|Cardiac output measurement has traditionally been performed using a pulmonary artery catheter. In recent years several alternative methods have been introduced including oesophageal doppler ultrasonography. Concerns have been raised over the use of pulmonary artery catheters and procedural complications associated with their use. This has contributed to the increasing interest and use of less invasive techniques to measure cardiac output.
The oesophageal doppler cardiac output monitor, described in the early 1970s, provides a safe and minimally invasive means of continuously monitoring the circulation. Since it was first proposed, it has undergone significant technological advancement and clinical evaluation. Initially, suprasternal transthoracic ultrasound/Doppler probes were used for determining cardiac output but they were not widely adopted because probe position instability limited their use for repeated measures over extended periods of time.
Oesophageal probes were recognised to have two significant advantages over suprasternal probes.
The smooth muscle tone of the oesophagus is a natural means of maintaining the probe in position for repeated measurements.
The oesophagus is in close anatomical proximity to the aorta, hence signal interference from bone, soft tissue, and lung is minimised.
The oesophageal doppler monitor measures blood flow velocity in the descending thoracic aorta using a flexible ultrasound probe. When combined with the aortic cross sectional area, it allows haemodynamic variables including stroke volume and cardiac output to be calculated.
Despite several potential sources of error, there is good correlation, between measures of cardiac output made simultaneously with the oesophageal Doppler monitor and conventional thermodilution. Oesophageal doppler ultrasonography has been used for intravascular volume optimisation in both the perioperative period and in the critical care setting. Its use in cardiac, general and orthopaedic surgery has been associated with a reduction in morbidity and hospital stay.
Indications for oesophageal doppler monitoring
Immediate access to comprehensive haemodynamic data allows clinicians to monitor, optimise and maintain end-organ perfusion.
Intensive Care Unit
Patients over 65
Fluid loss >500ml
Procedures exceeding 2 hours