Janet E. Steele

Somatosensory Evoked Potentials Help Prevent Positioning-Related Brachial Plexus Injury during Skull Base Surgery

Objective Evaluate the use of somatosensory evoked potentials (SSEP) monitoring to detect positioning-related brachial plexus injury during skull base surgery. Study Design Prospective cohort observational study. Setting University Hospital. Subjects and Methods Patients undergoing skull base surgery had a focused neurologic exam of the brachial plexus performed before and after surgery. Under stable anesthesia, brachial plexus SSEP values were obtained before and after surgical positioning. Significant SSEP changes required a readjustment of arm or neck positions. SSEPs were assessed every 30 minutes. If changes were noted, position was readjusted and SSEPs were reassessed until surgical completion. Demographic data, neurologic exams, SSEP latency, and amplitude values were recorded. Persistent changes were correlated with postoperative neurologic findings. Results Sixty-five patients, 15 to 77 years old, were studied. Six patients (9.2%) developed SSEP amplitude changes after positioning (average amplitude decrease 72.8%). One patient had a significant latency increase. The sensitivity of SSEP for detection of injury was 57%, while specificity was 94.7%. The average body mass index (BMI) of patients with normal and abnormal SSEPs was 28.7 ± 5.6 versus 29.2 ± 8.0, respectively. Average BMI of patients with postoperative symptoms regardless of SSEP findings was 33.8 ± 4.3. Two patients who had persistent SSEP changes after positioning had BMIs of 40.1 and 31.2 kg/m2, respectively. Improvement in neurologic findings occurred in all patients after surgery. Conclusions This study demonstrates that upper extremity nerve stress can be detected in real time using SSEP and may be of value in protecting patients from nerve injury undergoing lateral skull base surgery.

State-Dependent Expression of Pressure Diuresis in Conscious Rats

In 1967, Guyton and Coleman modeled pressure diuresis as the underlying, essential, long-term mechanism that regulates arterial pressure when sodium intake changes. Other mechanisms that influence renal function interact with pressure diuresis to achieve sodium balance and determine the blood pressure. Increases in sodium intake suppress sodium conserving mechanisms and activate natriuretic mechanisms; decreases in sodium intake have the opposite effect. If the Guyton-Coleman model is correct, then pressure diuresis should be more readily detected in animals on a high-salt diet than in animals on a low-salt diet. We measured spontaneous changes in arterial pressure and urine flow in conscious rats fed low-salt (0. 4% NaCl) and high-salt (8.0% NaCl) chow. For 10 rats fed a high-salt diet, arterial pressure and urine flow were positively correlated in 19 of 32 (59%) trials. In 10 rats fed a low-salt diet, a positive correlation was observed in 10 of 33 (30%) trials. Chi-square analysis revealed that differences in Na+ content of the diet were significantly associated with the probability of a positive relationship between blood pressure and urine flow. These results support the hypothesis that the expression of pressure diuresis across time is dependent on the state of sodium balance.

Gravimetric Method for the Dynamic Measurement of Urine Flow

Abstract The rate of urine formation is a primary index of renal function, but no techniques are currently available to accurately measure low rates of urine flow on a continuous basis, such as are normally found in rats. We developed a gravimetric method for the dynamic measurement of urine flow in anesthetized rats. Catheters were inserted directly into the ureters close to the renal pelves, and a siphon was created to collect all of the urine formed as rapidly as it was produced. Urine flow was determined by measuring the weight of the urine using a direct-reading analytical balance interfaced to a computer. Basal urine flow was measured at 2-sec intervals for 30 to 60 min. The dynamic response of urine flow to a rapid decrease in arterial pressure produced by a bolus intravenous injection of acetylcholine (0.5 μg) was also measured. Intrinsic drift, evaporative losses, and the responsiveness of the system to several fixed pump flows in the low physiologic range were evaluated in vitro. The gravimetric method described was able to continuously measure basal urine flows that averaged 37.3 ± 12.4 μl/min. Error due to drift and evaporation was negligible, totaling less than 1% of the measured urine flow. Acetylcholine-induced declines in arterial pressure were followed within 8 sec by a decline in urine flow. These data demonstrate that this new gravimetric method provides a simple, inexpensive, dynamic measurement of urine flow in the μl/min range.