Cardiovascular effects of 50 per cent nitrous oxide and 50 per cent oxygen mixture

Nitrous oxide in emergency medicine

Safe and predictable analgesia is required for the potentially painful or uncomfortable procedures often undertaken in an emergency department. The characteristics of an ideal analgesic agent are safety, predictability, non-invasive delivery, freedom from side effects, simplicity of use, and a rapid onset and offset. Newer approaches have threatened the widespread use of nitrous oxide, but despite its long history this simple gas still has much to offer. "I am sure the air in heaven must be this wonder-working gas of delight". Robert Southey, Poet (1774 to 1843)

Nitrous oxide for blood pressure control after coronary artery surgery: a dose-response hemodynamic study in postoperative patients

The inhaled anesthetic nitrous oxide is used for sedation in the coronary care unit and has been advocated as an antihypertensive agent in patients after a cardiac operation. To delineate the hemodynamic effects of nitrous oxide after cardiac surgery, we studied 11 patients with a mean age of 63 years who had undergone isolated coronary artery bypass grafting. All patients had major coronary artery disease and normal preoperative hemodynamic function. When patients were in stable condition in the intensive care unit, hemodynamic measurements were made at an inspired oxygen concentration of 40% with progressive doses of nitrous oxide (0, 10, 20, 40, and 60%). Measured and calculated hemodynamic data were compared using analysis of variance. Nitrous oxide caused significant decreases in mean arterial pressure in a dose-related fashion; pressure fell 6 mm Hg at 10% nitrous oxide and 11 mm Hg at 60% nitrous oxide (6 and 13% decreases). These changes were not associated with significant alteration in pulmonary artery pressure or cardiac index. Left ventricular stroke work index decreased significantly with the administration of nitrous oxide. We conclude that, in patients with normal hemodynamic and ventricular function, the administration of nitrous oxide for its sedative and antihypertensive effects is safe in the postoperative period and may be useful, especially in patients who are candidates for early extubation.

Nitrous oxide sedation/analgesia in emergency medicine

Nitrous oxide:oxygen mixtures are safe and effective sedative/analgesic agents for use in emergency medicine. They are suitable for prehospital care because of their safety, rapid onset, and short duration of action. The self-administered form of a 50:50 mixture is the most acceptable system, but other methods of administration should be investigated, particularly in the ED setting. While there are unresolved questions concerning the use of nitrous oxide as an anesthetic and analgesic agent, it is difficult to extrapolate many of the findings to the low-dose, patient-controlled 50:50 mixture used in emergency medicine. Use of the gas mixture in a variety of clinical settings has been associated with no major adverse occurrences, and experience continues to support its safety. Future possibilities for study include the use of the gas mixture in combination with other agents or TENS. No one drug can be considered a panacea for all painful states. Nitrous oxide:oxygen mixtures certainly do not meet all the criteria for the ideal analgesic. Those clinicians who seek a safe and effective sedative/analgesic for mild to moderate pain will be satisfied that we can now offer more to our patients in the early management of their problems.

Cardiovascular collapse associated with nitrous oxide anaesthetic: a case report

This is a case report of a one year old infant weighing 6.7 kilograms with a preoperative diagnosis of partial anomalous pulmonary venous drainage whose anaesthetic was complicated by cardiovascular collapse following addition of nitrous oxide to oxygen administered to the patient using a T-piece breathing circuit. The cardiovascular pathopharmacology of all the drugs administered to the patient before the event as well as that of nitrous oxide is discussed in detail. The drugs discussed are atropine 0.1 mg (0.01 mg . kg-1), glycopyrrolate 0.07 mg (0.01 mg . kg-1), morphine 0.9 mg (0.14 mg . kg-1) methylprednisone sodium succinate 165 mg (23.5 mg . kg-1) sodium thiopentone 40 mg (5.7 mg . kg-1), metocurine 3.4 mg (0.4 mg . kg-1).

Effects of nitrous oxide on liver haemodynamics and oxygen consumption in the greyhound

The effects of increasing concentrations of nitrous oxide 30, 50 and 70% on the hepatic circulation and hepatic oxygen consumption were investigated in seven normocapnic greyhounds. Hepatic arterial and portal venous blood flows were measured continuously using electromagnetic flow probes, and mean arterial pressure and cardiac output monitored. The administration of nitrous oxide caused linear decreases in hepatic arterial, portal venous and total liver blood flows. Increases in mean arterial pressure, hepatic arterial resistance and mesenteric vascular resistance were noted. There were no significant changes in hepatic oxygen consumption. It is suggested that the decrease in liver blood flow may result from alpha-adrenoceptor stimulation.

Haemodynamic changes with the administration of nitrous oxide during coronary artery surgery

Haemodynamic responses to 50% nitrous oxide in oxygen during coronary artery surgery were investigated in 10 patients. Morphine, diazepam and pancuronium were given intravenously as both induction and maintenance anaesthesia. A significant decrease was seen in heart rate, arterial pressure, pulmonary artery pressure, heart rate-systolic arterial pressure product, peripheral vascular resistance, left ventricular end-diastolic pressure, dP/dT of the left ventricle and resistance as measured in the graft to the left anterior descending coronary artery (LAD). No variations occurred in cardiac output, stroke volume, flow in the LAD-graft, pulmonary artery pressure or heart rate-systolic pulmonary artery pressure product or in dP/dT of the right ventricle. Pulmonary vascular resistance increased. It was concluded that left ventricular performance did not decrease and that no deterioration of the oxygenation to the myocardium seemed to occur.

Prehospital administration of nitrous oxide for control of pain

The effects of 50% nitrous oxide and 50% oxygen and of 30% nitrous oxide and 70% oxygen were evaluated in 88 patients with significant pain while in the prehospital setting. Under the telemetry physicians' supervision, EMT-Ps directed patients in the technique of self-administration of the analgesic gas. Fifty percent (36/72) reported complete or almost complete amelioration of pain; all but 15% (11/72) reported some degree of relief. No patients experienced clinically deleterious side effects, although 48% (41/85) reported side effects, of which 90% were mild. The analgesic properties, lack of complications, quick onset and short duration of action, and acceptance by patients, physicians, and paramedics make N2O a nearly ideal analgesic agent for advanced life support use in the prehospital relief of significant pain.

Effects of nitrous oxide on systolic time intervals

The effects of inhalation of oxygen, nitrous oxide/oxygen and nitrous oxide/nitrogen/oxygen on systolic time intervals (PEP (pre-ejection period) and LVET (left ventricular ejection time) were investigated in eight healthy persons. Nitrous oxide 40%, administered with oxygen or oxygen/nitrogen, prolonged PEP significantly by 25% and 22%, respectively. Inhalation of oxygen also prolonged PEP but to a significantly minor degree. LVET, heart rate and MAP were unchanged during the experiments. Derivatives from the systolic time intervals, i.e. PEP/LVET, I/PEP2 and ejection fraction changed significantly in the nitrous oxide groups. It is concluded that nitrous oxide depresses cardiac performance, to some degree, even when administered at a rather low concentration.

Physostigmine in open-heart surgery. Reversal of hyoscine supplementation of morphine-oxygen-air-relaxant anaesthesia

The use of physostigmine salicylate to reverse hyoscine supplementation of morphine-oxygen-air relaxant anaesthesia has been studied in twenty open-heart surgery patients and found to be both reliable and innocuous.

Administration of nitrous oxide in normal subjects. Evaluation of systems of gas delivery for their clinical use and hemodynamic effects

Nitrous oxide (concentrations of 30 to 50%) was administered to 22 healthy volunteer subjects via nasal prongs, rebreathing mask, and an airlines mask to assess the efficiency of systems of delivery and the hemodynamic effects. The end-expired concentration of nitrous oxide, expressed as a percentage of the inspired concentration of nitrous oxide, was 19% for nasal prongs, 34% for the rebreathing mask, and 95% for the airlines mask (most accurate delivery). The pulse rate fell from a mean of 79+/-3 beats per minute to 67+/-2 with nasal prongs, to 64+/-2 with the rebreathing mask, and to 64+/-2 with the airlines mask. Both systolic and diastolic pressures fell from means of 122+/-4 and 74+/-2 mm Hg, respectively, to 98+/-3 mm Hg and 64+/-2 mm Hg, respectively, with the airlines apparatus. The fall in systolic blood pressure (slope, -0.79) exceeded that in the diastolic (slope, -0.35). In additional eight normal subjects, administration of 30% nitrous oxide via airlines mask produced identical changes in blood pressure and pulse rate, but there was no effect from 30 minutes of administration of 30% nitrous oxide on the end-systolic volume index, cardiac index, ejection fraction, normalized wall velocity determined echocardiographically, ejection time, or the ratio of preejection period to ejection time.

The haemodynamic effects of nitrous oxide anaesthesia on systemic and pulmonary circulation in dogs

The haemodynamic effects of nitrous oxide in normoxia (20% oxygen) and in hyperoxia (50% oxygen) were investigated in 13 dogs. Nitrous oxide in hyperoxia caused a significant rise in total peripheral resistance and a significant decrease in cardiac output, heart rate, myocardial contractility (dP/dt max) and cardiac work. On the other hand, nitrous oxide in normoxia seemed to reverse these findings and did not exert negative inotropic effects on the myocardium. The results indicate that the earlier reported sympathetic activation of the circulation may be related to hyperoxia and not to nitrous oxide as such.

The haemodynamic effects of unsupplemented nitrous oxide-oxygen-relaxant anaesthesia in cardiac patients

The effect on the systemic and pulmonary circulation of unsupplemented nitrous oxide in about 30% oxygen, with and without IPPV, was investigated in five cardiac patients and compared with control periods at rest with air breathing. The results of this investigation show that nitrous oxide in conventional oxygen concentration does not cause any great haemodynamic changes. During tracheal intubation, however, significant increases in both systemic and pulmonary blood pressures were registered. However, these pressures reverted to the control values immediately after the tracheal intubation. The results of this study support many earlier clinicaly good experiences obtained when using this conventional nitrous oxide-oxygen mixture in cardiac patients.