Invasive methods - The "gold standard" CO monitor is the pulmonary artery catheter. This measures CO using the temperature change of an injectate (the thermodilution principle) as blood is passing through the cardiac chambers. It is accurate and allows measurement of pulmonary artery pressures, but the obvious drawback is the invasive nature of the device, coupled with the risks of injury to the heart and pulmonary vessels. Due to its invasive nature, this is used almost exclusively in patients who are severely unwell, necessitating central catheterisation. Even so, there is currently no evidence from randomised controlled trials supporting the use of the pulmonary artery catheters 2. Its use is not possible in normal pregnancies, as it would be unacceptable to subject patients to the risks of central catheterisation. Other invasive techniques such as angiography or conductance catheters cause radiation exposure to the developing fetus.