[Preoxygenation for anesthesia]. / Preoxigenación en anestesia.

Oxygenation, or rather denitrogenation, prior to apnea during anesthetic induction attempts to replace alveolar nitrogen with oxygen to achieve an intrapulmonary oxygen reserve that will allow apnea to be as prolonged as possible with the least possible desaturation. During apnea, the rate of arterial desaturation depends mainly on the volume of oxygen stored in the lung, on mixed venous oxygen saturation, and on the presence of intrapulmonary shunt. Together, these factors account for the higher rate of desaturation during apnea in children, obese individuals, postoperative patients, and pregnant women. Two approaches to preoxygenation have proven effective to date: ventilation with 100% oxygen at tidal volume for 3 minutes using a well-sealed face mask and the performance of 8 vital capacity maneuvers in 1 minute. The efficacy of preoxygenation can be assessed by expired oxygen fraction or by pulse oximetry. In a healthy adult, both methods described ensure sufficient oxygenation (pulse oximetry 90% to 95%) after a period of apnea lasting between 6 and 10 minutes.

[Quantification of dehydration of soda lime in clinical circumstances]. / Cuantificación de la desecación de la cal sodada en circunstancias clínicas.

HYPOTHESIS AND OBJECTIVES: When the CO2 absorbents, soda lime and baralime, have lost their normal level of hydration, they may react with certain halogenated anesthetics to produce appreciable levels of carbon monoxide. The degree of absorbent desiccation has been considered the limiting factor for this phenomenon. This study quantifies the level of dehydration of lime produced under clinical conditions and the influence of several factors. MATERIAL AND METHOD: Desiccation was determined: 1) at set periods of time (3, 7 and 14 days) after clinical use of fresh soda lime in general anesthesia using a fresh gas flow (FGF) of 6 l/min, and 2) after gas had been crossing the continuous flow (CF) oxygen reservoir at 7 l/min for 17 and 65 hours. Two anesthetic systems were used: a) the Ohmeda Excel-210, in which the continuous FGF did not cross the reservoir and b) the Siemens Ventilator 710, in which the FGF did cross the reservoir. The experiments were repeated with three types of lime. RESULTS: The clinical use of lime for 3, 7 and 14 days caused different levels of desiccation, with decreases in hydration of up to 50% and 14 days. Nevertheless, water content was always over 5%, a level at which no reaction with halogenated agents takes place. After 17 and 65 hours of CF in the circuit where continuous FGF did not pass through the canister, the water content did not change. With the Siemens 710 circuit, in which the continuous FGF crossed the canister, the dehydration level was 1.2 +/- 0.3% after 17 hours and 0.7 +/- 0.3% after 65 hours, a level that can produce CO upon reaction between lime and halogenated gases. The type of lime used had little effect. CONCLUSIONS: Lime does not desiccate to levels able to produce CO in daily use, regardless of the FGF system used. The phenomenon of desiccation depends on two factors: 1) use of anesthetic equipment in which continuous FGF conditions require gas to pass through the canister, and 2) the maintenance of CF for a sufficient period of time.

[Ondansetron in the prevention of postoperative nausea and vomiting in ambulatory surgery]. / Ondansetrón como profilaxis de las náuseas y vómitos postoperatorios en cirugía mayor ambulatoria.

OBJECTIVE: To evaluate the efficacy of antiemetic prophylaxis with ondansetron administered before major ambulatory surgery. PATIENTS AND METHOD: We retrospectively analyzed 1,881 patients (566 women, 31%) who underwent ambulatory surgery for various conditions between January and December 1993; 84% were ASA I, 15% ASA II and 1% ASA III: Mean age was 35.82 +/- 21.66 years; mean weight was 61.26 +/- 21.77 kg, and mean height was 155.13 +/- 19.27 cm. The techniques employed were total intravenous anesthesia in 64.8%, epidural in 22%, regional intravenous in 5.8%, local in 3.8%, peripheral blockade in 2.6% and intradural in 1%. One thousand thirty-three patients were premedicated with 4 mg ondansetron 30 minutes before the start of surgery; the remaining 848 received no antiemetic drug. We determined the incidences of nausea and vomiting during the 24 hours after surgery and the data were subjected to analysis of variance, covariance adjustment for quantitative variables, chi 2 test with continuity correction for nausea and vomiting. RESULTS: The incidences of nausea and vomiting were significantly lower in the patients premedicated with ondansetron. Only 36 (3.48%) of the premedicated patients experienced nausea, in comparison with 63 (7.4%) patients with no antiemetic premedication. Vomiting was experienced by 68 (6.58%) patients who received ondansetron, as compared to 112 (13.2%) patients who did not receive the drug. CONCLUSION: In our experience, administering 4 mg of ondansetron before major ambulatory surgery significantly reduces the incidence of nausea and vomiting in the immediate postoperative period.