Several years ago, when the dinosaurs among us were in training, we used to look upon each pulmonary artery catheter that we inserted with a sense of pride and fulfillment. Thankfully, the era of inflating balloons with the pulmonary artery, measurement of wedge pressures, and serial cold saline injections to measure cardiac output seem to be drawing to a close. Arterial lines are often inserted more for convenience than for precisely titrated therapeutic interventions. There is increasing realization that the central venous pressure may be just a ballpark number that may not reflect the preload to the heart that it is assumed to represent. Perhaps we need to look at technological refinement that will enable us to obtain maximal information with minimal invasion.
Non-invasive, continuous blood pressure measurements with display of an arterial waveform is possible today by the finger clamp and the applanation techniques. The former uses a finger cuff with inflation-deflation cycles to maintain the finger volume constant, while the counterpressure applied is reconstructed to obtain arterial blood pressures (Fig. 1). In the applanation technique, a miniaturized transducer placed on the surface exerts pressure on an artery and enables direct pressure measurement. Much work needs to be done to validate measurements using these techniques; however, it seems likely that reliable, continuous, non-invasive blood pressure measurements may be extensively available in the near future.
Fig. 1 The finger clamp method of continous blood pressure measurement
Lack of adequate windows, especially in ventilated patients can be a frustrating experience for the intensivist by the bedside. There are major limitations to performing repeated transesophageal echocardiographic examinations in critically ill patients. However, a miniaturized probe, no bigger than a nasogastric tube, has been introduced that can be placed in the esophagus for repeated examination over a period of 72 hours. Three standard views are utilized, including the mid-esophageal 4-chamber view, the transgastric short axis view, and the superior vena caval view to enable assessment of volume responsiveness, size and function of the ventricles, assessment of valvular function, and detection of pericardial effusion (Fig. 2)
Fig. 2 Hemodynamic transesophageal echocardiography using a miniaturized probe
Despite the debate surrounding the use of lactate levels as a guide to the efficacy of resuscitation, it is well-established that persisting, high lactate levels are associated with increased mortality. Continuous, real-time measurement of lactate levels may help assess the trajectory of illness, especially in critically ill, septic patients. Using a 5-lumen central venous catheter, with saline infusion as a microdialysate, continuous measurement of lactate levels can be carried out, with values displayed in a graphic format (Fig. 3). The direction of the lactate curve may help guide therapy and assess progress in septic patients.
Fig. 3 Continous lactate monitoring by microdialysis
The time to embrace non- or less invasive techniques of hemodynamic monitoring is probably overdue. Technological refinement, would, hopefully allow us in the near future to extract maximal information from our patients to tailor appropriate intervention with the least risk of damage.