Richard F. McLean

Evaluation of A ventilation strategy to prevent barotrauma in patients at high risk for acute respiratory distress syndrome

BACKGROUND A strategy of mechanical ventilation that limits airway pressure and tidal volume while permitting hypercapnia has been recommended for patients with the acute respiratory distress syndrome. The goal is to reduce lung injury due to overdistention. However, the efficacy of this approach has not been established. METHODS Within 24 hours of intubation, patients at high risk for the acute respiratory distress syndrome were randomly assigned to either pressure- and volume-limited ventilation (limited-ventilation group), with the peak inspiratory pressure maintained at 30 cm of water or less and the tidal volume at 8 ml per kilogram of body weight or less, or to conventional ventilation (control group), with the peak inspiratory pressure allowed to rise as high as 50 cm of water and the tidal volume at 10 to 15 ml per kilogram. All other ventilatory variables were similar in the two groups. RESULTS A total of 120 patients with similar clinical features underwent randomization (60 in each group). The patients in the limited-ventilation and control groups were exposed to different mean (+/-SD) tidal volumes (7.2+/-0.8 vs. 10.8+/-1.0 ml per kilogram, respectively; P<0.001) and peak inspiratory pressures (23.6+/-5.8 vs. 34.0+/-11.0 cm of water, P<0.001). Mortality was 50 percent in the limited-ventilation group and 47 percent in the control group (relative risk, 1.07; 95 percent confidence interval, 0.72 to 1.57; P=0.72). In the limited-ventilation group, permissive hypercapnia (arterial carbon dioxide tension, >50 mm Hg) was more common (52 percent vs. 28 percent, P=0.009), more marked (54.4+/-18.8 vs. 45.7+/-9.8 mm Hg, P=0.002), and more prolonged (146+/-265 vs. 25+/-22 hours, P=0.017) than in the control group. The incidence of barotrauma, the highest multiple-organ-dysfunction score, and the number of episodes of organ failure were similar in the two groups; however, the numbers of patients who required paralytic agents (23 vs. 13, P=0.05) and dialysis for renal failure (13 vs. 5, P= 0.04) were greater in the limited-ventilation group than in the control group. CONCLUSIONS In patients at high risk for the acute respiratory distress syndrome, a strategy of mechanical ventilation that limits peak inspiratory pressure and tidal volume does not appear to reduce mortality and may increase morbidity.

Metaanalysis of prophylactic drug treatment in the prevention of postoperative bleeding.

Prophylactic drug treatment is one of several strategies to reduce postoperative blood loss and potentially limit homologous blood use in open heart surgery. A computerized MEDLINE search supplemented with manual bibliography reviews was performed for randomized clinical trials published in peer-reviewed English-language journals from January 1980 to June 1993. A metaanalysis was conducted of trials evaluating desmopressin (group DD, n = 13), epsilon-aminocaproic acid or tranexamic acid (group EA, n = 4), and aprotinin (group AP, n = 16). Eligible studies used placebo controls and administered the drug in a prophylactic manner. The primary study end point was postoperative chest tube loss (mL, mean +/- standard deviation). There was a significant reduction in postoperative chest tube loss detected for each of the active treatments versus the placebo (DD versus controls: percent reduction 0.11, p = 0.0021; EA versus controls: percent reduction 0.30, p < 0.0001; and AP versus controls: percent reduction 0.36, p < 0.0001). Therapy with EA or AP was associated with a greater reduction in chest tube loss than DD (EA versus DD, p = 0.0033, and AP versus DD, p < 0.0001). Secondary study end points were transfusion requirements, chest reexploration, and perioperative mortality. The volume of postoperative red cell transfusion (mean +/- standard deviation) was reduced with EA (p < 0.0001) or AP treatment (p < 0.0001) compared with a placebo or DD, whereas the proportion of patients given transfusions was limited only in the AP-treated patients (odds ratio 0.23; 95% confidence interval, 0.16 to 0.33; p < 0.0001).(ABSTRACT TRUNCATED AT 250 WORDS)

Central-nervous-system dysfunction after warm or hypothermic cardiopulmonary bypass

The increasing popularity of warm heart surgery led us to assess the effect of temperature during cardiopulmonary bypass (CPB) on neuropsychological function after coronary surgery. 34 patients enrolled in a randomised trial of normothermic versus hypothermic CPB were subjected to a battery of psychomotor and memory tests before and after their operations. The mean nasopharyngeal temperature for warm CPB was 34.7 (SD 0.5) degrees C and that for hypothermic CPB was 27.8 (2.0) degrees C. In all seven neuropsychological tests the postoperative scores were better in the warm CPB than in the hypothermic group, although only one difference achieved significance (trial-making test A; p less than 0.023). Thus, neurological function after normothermic CPB seems to be no worse than that after hypothermic procedures.

Anaesthetic management of acute blunt thoracic trauma

Sunnybrook Health Science Centre is an adult regional trauma unit serving metropolitan Toronto and environs. We undertook a nvo-year retrospective review of patients admitted to our institution with blunt thoracic trauma. Three hundred and thirty-three patients with blunt trauma and an injury severity score (ISS) greater than 17 required emergency surgery. Of these, 208 had blunt thoracic injuries while 125 did not have chest injuries. Both groups were similar with respect to age but patients with thoracic trauma had a greater ISS. (P < 0.05) and greater intraoperative mortality (P < 0.01). The aetiology of the intraoperative deaths with one exception was exsanguination. Emergency thoracotomy or sternotomy indicated a poor prognosis with a mortality rate of 80%. The most common intraoperative problem was an elevated airway pressure. Awake intubation was undertaken in 77.5% of patients requiring anaesthesia and surgery because of the potentially compromised airways and difficult intubations due to the nature of the associated injuries. Finally, 74% of patients undergoing urgent surgery required mechanical postoperative ventilation. The presence of blunt chest trauma should be considered a marker of the severity of injury sustained by the patient.RésuméLe Sunnybrook Health Science Centre est le centre de traumatologie adulte desservant le Toronto métropolitain. Nous avons revu les cas de traumatisme s fermés du thorax qui y furent admis depuis deux ans. Nous avons recensé 333 cas de traumatisme fermé important chez qui une intervention chirurgicale urgenle s’avéra nécessaire. On à dénombré parmi eux 208 traumatismes du thorax. L’âge moyen était le même qu’il y ait ou non implication thoracique toutefois, cette dernière entraînait un score de gravité moins rejouissant (P < 0,05) et une mortalité peropératoire plus grande (P < 0,01). Sauf exception, les décès peropératoire survenaient par exsanguination. Les thoracotomies et sternotomies d’urgence étaient de mauvais augure avec une mortalité de 80%. En peroperatoire, les pressions des voies respiratoires étaient souvent très élevées. Des voies respiratoires supérieures déformées et d’autres blessures laissant anticiper des dijficultés lors de l’intubation, nous out fait opter pour une intubation éveillée dans 77,5% des cas nécessitant une anesthésie. Enfin, après l’intervention chirurgicale nous avons dû ventiler mécaniquement 74% des patients. La presénce d’ un traumatisme fermé du thorax assombrit le pronostic des victimes daccident.

Cardiopulmonary bypass, rewarming, and central nervous system dysfunction

BACKGROUND During cardiopulmonary bypass a nasopharyngeal temperature greater than 38 degrees C at the end of rewarming may indicate cerebral hyperthermia. This could exacerbate an ischemic brain injury incurred during cardiopulmonary bypass. METHODS In a cohort of 150 aortocoronary bypass patients neuropsychologic test scores of 66 patients whose rewarming temperature exceeded 38 degrees C were compared with those who did not. There were no differences between groups with respect to demographic and intraoperative variables. RESULTS A trend was seen for hyperthermic patients to do worse on all neuropsychologic tests in the early postoperative period but not at 3-month follow-up. By analysis of covariance hyperthermic patients did worse on the visual reproduction subtest of the Weschler memory scale at 3 months (p = 0.02), but this difference was not found by linear regression (p = 0.10). CONCLUSIONS We were unable to demonstrate any significant deterioration in patients rewarmed to greater than 38 degrees C in the early postoperative period. The poorer performance in the visual reproduction subtest of the Wechsler memory scale at 3 months in the group rewarmed to more than 38 degrees C is interesting but far from conclusive. Caution with rewarming is still advised pending more in-depth study of this issue.

Perioperative cardiovascular complications associated with blunt thoracic trauma

The purpose of this study was to examine the frequency and importance of intraoperative mortality, arrhythmias and hypotension in the presence of thoracic trauma and to determine the effect of myocardial contusion on these perioperative complications. Over a two-year period patients with evidence of blunt thoracic injury who required surgery within 24 hr of admission were studied. The anaesthetist filled in a questionnaire on intraoperative events. Patients were also studied for the presence of myocardial injury with radionuclide angiography (RNA), at autopsy or at thoracotomy. Two hundred and one patients were studied. The intraoperative and overall mortality was 7.9% and 22.9% respectively. Of the operating room survivors the incidence of intraoperative arrhythmias and hypotension was 3.8% and 26.5% respectively. Only 5.9% of patients had a suspected or confirmed myocardial contusion. Patients were divided into two groups, those without myocardial injury were designated Group I, while those with myocardial contusion were designated Group II. The Group II patients had a greater severity of injury and intraoperative mortality (54.4%) than those in Group I (4.6%) P<0.05. Intraoperative deaths were attributed to, with one exception, non-cardiac causes. There were no differences in the incidences of arrhythmias and hypotension between patients with or without myocardial injury surviving the operating room. All patients with blunt thoracic injury may develop intraoperative arrhythmias or hypotension.RésuméCette étude porte sur le taux de mortalité, la fréquence et l’importance des dysrythmies et de l’hypotension peropératoires dans le trauma thoracique et vise à déterminer l’influence de la contusion thoracique sur ces complications périopératoires. Sur une période de deux années, on a étudié, à l’aide d’un questionnaire rempli par l’anesthésiste pendant l’opération, les malades porteurs d’un traumatisme thoracique fermé nécessitant une intervention chirurgique dans les 24 heures de l’admission. On a recherché aussi chez ces malades les lésions du myocarde soit par angiographie isotopique, soit à l’autopsie soit lors de la thoracotomie. L’étude regroupait deux cents et un patients. La mortalité peropératoire et la mortalité totale se situent à 7.9% et 22.9% respectivement. Pour les survivants de la chirurgie, l’incidence peropératoire des dysrythmies est de 3,8% et de l’hypotension 26,5%. Seulement 5,9% des patients souffraient de contusion myocardique suspecie ou confirmée. Les patients ont été répartis en deux groupes: ceux qui ne présentaient pas de contusion myocardique désignés comme groupe I; ceux qui en présentaient une désignés comme groupe II. Les patients du groupe II étaient plus gravement blessés et avaient un taux de mortalité peropératoire plus élevé (54,4%) que ceux du groupe I (4,6%), P<0.05. A l’exception d’un cas, la mortalité peropératoire est attribuée à des causes non cardiaques. Chez les survivants, on n’a pas trouvé de différence en ce qui regarde l’incidence des dysrythmies et de l’hypotension entre les patients qui présentaient une blessure myocardique et ceux qui n’en présentaient pas. Tous les traumatisés avec des blessures fermées du thorax peuvent développer des dysrythmies ou de l’hypotension.

Ketamine concentrations during cardiopulmonary bypass

PurposeTo describe the serum concentrations of ketamine following a clinically relevant dosing schedule during cardiopulmonary bypass (CPB).MethodsDesign: Prospective case series. Setting: Tertiarycare teaching hospital. Patients: Six patients undergoing coronary artery bypass grafting and over age 60 yr. Intervention: Following induction of anaesthesia each patient received a bolus of ketamine 2 mg· kg−1 followed by an infusion of 50 μg· kg−1 · min−1 which ran continuously until two hours after bypass. Main Outcome Measures: Ketamine serum concentrations were measured at five minutes after bolus, immediately following aortic cannulation, 10 and 20 min on CPB, termination of CPB, termination of the drug infusion and three and six hours after infusion termination.ResultsAt the time of aortic cannulation, ketamine concentrations were 3.11 ± 0.81μg · ml−1, these levels decreased by one third with the initiation of CPB. By the end of CPB the concentrations had returned to levels roughly equivalent to those observed at the time of aortic cannulation. Following cessation of the infusion, ketamine concentration declined in a log-linear fashion with a half-life averaging 2.12 hr. (range 1.38–3.09 hr).ConclusionsThis dosage regimen maintained general anaesthetic concentrations of ketamine throughout the operative period. These levels should result in brain tissue concentrations in excess of those previously shown to be neuroprotective in animals. Thus we conclude that this infusion regimen would be reasonable to use in order to assess the potential neuroprotective effects of ketamine in humans undergoing CPB.RésuméObjectifFaire connaître les concentrations sériques de la kétamine procurées par un schéma posologique approprié à la circulation extracorporelle (CEC).MéthodesType d’étude: Prospective. Endroit: Hôpital de soins tertiaires et d’enseignement. Patients: revascularisation du myocarde chez six patients âgés de plus de 60 ans. Intervention: Après l’induction de l’anesthésie chacun des patients a reçu un bolus de kétamine 2 mg· kg−1 suivi d’une perfusion de 50 μg· kg−1· h−1 en permanence, arrêtée deux heures après l’intervention. Principales mesures de résultats: Les concentrations sériques de kétamine mesurées après la canulation de l’aorte, 10 et 20 min après le début de la CEC, à l’arrêt de la CEC et trois et six heures après l’arrêt de la perfusion.RésultatsLes concentrations de kétamine qui étaient de 3,11 ± 0,81 μg· ml−1 au moment de la canulation de l’aorte ont diminué du tiers avec le début de la CEC. A la fin de la CEC, elles sont revenues à peu près à ce qu ’elles étaient au moment de la canulation de l’aorte. Après l’arrêt de la perfusion, la concentration de la kétamine a diminué de façon linéaire logarithmique avec une demi-vie moyenne de 2,12 h (écart de 1,38 à 3,09h).ConclusionsCe schéma posologique a permis de maintenir des concentrations anesthésiques de kétamine pendant l’intervention. Ces niveaux devraient produire des concentrationscérébrales plus élevées que celles qui ont été démontrées comme neuroprotectrices chez l’animal. Les auteurs concluent que ce schéma devrait être pertinent pour l’évaluation des propriété neuroprotectrices de la kétamine chez les humains qui subissent une CEC.

Pulmonary pressures at high flows in the intact pulsatile flow perfused lung.

Pulmonary pressure-flow curves can be easily generated in the intact animal by using a combination of systemic arteriovenous (a-v) fistulas and inferior vena cava (IVC) occlusion. By combining this technique with pulmonary artery occlusion, pulmonary pressure-flow curves may be studied over a broader range of pressures than has been previously been done in the intact, resting animal using pulsatile flow. Pressure-flow curves were generated by varying flow through opening and closing of the a-v fistulas in conjunction with inflating and deflating a balloon in the inferior vena cava. The pressure-flow curves were done under two conditions; (1). with both lungs perfused; (2) with the right lung excluded from the circulation (PA occlusion). PA occlusion resulted in no change in alveolar arterial oxygen tension gradient. The pressure-flow relationships for one lung and two lungs were well described by linear equations (r2 =0.83 ±0.03 and 0.82 ±0.04 respectively). The slope of the equations increased with PA occlusion (3.6 ±0.4 mmHg ·L−1 to 5.9 ±0.9 mmHg ·L−1). There was no change in the pressure axis intercept with PA occlusion (8.34 ±0.8 mmHg pre-occlusion and 8.9 ±1.3 mmHg post-occlusion). It is concluded that the pulmonary pressure-flow relationship is well described by a linear function above a mean pulmonary artery pressure (PAP) of 10–12 mmHg.RésuméDes courbes pression-debit pulmonaires peuvent être obtenues facilement chez l’animal intact en utilisant une combinaison de fistules artérioveineuses (a-v) systémiques et d’occlusions de la veine cave inférieure (IVC). En combinant cette technique avec l’occlusion de l’artère pulmonaire, les courbes pression-débit pulmonaires peuvent etre étudiées sur une gamme plus étendue de pression qu’auparavant chez l’’animal intact au repos par l’utilisation d’un débit pulsatile. Les courbes pression-débit ont été produites en variant le débit à trovers les fistules a-v à différents degrés d’ouverture et de fermeture, pendant temps qu’un ballon était gonflé puis dégonflé dans la veine cave inférieure. Les courbes pression-débit ont été produites sous deux conditions: 1) avec les deux poumons perfusés; 2) avec le poumon droit exclu de la circulation (occlusion PA). L’occlusion de la PA n’a produit aucun changement dans le gradient alvéolo-artériel de la tension en oxygène. Les relations pressiondébit pour un ou deux poumons étaient bien décrites par des équations linéaires (r2 = 0,83 ±0,03 et 0,82 ±0,04 respectivement). La pente des equations augmentait avec l’occlusion de la PA (3,6 ±0,4 mmHg ·L−1 à 5,9 ±0,9 mmHg ·L−1). Il n’y avait aucun changement avec l’occlusion de la PA quant à la valeur de l’intersection de la droite avec l’axe depression (8,34 ±0,8 mmHg avant l’occlusion et 8,9 ±1,3 mmHg après l’occlusion). En conclusion, la relation pression-debit pulmonaire est bien représentée par une fonction linéaire lorque la pression de l’artère pulmonaire moyenne (PAP) est au-dessus de 10 à 12 mmHg.