Eugene Y. Cheng

Direct Relaxant Effects of Intravenous Anesthetics on Airway Smooth Muscle

Ketamine, at concentrations achieved with the usual clinical doses, has a direct relaxant effect on airway smooth muscle (ASM).This study investigates the dose-dependent direct relaxation effects of midazolam and propofol on both proximal and distal ASM compared with ketamine. The proximal and distal airways were dissected from eight mongrel dogs and cut into 2-mm rings. The rings were attached to pressure transducers and equilibrated in a Krebs-Ringer bicarbonate bath kept at 37 degrees C, pH 7.4, CO2 37 mm Hg, and PaO2 > 100 mm Hg. Optimal length was determined, a dose-response curve to acetylcholine was established, and the 50% effective dose (ED50) of acetylcholine was calculated. Ketamine, midazolam, or propofol were given in random order to each ring preconstricted with ED50 of acetylcholine in cumulative log incremental doses from 10-6 to 10-4 M. Relaxation response was the tension during anesthetic equilibrium, expressed as a percentage of the tension from ED50 of acetylcholine. The drug vehicles were tested for their effects on the ASM. No bronchorelaxation was seen with any of the intravenous anesthetics at 10-6 M. Ketamine 10-5 M produced a 17.9% +/- 2.1% relaxation in the distal ASM but had no effect on the proximal ASM. Neither propofol nor midazolam affected the ASM at 10-5 M. The distal ASM was significantly (P < 0.005) more sensitive to 10-4 M of all three drugs compared with the proximal ASM. In the proximal ASM, 10-4 M of ketamine, midazolam and propofol reduced ASM tension by 14.9% +/- 4.4%, 19.0% +/- 8.8%, and 14.7% +/- 5.5%, respectively, versus 36.4% +/- 3.2%, 58.6% +/- 6.1%, and 64.4% +/- 9.0% in the distal ASM. The drug vehicles had no effect on the ASM. We conclude that ketamine, midazolam, and propofol have direct relaxant effects on ASM. All three intravenous anesthetics have a greater direct relaxant effect on distal ASM than on proximal ASM. Only ketamine showed significant direct bronchorelaxing effects at concentrations that are likely to be achieved with the usual clinical dosing patterns. (Anesth Analg 1996;83:162-8)

Effects of acute hypothermia and β-adrenergic receptor blockade on serum potassium concentration in rats

Background and MethodsWe hypothesized that β-adrenergic receptor blockade would result in an increase in serum potassium concentration in hypothermic rats given a potassium load compared to non-β-blocked, hypothermic, potassium-loaded rats. To test this hypothesis, we investigated the interaction between body temperature and β-adrenergic receptor blockade on serum potassium concentrations in ureter-ligated rats with and without potassium loading. To acheive this goal, we performed three experiments. In the first experiment, serum potassium concentrations were determined in 16 rats as they were continuously cooled from 37° to 22°. In the second experiment, 12 ureter-ligated rats were cooled to 31°, after which they were rewarmed to 37°. Serum potassium concentrations were determined before and after cooling and on rewarming. Twelve other ureterligated rats were cooled to 31°, then given a potassium load until their serum potassium concentrations returned to their baseline values, after which they were rewarmed to 37°. Serum potassium concentrations were determined before and after cooling, during the potassium infusion, and on rewarming. In the third experiment, 14 rats were pretreated with propranolol and 14 rats served as controls. Half of the rats in each of these two groups were kept at 37° and half were cooled to 25°. All rats were then given a 690-μmol potassium chloride infusion. Serum potassium concentrations were determined before and after the potassium infusion. ResultsThe rats developed hypokalemia with cooling, which spontaneously resolved in the rats without supplementation on rewarming to 37°. The hypothermic hypokalemic rats that had their serum potassium concentrations corrected to normothermic status (2.93 ±PT 0.17 mmol/ L) had marked increases in serum potassium concentrations (4.22 ±PT 0.15 mmol/L) on rewarming. In the normothermic rats, potassium loading after β-adrenergic receptor blockade resulted in even higher serum potassium concentrations (5.65 ±PT 0.36 mmol/L) compared with non-β-blocked rats given equal potassium loads (4.6 ±PT 0.4 mmol/L). However, in hypothermic (25°) rats given the same potassium load, there was no difference in serum potassium concentrations in β-blocked (6.5 ±PT 0.35 mmol/L) and non-β-blocked rats (6.63 ±PT 0.3 mmol/L). ConclusionsThese results suggest that acute hypothermia causes a decrease in serum potassium, probably secondary to redistribution, which is reversible on rewarming. Supplementation of potassium during hypothermia can cause a significant increase in serum potassium concentration on rewarming. Blocking β-adrenergic receptors with propranolol did not effect hypothermia-induced hypokalemia, suggesting that the β-adrenergic mechanism may not be functional in hypothermia. (Crit Care Med 1991; 19:1545)

Glass particle contamination: influence of aspiration methods and ampule types

Glass particle contamination of the contents of single-dose glass ampules can occur upon opening. In our study we determined if different aspirating techniques or different ampule types had any effect on glass particle contamination. In part 1 of this study different aspiration techniques were evaluated. The four groups included a control group of 3 mm tubing, an 18-g 1.5-inch needle, a filter needle, and an in-line filter. A significant reduction in glass particle contamination was found when using either an in-line filter or a filter needle compared with the control group or when aspirating through an 18-g needle. The average number of glass particles found per ampule for each group was 100.6 +/- 16.3, 65.6 +/- 18.7, 1.3 +/- 0.3, and 1.2 +/- 0.3, respectively, for the control group, 18-g needle, filter needle, and in-line filter. In part 2 we examined four types of glass ampules: transparent metal etched, transparent chemically etched, amber metal etched, and amber chemically etched. There was a significantly greater number of glass particles found in the transparent metal etched ampules compared with that found in the other three ampule types. Transparent metal etched ampules yielded an average total number of particles per ampule of 45.9 +/- 15.4 compared with 3.2 +/- 0.9, 6.0 +/- 1.7, and 3.1 +/- 0.6, in the transparent chemically etched, amber metal etched, and amber chemically etched ampule types, respectively. This study demonstrates that using drugs supplied in ampules other than transparent metal etched type and by using filters will decrease the risk of parenteral injection of glass particles.

Forehead pulse oximetry compared with finger pulse oximetry and arterial blood gas measurement

Usual monitoring sites for pulse oximetry involve the fingers, toes, ear lobe, and nasal septum. This study examined the performance of a forehead sensor compared with a finger sensor for the pulse oximeter and arterial blood gas (ABG) analysis. Ten healthy adult volunteers and 22 ventilator-dependent patients were studied. The arterial oxygen saturation detected by forehead pulse oximetry (SpO2) correlated well with finger SpO2 and arterial oxygen saturation (SaO2) determined by arterial blood gas analysis in the healthy volunteers. Forehead SpO2 in mechanically ventilated patients correlated well with finger SpO2 and SaO2 when heart rate detected by pulse oximeter differed less than 10% from apical heart rate. Factors that caused a difference in oximeter-detected heart rate and apical heart rate were extensive tissue edema, head movement, and difficulty securing good tape placement. This suggests that when signal strength is weak, causing poor pulse rate detection, there will also be problems associated with accurate SpO2.The forehead pulse oximeter sensor works well on healthy, well-oxygenated volunteers. Difficulty was experienced when applying and using the sensor on critically ill patients. The reliability of the forehead pulse oximeter sensor has not been established at low saturations.

The effect of acute hypothermia and serum potassium concentration on potassium cardiotoxicity in anesthetized rats

We examined the effects of hypothermia on serum K+ concentration and the interaction of body temperature and K+ load on cardiac toxicity in anesthetized rats. Serum K+ concentration significantly decreased to 2.61 ± 0.13, 2.59 ± 0.19 and 2.39 ± 0.14 mmol/1 at 31.0°C, 28.0°C and 25.0°C, respectively, from the control value of 2.80 ± 0.15 mmol/l at 37.0°C. We used a 300% increase in baseline QRS duration as evidence of cardiac toxicity. Serum K+ concentrations of 4.95 ± 0.12, 4.71 ± 0.10, 4.45 ± 0.14 and 4.07 ± 0.14 mmol/l resulted in cardiac toxicity at 37.0°C, 31.0°C, 28.0°C, and 25.0°C, respectively. These data indicate that the level at which an elevation of serum K+ concentration causes cardiac toxicity diminishes with progressive hypothermia. We conclude that hypothermia induces hypokalemia, possibly through redistribution, and that the myocardium appears to be more sensitive to the toxic effects of K+ as hypothermia deepens.

Use of active noise cancellation devices in caregivers in the intensive care unit.

Objective: Recent development of noise cancellation devices may offer relief from noise in the intensive care unit environment. This study was conducted to evaluate the effect of noise cancellation devices on subjective hearing assessment by caregivers in the intensive care units. Design: Randomized, double‐blind. Setting: Adult medical intensive care unit and pediatric intensive care unit of a teaching hospital. Subjects: Caregivers of patients, including nurses, parents, respiratory therapists, and nursing assistants from a medical intensive care unit and pediatric intensive care, were enrolled in the study. Intervention: Each participant was asked to wear the head‐phones, functional or nonfunctional noise cancellation devices, for a minimum of 30 mins. Measurements: Subjective ambient noise level was assessed on a 10‐point visual analog scale (VAS) before and during headphone use by each participant. Headphone comfort and the preference of the caregiver to wear the headphone were also evaluated on a 10‐point VAS. Simultaneously, objective measurement of noise was done with a sound level meter using the decibel‐A scale and at each of nine octave bands at each bedspace. Results: The functional headphones significantly reduced the subjective assessment of noise by 2 (out of 10) VAS points (p < 0.05) in environments of equal objective noise profiles, based on decibel‐A and octave band assessments. Conclusion: Noise cancellation devices improve subjective assessment of noise in caretakers. The benefit of these devices on hearing loss needs further evaluation in caregivers and critically ill patients.

Early response to pulse oximetry alarms with telemetry

Twenty ICU patients were monitored for an average of 45 hr each, with both bedside and nursing station monitors, which were set to alarm audibly if the patient’s oxygen saturation dropped below 90%. Bedside alarms alerted caregivers to 51 of the 74 hypoxemic events; central alarms alerted personnel to the other 23 events. The alarms led to a change in treatment in 35 of the 48 (73%) true desaturation episodes. We conclude that central oximetric monitoring may help with detection of arterial desaturation events even in a well-staffed ICU.RésuméVingt malades de soins intensifs furent monitores pour une moyenne de 45 heures tant à l’aide de moniteurs de chevet que de moniteurs d’une station de soins centrale programmes pour sonner l’alarme des que la saturation d’oxygène tomberait en dessous de 90%. Les signaux d’alarme de chevet alerterent le personnel infirmier de l’occurrence de 51 des 74 épisodes hypoxémiques. Les signaux d’alarme centraux alerterent le personnel infirmier des 23 autres épisodes. Grâce aux signaux d’alarme, le traitement de 35 des 48 épisodes de véritable désaturation fut changé. Nous concluons que lc monitorage central oximétrique aide la détection d’épisodes de désaturation artérielle même dans une unité de soins intensifs bien fournie.AbstraktZwanzig Intensivpatienten wurden durchschnittlich jeweils 45 Stunden überwacht, jeder sowohl mit Monitoren am Patientenbett wie auch im Stationszimmer. Die Monitore gaben akustischen Alarm, wenn die Sauerstoffsättigung unter 90 % fiel. Alarme am Bett riefen Pflegekräfte zu 51 der 74 hypoxämischen Ereignisse, während zentrale Alarme Personal in den übrigen 23 Fallen alarmierten. Die Alarme führten bei 35 der 48 (73 %) echten Untersättigungscreignisse zu einer Anderung der Behandlung. Wir folgern daraus, daß zentrale Überwachung der Sauerstoffsättigung bei der Entdeckung von arteriellen Untersättigungsereignissen sogar auf einer gut besetzten Intensivstation helfen könnte.ResumenVeinte pacientes de una unidad de cuidados intensivos fueron monitorizados durante un promedio de 45 horas cada uno, con monitores tanto al lado de la cama como en la estación de enfermería. Las alarmas audibles de los monitores se regularon de modo de ser activadas si la saturatión de oxigeno del paciente descendía bajo 90%. Las alarmas al lado de la cama alertaron al personal a cargo del paciente en 51 de 74 eventos hipóxicos, mientras que las alarmas centrales alertaron al personal en los restantes 23 eventos. Las alarmas condujeron a cambios en la terapia en 35 de los 48 eventos (73%) en que hubo verdadera desaturatión arterial. Concluimos que la monitorización central de la oximetría puede ser de ayuda en la detection de eventos de desaturación arterial, incluso en una unidad de cuidados intensivos adecuadamente dotada de personal.

Recall in the sedated ICU patient

The effect of sedation on ICU patient recall is uncertain. Ensuring suppression of awareness and, particularly, recall, may prevent post-ICU psychological problems. Development of ICU sedation pathways and improved neurophysiologic monitoring techniques may help clinicians provide good levels of recall suppression and sedation when they are most needed by ICU patients.

Topographical Differences in the Direct Effects of Isoflurane on Airway Smooth Muscle

Volatile anesthetics have a direct relaxant effect on airway smooth muscle, but it is not known whether this effect is similar throughout the bronchial tree. We studied the direct relaxation effect of isoflurane on isolated proximal (outer diameter [OD] 4–6 mm) and distal (OD 0.7–1.5 mm) canine airways precontracted with acetylcholine. Proximal and distal airway rings were suspended in tissue baths and stretched to their optimum length. A dose-response curve was obtained for each airway ring with log increments of acetylcholine. Maximum contraction was reached with 10 −2 mol/L of acetylcholine for the proximal airway smooth muscle (7.0 ± 0.3 g of tension) and 10 −3 mol/L of acetylcholine for the distal airway smooth muscle (2.3 ± 0.1 g of tension). Based on the dose-response curve, the ED50 of acetylcholine was calculated (1.26 ± 0.37 × 10 −4 mol/L for proximal airway smooth muscle; 2.12 ± 1.14 × 10 −5 mol/L for distal airway smooth muscle) and administered to each tissue bath, after which the stabilized response was recorded. A randomly selected dose of isoflurane (1,2, or 2.6 dog minimum alveolar anesthetic concentration [MAC]) was then administered to each bath and the relaxant responses were recorded. The proximal and distal airways relaxed with increased doses of isoflurane in a dose-related manner. The average distal airway relaxation was three times greater than the proximal airway relaxation at all isoflurane levels. The mechanism of this differential response is not known, but may be due to an epithelium dependent effect, sensitivity of contractile elements to Ca2+; and/or differences in ionic fluxes.

Antibiotic therapy and the anesthesiologist.

The anesthesiologist is frequently responsible for administering antibiotics in the immediate preoperative and intraoperative periods. Anesthesiologists often are not trained in the administration of antibiotics, which can be associated with both acute and long-term complications including potentiation of neuromuscular blocking agents, allergic reactions, and end-organ toxicity. The indications for perioperative antibiotics, proper method of administration, and occurrence and treatment of major side effects of the more commonly recommended prophylactic antibiotics are discussed.