Semi-invasive methods include LiDCO and PiCCO. The LiDCO monitor uses transpulmonary lithium dilution via a central line catheter and arterial sampling line for calibration. It then uses arterial waveform analysis to provide continuous CO measurements. PiCCO uses transpulmonary thermodilution, again via a central line and arterial sampling line. The disadvantages of these are that lithium dilution should be avoided in the first trimester and normal pregnancies do not require central or arterial lines. This negates their use in the normal pregnant population. However, newer LiDCO products such as the LiDCOrapid have been used in the literature 3.