Despite recent controversies regarding its safety and efficacy, pulmonary artery catheterization (PAC) remains a widely used tool for the management of patients with cardiovascular instability. In addition to providing measurements of cardiac output (CO), several other potentially useful pieces of data can be obtained, including estimates of preload, afterload, and oxygen utilization. However, many practitioners feel that CO is the most useful parameter obtained with PAC.
The desire to measure CO without the risks of PAC has driven the search for other, less invasive measurement methods, such as esophageal Doppler measurements, lithium dilution, and carbon dioxide-based techniques. Esophageal Doppler monitoring involves inserting a flexible probe, similar to an esophageal stethoscope, into the midthoracic esophagus. A pulse-wave Doppler transducer in the probe tip calculates blood flow velocity from the Doppler frequency shift of RBCs in the descending aorta. By entering the age, gender, height, and weight of the patient, the aortic diameter can be estimated. From this and the blood flow velocity, aortic blood flow is calculated, representing approximately 70% of the total CO. Estimates of preload and afterload can be derived from the shape of the velocity waveforms. Modifications of this technique allow for the actual measurement of aortic diameter using M-mode ultrasound, eliminating the error associated with nomogram-based estimates. The resulting values for aortic blood flow correlate well with those of thermodilution CO,1–2 but the limits of agreement between the two methods are fairly wide.2