Increasingly these days, blood flow is being assessed by Doppler techniques, which are very convenient because they are non-invasive. These machines transmit an ultrasonic vibration into the body and record the change in the frequency of the signal that is reflected off the red blood cells, so doppler techniques measure velocity, not flow; the flow could be obtained by integrating the signal over the cross-sectional area of the vessel. Transoesophageal echocardiography (TOE) can measure the length of blood in the ascending aorta in unit time. This is multiplied by the cross-sectional area of the aorta to give stroke volume. Consequently, the results of these devices should be assessed critically, and their main uses are confined to looking for changes or seeing whether the frequency shift is within the range found in normal people.
TOE provides diagnosis and monitoring of a variety of structural and functional abnormalities of the heart (for review see Poelaert et al. 1998). It can be used to derive cardiac output from measurement of blood flow velocity by recording the Doppler shift of ultrasound reflected from the red blood cells. The time velocity integral, which is the integral of instantaneous blood flow velocities during one cardiac cycle, is obtained for the blood flow in the left ventricular outflow tract (other sites can be used). This is multiplied by the cross-sectional area and the heart rate to give cardiac output. In a study of patients having coronary artery revascularisation (Krishnamurthy et al. 1997), the authors concluded that 'undue reliance placed on the absolute values may be unwise'. Others have compared TOE with thermodilution and reported agreements ranging from good (Perrino et al. 1998) to poor (Estagnasie et al. 1997). The main disadvantages of the method are that a skilled operator is needed (Lefrant et al. 1998), the probe is large and therefore heavy sedation or anaesthesia is needed, the equipment is very expensive, and the probe cannot be fixed so as to give continuous cardiac output readings without an expert user being present.
Oesophageal Doppler-portable devices
The velocity of blood in the ascending aorta may be measured using the doppler effect. This allows estimation of the length of a column of blood passing through the aorta in unit time. This is multiplied by the cross-sectional area of the aorta to give stroke volume. This equipment is described in detail later in this resource.
Use of a doppler ultrasound probe in the suprasternal notch to measure blood velocity and acceleration in the ascending aorta. Generally an inaccurate method of measuring cardiac output.
Can be measured across externally applied electrodes. Impedance changes with the cardiac cycle (changes in blood volume). The rate of change of impedance is a reflection of cardiac output. It is thought to be useful in estimating changes but not for absolute measurements.
Contraction of the heart produces a cyclical change in transthoracic impedance of about 0.5%, unfortunately giving a rather low signal to noise ratio. Although the method has been reported to give accurate results in normal subjects, several studies have some inaccuracy in critically ill patients, e.g. Genoni et al. (1998), Marik et al. (1997) and Imhoff et al. (2000).
Two-dimensional echocardiography can be used to measure the movement of the anterior and posterior ventricular walls and the ejection fraction.
Application of Fick principle
The NICO system is a non-invasive device that applies Fick’s principle on CO2 and relies solely on airway gas measurement. The method actually calculates effective lung perfusion, i.e. that part of the pulmonary capillary blood flow that has passed through the ventilated parts of the lung. The effects of unknown ventilation/perfusion inequality in patients may explain why the performance of this method shows a lack of agreement between thermodilution and CO2-rebreathing cardiac output.
Arterial pulse contour analysis
A technique combining transpulmonary thermodilution and arterial pulse contour analysis. The estimation of cardiac output based on pulse contour analysis is an indirect method, since cardiac output is not measured directly, but is computed from a pressure pulsation on basis of a criterion or model. The origin of the pulse contour method for estimation of beat-to-beat stroke volume is based on the Windkessel model described by Otto Frank in 1899. Most pulse contour methods are, explicitly or implicitly based on this mode. They relate an arterial pressure or pressure difference to a flow or volume change. Nowadays, three pulse contour methods are available; PiCCO, PulseCO and Modelflow. All three pulse contour methods use an invasively measured arterial blood pressure and need to be calibrated.
Distension of the aorta when blood is ejected from the left ventricle. The aorta recoils and smoothes out pressure and blood flow, this helps perfuse coronary arteries by pushing blood back to the opening of the coronary arteries.
PiCCO is calibrated by transpulmonary thermodilution, PulseCO by transpulmonary lithium dilution and Modelflow by the mean of 3 or 4 conventional thermodilution measurements equally spread over the ventilatory cycle. Output of these pulse contour systems is calculated on a beat-to-beat basis, but presentation of the data is typically with a 30 seconds window.