The oesophageal Doppler monitor

A safe means of monitoring the circulation

The oesophageal Doppler monitor, described in the early 1970s1 and subsequently refined by Singer,2 provides a safe and minimally invasive means of continuously monitoring the circulation. A paper in this week's issue describes using the oesophageal Doppler monitor to guide intraoperative fluid resuscitation in elderly patients undergoing repair of proximal femoral fractures (p 909).3 These patients are typically managed with only minimal intraoperative monitoring, and the paper thus raises questions about the role of invasive intraoperative monitoring. Similar questions have recently been raised about the role and use of pulmonary artery catheters.4

The oesophageal Doppler monitor measures blood flow velocity in the descending thoracic aorta using a flexible ultrasound probe about the size of a nasogastric tube. When combined with a nomogram based estimate of aortic cross sectional area (derived from the patient's age, height, and weight), it allows haemodynamic variables, including stroke volume and cardiac output, to be calculated. Despite several potential sources of error, there is good correlation, at least in adults, between measures of cardiac output made simultaneously with the oesophageal Doppler monitor and conventional thermodilution.2 5 6

In contrast to the pulmonary artery catheter, the probe of an oesophageal Doppler monitor can be inserted within minutes, requires minimal technical skill, and is not associated with major complications.7 In our experience the probe may be safely left in situ for over two weeks. The monitor should not, however, be regarded as a replacement for a pulmonary artery catheter. It cannot provide direct measurement of pulmonary artery and pulmonary artery occlusion pressures (though changes in the corrected systolic flow time, measured by the oesophageal Doppler monitor, have been shown to reflect qualitative changes in pulmonary artery occlusion pressures, allowing optimisation of left ventricular filling8). Also, the turbulent aortic blood flow associated with use of the intra-aortic balloon pump and in aortic coarctation causes the oesophageal Doppler monitor to be unreliable in these settings.

In the study by Sinclair et al patients given intravascular colloid as guided by the oeosphageal Doppler monitor had a significantly reduced (44%) hospital stay compared with controls, but too few were studied to make any useful assessment of differences in postoperative morbidity and mortality.3 Nevertheless, this study adds to the growing body of evidence supporting the notion that early goal directed resuscitation reduces both morbidity and mortality after surgery.9 10 11 Applying the 44% reduction in hospital stay to the 57 000 patients undergoing femoral fracture repair in Britain each year12 translates into 450 000 patient hospital days. Extrapolating beyond Britain is harder: in America, for example, lengths of stay are already shorter. At Duke University Medical Centre 103 patients underwent repair of proximal femur fractures in 1996. The median length of hospital stay was 6 days (range 2-45), in contrast to 18 days in the control group in Sinclair and other's study. Mortality in hospital was 3%. Of the survivors, however, only 37% were discharged home. The rest were discharged to skilled nursing facilities, and among these patients the length of stay and the proportion who eventually returned home is not known.

In light of the recent debate on the safety and efficacy of pulmonary artery catheters, could the oesophageal Doppler monitor be a viable alternative? Although it is not known how often pulmonary artery catheters are used perioperatively—though over 2 million are sold each year world wide13—they are probably used less often in Britain than in America.14 Despite the absence of large, randomised trials unequivocally showing efficacy, pulmonary artery catheters remain the de facto standard for invasive cardiovascular monitoring, and it will be interesting to see what effect the widely endorsed consensus statement from the US Society of Critical Care Medicine has on their use.4 This rejected the idea of a moratorium on the use of pulmonary artery catheters but recommended randomised controlled trials for most indications. Our belief is that in the perioperative period the oesophageal Doppler monitor can be used as an alternative to the pulmonary artery catheter.

A second question is whether the oesophageal Doppler monitor has a place in the intraoperative management of patients who would not normally be monitored with either a central venous pressure monitor or a pulmonary artery catheter? While most anaesthetists in Britain would agree that some means of optimising the circulation in high risk surgical patients would be desirable, the evidence suggests that many of these patients receive no invasive monitoring. Of the 1802 patients investigated by the National Confidential Enquiry into Perioperative Deaths 73% were classified as American Society of Anesthesiology physical status grade III or above,15 but direct monitoring of arterial, central venous, and pulmonary artery pressure was used in 24%, 31%, and 4% of cases respectively. Unfortunately this type of study cannot determine if perioperative monitoring improves patient outcome. The oesophageal Doppler monitor undoubtedly offers a safe means of monitoring the circulation, though it has failed to attain widespread use. Larger studies in a variety of settings will have to show both efficacy and cost effectiveness if this form of monitoring is to become established as a routine component of perioperative care.