waveform.

Schematic diagram of pressure waveform generator
Schematic diagram of pressure waveform generator


The following two figures show the high degree of waveform fidelity achieved for both radial and femoral arterial waveforms. Each figure plots the waveform originally recorded from the patient and the waveform generated by the PWG on the primary y-axis - at each sampling point, the difference between the two waveform values is also plotted, on the secondary y-axis.

Difference between the recorded and generated radial artery waveforms
Difference between the recorded and generated radial artery waveforms

 

Difference between the recorded and generated femoral artery waveforms
Difference between the recorded and generated femoral artery waveforms

 

Last Updated: 6 Oct 2016
Created: 17 Jan 2011

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