Michael S. Gorback

The relative accuracies of two automated noninvasive arterial pressure measurement devices

We compared the accuracies of two types of noninvasive blood pressure devices. Thirty-two patients requiring an intraarterial catheter for anesthetic management underwent simultaneous monitoring with Dinamap 1846SX and Ohmeda Finapres 3700 devices. For the first 10 minutes of recording, new Dinamap determinations were performed every 60 seconds; subsequent recordings were made at 3-minute intervals. Data were obtained at the time of new Dinamap readings, and twice between new readings to approximate the real-time performance of the two monitors. We defined superior accuracy as a statistically significant difference in mean absolute error greater than 5 mm Hg. With these criteria, pooled data from all patients revealed no difference in performance, even in real time. Pooled data can be misleading since there was a significant amount of variation in accuracy for both monitors. Therefore, we used nonparametric analysis to determine how many individual patients were monitored better by either device. When we compared only data from new Dinamap readings, the Finapres monitor showed superior performance for systolic readings in 13 patients, versus 6 patients for the Dinamap (P<0.05, chi-square test). Similar analysis for diastolic and mean pressure performance did not reach statistical significance. However, in real time, the Finapres unit monitored more patients more accurately for systolic (14 Finapres versus 3 Dinamap), diastolic (11 Finapres versus 3 Dinamap), and mean (10 Finapres versus 3 Dinamap) pressure determinations. The magnitude of these differences were, however, less dramatic than expected. This was probably due to stabilization of arterial pressure during the anesthetic, which minimized the error due to intermittent sampling. We conclude that continuous Finapres readings and new Dinamap determinations are equally accurate for diastolic and mean arterial pressures. The accuracy of Finapres appears to be slightly superior for systolic pressure. The intermittent sampling of oscillometric devices compromises their performance relative to the Finapres in many, but not all, cases.

Management of the challenging airway with the Bullard laryngoscope

The Bullard laryngoscope is a rigid fiber-optic device that offers a new approach to oral laryngoscopy. In this report, I present five cases illustrative of the use of the Bullard laryngoscope in patients with demonstrated or suspected difficult laryngoscopy and intubation.

CLINICAL RESPONSES TO ORG 9426 DURING ISOFLURANE ANESTHESIA

&NA; To determine average dose requirements and pharmacodynamic characteristics before general clinical use, the doseresponse curve, onset time, and recovery time for the neuromuscular relaxant ORG 9426 were determined in 72 adult patients given doses of 120, 160, 200, or 240 μg/kg after establishment of a steady‐state expired isoflurane concentration of approximately 1%. Neuromuscular blockade was continuously recorded using the ulnar evoked electromyogram. Using the log probit method, ED95 was 268 μg/kg, ED90 was 251 μg/kg, and ED50 was 144 μg/kg. The time until 80% blockade was 1.9 min at 240μg/kg, and the average time to peak effect was 4.6 min, which did not vary with dose. The clinical duration (injection until T1 returned to 25%) was 20.5 min, and the recovery index (T1 increased from 25% to 75% of control) was 15.4 min, after a total dose of 300 μg/kg. The duration of 75‐μg/kg and 100‐μg/kg repeat (maintenance) dose was 14.6 and 17.8 min, respectively, and no cumulative effect was apparent after as many as five maintenance doses. No cardiovascular side effects were seen at doses used in the study. We conclude that ORG 9426 is a nondepolarizing muscle relaxant with a rapid onset and short duration of action that deserves further clinical evaluation.

Problems associated with the determination of pulmonary vascular resistance

The presence of critical pressures in the pulmonary circulation complicates the traditional use of pulmonary vascular resistance (PVR). The recruitable nature of the pulmonary circulation violates a basic assumption of the PVR formula, that is, that the involved vessels arc rigid-walled. Flow through collapsible blood vessels is subject to the influence of critical opening pressures in addition to inflow and outflow pressures. As a result, PVR has a variable relationship to the Poiseuille resistance, approximating it better when zone 3 conditions predominate. In addition to being flow-dependent, PVR cannot easily distinguish among vasodilation, recruitment, and rheologie changes. PVR may be viewed as an index of steady-state power dissipation by the circulation, describing the relationship between power dissipation and flow, but it will still underestimate power dissipation by as much as 50%, since it cannot express oscillatory and kinetic power components. Laboratory data regarding the pulmonary circulation are predicted and explained by positing the existence of critical pressures in the pulmonary circulation and allow estimation of Poiseuille resistance. Unfortunately, clinical application of this approach is difficult owing to the necessity of generating pressure-flow plots under very stringent conditions. The clinical use of both pressure-flow and PVR-flow plots is impaired by shifting to different curves during hemodynamic manipulation. PVR must be interpreted in light of its considerable limitations.

Oscillometric blood pressure determination from the adult thumb using an infant cuff.

Automatic digital noninvasive blood pressure (NIBP) monitors have become commonly used during anesthesia within the past 10 years. The majority of these are oscillometric units that require the intermittent inflation of a cuff applied to the proximal arm. During the time the cuff is inflated, intravenous access and pulse oximetry monitoring on the same arm are essentially precluded. This limitation can be significant during surgery where only one arm is available. We observed that a small (neonatal) cuff applied to the adult thumb seemed to function well as a way of measuring oscillometric blood pressure. We now report the quantitative performance of a neonatal blood pressure cuff applied to the thumb when used for the noninvasive monitoring of blood pressure in adults.

Gastroesophageal reflux during anesthetic induction with thiopental and succinylcholine

The effects of patient physiology and the prior administration of a nondepolarizing muscle relaxant on the frequency of gastroesophageal reflux during induction with thiopental sodium and succinylcholine were investigated. Forty patients underwent anesthetic induction during continuous esophageal pH monitoring. Twenty patients had preoperative symptoms of gastroesophageal reflux, and 20 asymptomatic patients served as controls. Half the patients in each group received a small dose of nondepolarizing muscle relaxant prior to induction. Five patients (25%) with gastroesophageal reflux and none of the control patients showed significant decreases in esophageal pH during induction. Two of these patients received prior administration of a nondepolarizing drug. In all five patients, reflux occurred during laryngoscopy and intubation, suggesting the possibility that succinylcholine did not play a role in the generation of reflux. Preoperative gastroesophageal reflux symptomatology is associated with an increased frequency of reflux during induction, and the vulnerable period seems to occur after the achievement of neuromuscular blockade during laryngoscopy. Preinduction administration of a non-depolarizing muscle relaxant prior to succinylcholine use had no demonstrable effect on the frequency of reflux during induction, although the numbers studied were too small to be conclusive.