EXTRA-CREDIT QUESTIONS

22. Four hours after your patient undergoes an exploratory laparotomy following a motor vehicle accident, the nurse reports that the patient’s vital signs, urine output, and oxygen transport numbers are normal. Can the patient still be in trouble?

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The Advanced Trauma Life Support Program defines shock as an abnormality of the circulatory system that results in inadequate organ perfusion and tissue oxygenation. This definition is easy to understand with uncompensated shock. Critically injured patients rarely make the transition from uncompensated shock to normal physiology, however, without some evidence of continued suboptimal tissue perfusion. This altered physiologic state may exist in 85% of patients who exhibit normal blood pressure, heart rate, and urine output. Assessing the sufficiency of blood flow to vital organs based on normal oxygen transport indices offers incomplete information about the adequacy of flow distribution and whether cellular oxygen use is appropriate. To correct fully an oxygen debt in a high-risk patient, oxygen transport variables should be maximized with simultaneous monitoring of indirect biochemical indices of perfusion, such as lactate, base deficit, and gastric mucosal pHi. (Good urine output and a warm big toe are reassuring.)

Figure 11-2 ICU hypoxic events. (Courtesy of Denver Hospital.)

23. Do supranormal oxygen transport indices (Do2, Vo2, cardiac index) serve as useful resuscitation end points?

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Probably, but this is controversial. In a meta-analysis of severely ill patients who received early resuscitation (8-12 hours postoperatively or before organ failure), there was a 23% decrease in mortality with early goal-oriented (supranormal oxygen delivery) resuscitation. The supranormal oxygen delivery targets are a cardiac index of 4.5 L/min/m2, oxygen delivery index > 600 mL/min/m2, and oxygen consumption index > 170 mL/min/m2. Most young, otherwise healthy patients achieve these objectives with little extra assistance. Conversely, cardiovascular disease may place the older patient at higher risk, and attempts to achieve supranormal oxygen transport goals (by whipping an old heart) may increase mortality. Although the final answer about the efficacy of supranormal resuscitation is unknown, the message is clear: All patients should be rewarmed promptly, mechanical ventilation must be optimized, adequate sedation and pain control must be achieved, and the patient must be volume-resuscitated appropriately.

24. Are there any organ-specific indicators of the adequacy of blood flow?

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ECG and urine output assess cardiac and renal perfusion. Cerebral perfusion is probably adequate if the patient can recall “how many dudes jumped me.” Gastric tonometry is a method of assessing the adequacy of the splanchnic circulation. Splanchnic hypoperfusion occurs early in the course of shock (see Chapter 4) and may precede changes in systemic hemodynamic indices, oxygen transport variables, and acid-base balance.

25. How is gastric tonometry performed?

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The gastric tonometer consists of a CO2-permeable balloon secured to the distal end of a nasogastric tube. CO2 in the adjacent gastric mucosa is allowed to equilibrate with the saline-filled balloon. After 60 minutes of equilibration, the saline is aspirated as a measure of the gastric mucosal Pco2, and arterial blood gases are obtained for bicarbonate concentration [HCO32]. Gastric intramucosal pH is calculated from the Henderson-Hasselbach equation:

pHi = 6.1 + log10 (arterial[HCO3-] ÷ NG tube PCO2 x 0.03)

Normal pHi is approximately 7.38 (range = 7.35-7.41). Survival benefits in critically ill patients have been shown if admission pHi can be corrected and maintained at values > 7.32 within the initial 24 hours.