Perhaps one of the greatest challenges in shock management facing both clinicians and researchers today is the identification of valid and reliable shock parameters to assess the severity of hemorrhage and the adequacy of resuscitation. 213 In the past, gross physiologic indices such as heart rate, blood pressure, capillary refill, urine output, and central venous pressure have been used to assess the severity of hemorrhage. Despite a long history of frequent clinical use, these parameters have repeatedly been shown to be unreliable. More reliable indicators include mixed venous oxygen saturation and serum lactate concentrations, both of which may reflect the presence of hypoperfusion, even in the face of normal vital signs. 1 22
Several different parameters have been introduced as potential markers for the end point of resuscitation, including supernormal oxygen transport variables, mixed venous oxygen saturation (Smvo2), central venous oxygen saturation (Scvo2), gastric intramucosal pH, end-tidal CO2, serum lactate, and base deficit.13 Oxygen transport variables of particular interest include cardiac index (CI), oxygen delivery (D o2), and oxygen consumption(Vo2). It has been proposed that the measurement of such indices would supply an accurate reflection of oxygen debt and therefore tissue perfusion. Adjusting therapeutic interventions in an effort to achieve supernormal values would therefore presumably be indicative of an adequate level of resuscitation. Studies have demonstrated, however, that although attainment of supernormal oxygen transport variables may have some prognostic significance, it is not an appropriate resuscitation end point. 13 This may also be true for Smvo2. Although it also provides a measure of tissue oxygen extraction, it does not provide information about regional hypoperfusion, and normal or elevated values do not necessarily imply the absence of shock, particularly in the face of sepsis.
The measurement of oxygen transport variables and mixed venous oxygen saturation requires the placement of a pulmonary artery catheter, a procedure that is both time-consuming and has inherent risks. Central venous oxygen saturation monitoring has therefore been proposed as an alternative, since Scv o2 measurements have been shown to correlate with Smvo2. Although it still requires invasive monitoring and its use is limited by similar factors as Smv o2 measurements, placement is certainly less time-consuming. Until prospective studies demonstrate its validity as a potential guide for resuscitation though, its use cannot be unequivocally advocated.
Gastric intramucosal pH (pHi) has been investigated as a potential minimally invasive marker of regional perfusion. The splanchnic bed is particularly sensitive to hypoperfusion states because blood is shunted from its mucosa to preserve flow to the brain, heart, and kidneys. Theoretically, restoration of splanchnic perfusion should indicate adequacy of resuscitation. To date though, studies evaluating gastric intramucosal pH have been limited and do not demonstrate a reliable correlation with indices of global hypoperfusion, such as serum lactate and base deficit.
End-tidal CO2 monitoring has also been recommended as a noninvasive monitoring tool during shock.23 The reduction in cardiac output and pulmonary blood flow should be accompanied by reduced production of CO2 and reflected in low end-tidal CO2 measurements. Similarly, restoration of normal flow should be accompanied by increased end-tidal CO2 measurements. Preliminary studies have shown that persistently low end-tidal CO2 measurements are associated with poor outcome, but no prospective trials exist evaluating its contribution to clinical decision making.
Serum lactate concentrations have gained prominence as an indirect measure of tissue perfusion and oxygenation, because shock ultimately results in a shift from aerobic to anaerobic metabolism and increased production of lactate. Elevated serum lactate levels have been shown to correlate with both the severity of shock and the adequacy of resuscitative measures. In addition, changes in serial lactate values during resuscitation have provided prognostic information about patients in shock.22 Although the use of lactate has been limited by its lack of real-time availability, the development of accurate bedside lactate analyzers may eliminate this disadvantage in the future.
Base deficit has also been utilized as a measure of global tissue acidosis due to inadequate tissue perfusion. Base deficit is defined as the amount of base, in millimoles, required to titrate 1 L of whole arterial blood to a pH of 7.40, with the sample fully saturated with O 2 at 37° and a Pco2 of 40 mmHg. Although some studies have found a correlation between serum lactate and base excess, the relationship is not always predictable. 24
One element that is certainly common to all potential markers for the end points of resuscitation is that isolated values in time have significantly less importance than the identification of a trend. It is clear that both the development and resuscitation of a shock state are dynamic processes and in many respects remain elusive ones to monitor.
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