Session 5: Monitoring Techniques

T he patient in shock often sets into motion a complex series of activities and interventions in the emergency

department. The emergency physician must, as a result, be comfortable with a wide range of disease entities and diag- nostic and therapeutic interventions.

This session of the Winter Symposium was devoted to the monitoring of such patients. The topic of monitoring was covered very broadly, ranging from simple, noninvasive monitoring to more complex and less commonly used in- vasive and experimental monitoring techniques. The theme was stated simply: How can the practitioner know who to treat, how to determine the effects of therapy, and when to stop?

There is an old adage in medicine that whenever you are about to do something to someone, know why you want to do it, and know what you hope it will change. After you are finished, go back and look to see that you made the intended changes. This is true whether you are intubating someone to correct hypercarbia, inserting a needle in the chest to relieve a tension pneumothorax, or administering fluid to resuscitate a hypotensive patient.

Dr Richard Cales began with a discussion of injury sever- ity scoring. This simplest of monitoring methods unfortu- nately can be imprecise and often less accurate than we would like. This impreciseness is obvious when one real- izes that 27 different scoring schemes have been proposed and are reviewed in his article. For patients undergoing re- suscitation, physiologic scoring systems are preferred. Pre- hospital physiologic scores such as CRAMS may be useful for patient triage to a trauma center or hospital providing specialized care.

Dr Cales noted the need for a comprehensive scoring sys- tem for ED patients that would correlate with case mix and disease severity and allow comparison of ED activity. Such systems developed for the intensive care unit (APACHE, TISS) have proven to be useful research and resource de- vices.

He emphasized that any monitoring method, to be useful, must be reliable, valid, and obtainable with available data. These simple concepts apply to the other presentations as well.

Dr MH Weil, whose manuscript is not included here, re- viewed the three techniques available for invasive hemo- dynamic monitoring: central venous pressure measure- ment, direct arterial pressure measurement, and Swan-Ganz catheter monitoring of pulmonary artery pressures. Central venous pressure monitoring is the only one of these tech- niques commonly used in EDs. Its limitation in patients with altered cardiac function was stressed. Although the in- formation available from direct arterial pressure monitoring

or Swan-Ganz catheters is considerably greater, the tech- niques often are not immediately available. Dr Weil also stressed the need for caution when interpreting sphyg- momanometer readings during profound shock or vaso- pressor use.

Dr William Shoemaker presented an algorithmic ap- proach to the resuscitation of hypotensive patients. Using a standardized protocol with monitoring of only blood pres- sure, central venous pressure, and hematocrit, resuscitation was accomplished in a shorter period of time and with fewer complications than in a similar group of nonprotocol patients.

Dr Shoemaker discussed the topic of "what is appropriate resuscitation." Severely ill patients, in his view, may require resuscitation to supranormal or "optimal" hemodynamic values that allow for more adequate restoration of tissue oxygenation and ultimately better tissue function and sur- vival. Such "optimal" values in the fully monitored patient include blood volume 500 mL in excess of normal, cardiac index 50% higher than normal, and oxygen consumption 25% higher than normal, and provide the increased supply of tissue oxygen required to meet the severe metabolic de- mands of the critically ill patient. Survival in patients re- suscitated to such end points is significantly better than in patients treated with "normal values" as their resuscitation end point.

Dr Shoemaker is an advocate of early colloid administra- tion. His sequence of resuscitation, including fluid resusci- tation, inotropes, vasodilators, and finally vasopressors, is logical and well worth remembering.

Finally, he whetted our appetites with early data regard- ing noninvasive cardiac output measurements using a bio= impedance technique. This technique has been available for some time, but is not used routinely in the ED or critical care setting. With further refinement, it may be extremely valuable for early monitoring in fluid resuscitation.

Finally, Dr Kenneth Waxman reviewed techniques for monitoring peripheral oxygen and carbon dioxide exchange. Both transconjunctival and transcutaneous oxymetry allow assessment of oxygenation at the tissue level. The transcon- juncti.val technique has some practical advantages for ED u s e .

Both techniques use the premise that the tissue oxygen level (measured at skin or conjunctiva) is approximately 80% of the arterial oxygen. Only two physiologic derange- ments alter this measurement: arterial hypoxia (easily mea- sured by arterial blood gas), and/or decreased cardiac output, which may preclude hypotension in the acutely ill patient. Conjunctival or transcutaneous oxygen measurement is thus a sensitive, indirect measurement of cardiac output,

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and in the patient with acute hemorrhage may be valuable for early detection of altered hemodynamic function.

The method of end tidal CO 2 monitoring also has wide clinical utility. At normal flow rates, it gives a good approx- imation of arterial CO 2. It also may be useful as a monitor of pulmonary flow, and hence cardiac output, during CPR.

Ideally, the injured patient of the future could be triaged appropriately in the field by paramedics using a scoring sys- tem. This would allow transport to an appropriate facility where resuscitation that begins using simple, noninvasive techniques can be guided as it continues by such simple invasive measurements as central venous pressure, and by peripheral oxymetry. Ultimate recovery would be assisted

by full invasive monitoring to achieve "optimal values" of resuscitation.

Paying close attention to basic principles of physiology, remembering the indications and consequences of each in- tervention, and monitoring resuscitation carefully and ap- propriately ultimately will improve patient outcome. These principles are the same in the field, the ED, and the inten- sive care unit.

John B McCabe, MD, FACEP Department of Emergency Medicine Wright State University School of Medicine Dayton, Ohio

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