EVALUATION OF INTRAVASCULAR VOLUME
Indications of hypovolemia include abnormal skin turgor, dehydration of mucous membranes, thready peripheral pulses, increased resting heart rate, decreased blood pressure, decreased urine output, or orthostatic heart rate and blood pressure changes from the supine to sitting or standing positions. Unfortunately, medications administered during anesthesia, as well as the neuroendocrine stress response to surgery and anesthesia, frequently alter these signs and render them unreliable in the immediate postoperative period. Intraoperatively, in addition to heart rate and blood pressure, the fullness of a peripheral pulse, urinary flow rate, and indirect signs, such as the blood pressure response to positive-pressure ventilation and to vasodilating or negative inotropic effects of anesthetics, are often used for guidance.
Pitting edema—presacral in the bedridden patient or pretibial in the ambulatory patient—and increased urinary flow are signs of excess extracellular water and likely hypervolemia in patients with normal cardiac, liver, and kidney function. Late signs of hypervolemia in settings such as congestive heart failure may include tachycardia, tachypnea, elevated jugular pulse pressure, lung crackles, wheezing, cyanosis, and pink, frothy pulmonary secretions.
Several laboratory measurements may be used as indicators of intravascular volume and adequacy of tissue perfusion, including serial hematocrits, arterial blood pH, urinary specific gravity or osmolality, urinary sodium or chloride concentration, serum sodium, and the blood urea nitrogen (BUN)-to-serum creatinine ratio. Laboratory signs of dehydration may include increasing hematocrit and hemoglobin, progressive metabolic acidosis (including lactic acidosis), urinary specific gravity greater than 1.010, urinary sodium less than 10 mEq/L, urinary osmolality greater than 450 mOsm/L, hypernatremia, and BUN-to-creatinine ratio greater than 10:1. The hemoglobin and hematocrit are usually unchanged in patients with acute hypovolemia secondary to acute blood loss because there is insufficient time for extravascular fluid to shift into the intravascular space. Ultrasonography can reveal a nearly collapsed vena cava or incompletely filled cardiac chambers. Radiographic indicators of volume overload include increased pulmonary vascular and interstitial markings (Kerley “B” lines), diffuse alveolar infiltrates, or both.
Central venous pressure (CVP) monitoring has been used when volume status is difficult to assess by other means or when rapid or major alterations are expected. However, single CVP readings do not provide an accurate or reliable indication of volume status.
Pulmonary artery pressure monitoring has been used in settings where CVP readings do not correlate with the clinical assessment or when the patient has primary or secondary right ventricular dysfunction; the latter is usually due to pulmonary or left ventricular disease, respectively. Pulmonary artery occlusion pressure (PAOP) readings of less than 8 mm Hg may indicate hypovolemia in patients with normal left ventricular compliance; however, values less than 15 mm Hg may be associated with relative hypovolemia in patients with poor ventricular compliance. PAOP measurements greater than 18 mm Hg are ...