How should postoperative hypotension be approached?

How should postoperative hypotension be approached?

Keeping in mind the physiologic principles outlined above will standardize the approach to the diagnosis and management of hypotension.

    Preload can be decreased either by a decrease in the blood volume (hypovolemia due to inadequate fluid replacement, continued postoperative third-spacing or, most critical to diagnose, postoperative bleeding) or because of obstruction of venous return leading to decreased cardiac filling (cardiac tamponade, tension pneumothorax, positive pressure ventilation, positive end-expiratory pressure). Hypovolemia is the most fre-quent cause of postoperative hypotension and, therefore, the first steps are to assess fluid intake and output, and to examine the operative site, wound, and drains for signs of bleeding.

A few facts regarding hypovolemia deserve special mention:

·        Acute hemorrhage will not cause a fall in hematocrit until there is intravascular absorption of interstitial water or intravenous volume administration; there-fore, early failure of the hematocrit to fall is not strong evidence against acute hemorrhage.

·        Blood loss alone without hypovolemia usually does not cause hypotension.

·        In a patient with a nonfailing (i.e., preload-sensitive, afterload-insensitive) heart, decreased preload usually leads to a wide systolic variation of the blood pressure with the inspiratory cycle that can easily be detected on the arterial line tracing.

·  A decrease in contractility may result from anesthetic administration, electrolyte imbalance (particularly hypo-calcemia), ischemia, myocardial infarction, dysrhyth-mias, hypothermia, and any disease causing chronic cardiomyopathy. Therefore, the electrolytes and electro-cardiogram should be checked in all hypotensive patients. On occasion, despite the lack of clear causative factors, postoperative echocardiography will reveal a major change in contractility. In patients with renal fail-ure or mediastinal manipulation, suspicion should be maintained for the occurrence of pericardial effusions that cause tamponade and require rapid treatment with pericardiocentesis.

·  A decrease in afterload may result from general or regional anesthesia, vasodilator administration, anaphy-laxis, sepsis, and neurologic injury.

·  Rate (extreme bradycardia or tachycardia) or rhythm alterations can also lead to decreases in blood pressure through a decrease in cardiac output.

If physical examination, review of the pertinent labora- tory data, electrocardiogram, chest radiograph, and empiric fluid challenge do not lead to diagnosis and improvement, echocardiography or insertion of a pul-monary artery catheter (PAC) should be considered. A PAC will yield data on filling pressures, cardiac output, and sys-temic vascular resistance, as well as on oxygen delivery and consumption, that may help in further management. Data analysis by an experienced physician is needed if beneficial interventions are to result. Transthoracic or transesophageal echocardiography can provide helpful information as well, is less invasive than a PAC, but is more operator-dependent and does not allow for continuous monitoring.

In patients who develop hypotension when mechanically ventilated, consideration must be given to the diagnosis of pneumothorax.