Clinical Case of the Month: You’re medical director for a busy outpatient surgery center. An RN routinely does the preoperative screening by telephoning each patient two days prior to surgery. The RN pages you with this question: A 48-year-old patient scheduled for anterior cruciate ligament (ACL) reconstruction surgery takes hydrochlorothiazide for hypertension, and has not had electrolytes checked for six months. His last labs show a low potassium = 3.4 mEq/L. The patient is asymptomatic except for knee pain. The nurse asks you whether this patient needs to have his potassium rechecked now, before surgery. What do you do?
Discussion: Pre-op evaluation will never be the topic of a Hollywood thriller — you’ll never see Tom Cruise or Brad Pitt rubbing their temples worrying about whether they need to recheck the electrolytes. But for you and me, it’s a question worth discussing. How important is it to diagnose hypokalemia in this asymptomatic patient on chronic diuretic therapy? If the K=3.0 mEq/L, will you cancel the surgery? What about if the K=2.9 mEq/L? Experienced anesthesiologists know standards of care for their specialty, and also develop a gut impression about which patients are prepared for surgery, and which ones are not. Do you sense this patient is at risk for sudden death or a cardiac arrhythmia? Let’s examine this question.
First off, why didn’t you see this patient in your pre-op clinic? The answer is because you won’t find the Stanford model of a well-staffed Pre-Anesthesia Clinic in the private practice community. The Pre-Anesthesia Clinic is important at Stanford because many patients suffer from significant medical comorbidities, and because of the invasive nature of many of the inpatient surgeries. In a community practice with healthier patients and less invasive procedures, there is neither the money nor the need to physically meet and examine every patient several days prior to surgery. Adam Smith’s economic dictum of the invisible hand pertains to clinical medicine as well — anesthesiologists are paid to give anesthetics. Neither insurers nor Medicare will reimburse you for routine pre-operative clinic encounters with patients.
In 2002, the American Society of Anesthesia published Practice Advisory for Preanesthesia Evaluation: A Report by the American Society of Anesthesiologists Task Force on Preanesthesia Evaluation. Their recommendations for the timing of preanesthesia evaluation differ, depending on the severity of disease and also on the surgical invasiveness. Our patient’s surgery involves a non-severe comorbidity (well-controlled hypertension) and a non-invasive surgery (knee arthroscopy). For patients such as this, the ASA Practice Advisory states, “preoperative assessment may be done on or before the day of surgery. “ In our community outpatient practice in Palo Alto, a surgery-center RN calls the patient two days prior to surgery to ask pertinent questions. This telephone call helps avoid day-of-surgery surprises (e.g. patients still on aspirin, patients with undiagnosed chest pain or dyspnea). The physical evaluation by the anesthesia attending occurs on the day of surgery.
Outpatient surgery centers rarely have the ability to do lab tests other than blood glucose measurements or a 12-lead ECG. Tests such as the measurement of electrolyte concentrations need to be done at an outside lab, at least one day prior to surgery. Regarding preanesthesia serum chemistries (i.e., potassium, glucose, sodium, renal and liver function studies), the ASA Practice Advisory gives no specific recommendation to check preoperative electrolytes during chronic diuretic therapy. The recommendation on checking pre-op electrolytes states “Clinical characteristics to consider before ordering such tests include likely perioperative therapies, endocrine disorders, risk of renal and liver dysfunction, and use of certain medications or alternative therapies.”
Might “perioperative therapies” include potassium replacement? Consider this: potassium is predominantly an intracellular ion. Per Miller’s Anesthesia, “Only 2% of total-body potassium is stored in plasma. . . . a 20% to 25% change in potassium levels in plasma could represent a change in total-body potassium of 1000 mEq or more if the change were chronic or as little as 10 to 20 mEq if the change were acute. . . . Chronic changes are relatively well tolerated because of the equilibration of serum and intracellular stores that takes place over time to return the resting membrane potential of excitable cells to nearly normal levels.” (Miller’s Anesthesia, 2005, pp.1105-6)
The same textbook states, “Retrospective epidemiologic studies attribute significant risk to the administration of potassium (even chronic oral administration). In one study, 1910 of 16,048 consecutive hospitalized patients were given oral potassium supplements. Of these 1910 patients, hyperkalemia contributed to death in 7, and the incidence of complications of potassium therapy was 1 in 250.” (Miller’s Anesthesia, 2005, p. 1107).
Given this information, what should we do?
Here’s the answer: Per Miller’s Anesthesia, p. 1107, “As a rule, all patients undergoing elective surgery should have normal serum potassium levels. However, we do not recommend delaying surgery if the serum potassium level is above 2.8 mEq/L or below 5.9 mEq/L, if the cause of the potassium imbalance is known, and if the patient is in otherwise optimal condition.”
The same textbook points out an additional problem in ordering lab tests: “the failure to pursue an abnormality appropriately poses a greater risk of medicolegal liability than does failure to detect that abnormality. In this way, extra testing increases the medicolegal risk to physicians.” (Miller’s Anesthesia, 2005, p. 945)
Regarding the timing of lab testing, the ASA Practice Advisory on Preanesthesia Evaluation states “test results obtained from the medical record within 6 months of surgery are generally acceptable if the patient’s medical history has not changed substantially. More recent test results may be desirable when the medical history has changed, or when test results may play a role in the selection of a specific anesthetic technique (e.g., regional anesthesia in the setting of anticoagulation therapy.)”
For all the reasons stated above, you tell the RN that you won’t recheck the potassium lab value for this patient, and you won’t delay or cancel the ACL surgery. The surgery is completed two days later, without complication. Your two clients, the patient and the surgeon, are both happy, and you’ve practiced sound, evidence-based medicine.
For further details on the management of hypokalemia and hyperkalemia before, during, and after surgery, see the chapter I wrote entitled Disorders of Potassium Balance, in Complications in Anesthesia, 3rd Edition, 2017, edited by Lee Fleisher and Stanley Rosenbaum, Elsevier Press, Philadelphia.