Can we be intensive and non-invasive?

The goal of hemodynamic monitoring is to assess the cardiovascular state of the patient, define their reserve and monitor response to treatments and time.  Resuscitation efforts are essentially aimed at restoring and sustaining tissue wellness through maintaining an adequate amount of oxygenated blood flow to the metabolically active tissues. We need to monitor pressure, flow and function. To accomplish these goals one must be able to measure arterial pressure and all its components (i.e. waveforms), cardiac output and stroke volume as well as the adequacy of flow. Presently, there are several devices that can estimate the arterial pressure waveform from a finger plethysmographic device. They are very accurate until profound circulatory collapse makes peripheral pulse not representative of central pressures. These devices can also estimate stroke volume by intuiting the arterial pressure waveform in a fashion similar to that performed by the numerous minimally invasive hemodynamic monitoring devices we now have now. These non-invasive devices can quantify functional hemodynamic monitoring dynamic parameters. Also, pulse oximeter pleth density signals vary with pulse volume into the finger or skin and the pleth variability can also be used as a surrogate of pulse pressure variation. Furthermore, bioreactance can measure both cardiac output and intrathoracic fluid content through surface electrodes. Finally, end-tidal CO2 transiently varies with venous return, increasing if blood flow increases. So both eh bioreactance device and end-tidal CO2 can be used to identify cardiac output changes in response to a passive leg raising maneuver. Thus, one can measure arterial pressure waveforms and cardiac output continuously, assess volume responsiveness and monitor therapy. Finally, the dynamic changes in tissue O2 saturation (StO2) measured by near infrared spectroscopy of the thenar eminence during a vascular occlusion test defines peripheral circulatory insufficiency and local blood flow independent of arterial pressure.  Furthermore, heart rate variability decreases with increasing cardiovascular stress and can be readily measured in real time from the R-R intervals of the surface ECG signal.  Finally, the measure of urine output, skin temperature and sensorium all define effective tissue blood flow as reasonable end-points to resuscitation, if the patient is not overwhelmingly ill. When these measures are coupled to a treatment approach know to improve outcome, there is little reason to believe that such completely non-invasive approaches will be inferior to invasive ones in the management of the critically ill patient.