An 80-year-old female was admitted to the ICU from the emergency department with the diagnosis of septic shock secondary to urosepsis. She was noted to be in her usual state of good health 12 hours earlier, but was then found at home lying in bed with altered mental status by her daughter. In the emergency department she was found to be hypotensive with a mean arterial blood pressure (MAP) of 45, lethargic, and was mostly incoherent on physical examination. She was given 2 L of IV fluids with improvement in her BP and mental status. Blood cultures were obtained and a urinalysis revealed E. coli in her urine. She was started on broad-spectrum antibiotics and was admitted to the ICU for further management of her urosepsis.
The resident has seen the patient on admission and is concerned because she continues to require aggressive volume resuscitation to maintain her pressures and is now on a norepinephrine infusion at high doses. On physical examination she is cold to the touch, quite pale, and has a moderately distended abdomen. Your resident is concerned that her initial diagnosis may be incorrect, is worried about a retroperitoneal hemorrhage causing hypovolemic shock, and elects to place a pulmonary artery catheter to help further elucidate the etiology of her shock.
Which of the following hemodynamic parameters obtained from the pulmonary artery catheter would be most helpful in distinguishing hypovolemic shock from septic shock?
A. COCorrect Answer: D
Shock is the physiologic state that results from cell dysfunction or death due to inadequate oxygen delivery or uptake. There are multiple causes of shock, including low circulatory volume (hypovolemic shock), severe vasodilation (septic or anaphylactic shock), low CO from heart failure (cardiogenic shock), or obstruction to forward blood flow (obstructive shock). In all cases, shock is manifested by low MAP. The diagnosis of the type of shock may be challenging, but it is important as the treatment of shock differs between the various etiologies. The hallmark of septic shock is low blood pressure due to profound vasoplegia from bacterial endotoxin. Early septic shock is characterized by high CO, low circulating volume, and low SVR due to vasoplegia, low cardiac filling pressures (CVP and PCWP), and an elevated mixed venous oxygen content due to the inability of cells to utilize delivered oxygen due to poisoning from the bacterial infection. The hallmark of hypovolemic shock, in contrast, is low MAP due to low effective circulating volume only. In this case, the body’s usual homeostatic mechanisms to increase blood pressure by increasing both CO and SVR are intact, and both of these numbers will be elevated. Owing to volume loss, the cardiac filling pressures (CVP and PCWP) will all be low. Therefore, the greatest contrast to septic shock is an elevated SVR.
Note that . Raising both CO and SVR will work to raise blood pressure back toward normal. In septic shock the primary problem is low SVR, so the body has no way to raise vascular resistance and can only rely on increased CO to move MAP toward a more normal value. Because of this difference, patients in septic shock will usually feel warm to the touch, whereas patients in hypovolemic shock will usually feel cold to the touch because of the high SVR state.
Of note, the use of a pulmonary artery catheter has never been shown to improve survival in randomized controlled clinical trials of patients in shock.
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