Doppler and ultrasound
The Doppler principle has been applied to monitoring in anesthesia. We can place a Doppler pencil probe over a vessel to identify blood flow or, with a broader emitter/receiver head, place it over the chest to detect the blood flow in the right atrium.
When air appears in the blood flowing into the heart, it changes the reflective characteristics of the blood, easily detected by the Doppler signal, which is transformed into a swooshing noise.
While ultrasound has been used for many years to spy on babies still in the womb and to view the functioning heart through the chest wall, more recently the equipment has been miniaturized into a finger-sized probe that views the heart from behind, through the wall of the esophagus (Fig. 7.5). This advance gives us a hands-free, relatively stable (and relatively non-invasive) view of the heart that does not impinge on the operative field. The technology continues to advance but currently allows views from multiple angles and Doppler analysis of flow through the valves and even the coronaries (for the experienced ultrasonographer). While invasive pressure monitoring can give indirect insights into cardiac physiology, with TEE we can actually see the heart doing its work. We can assess preload (how full is the ventricle?), contractility (how much are the walls thickening?), and ischemia (are there sections of the ventricular walls that lag behind?). During cardiac surgery, we can evaluate valve repairs and ASD closures. TEE is also a great way to detect air emboli.
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