The cardiovascular and sympathomimetic responses to the addition of nitrous oxide to halothane in man

[Inhalation anesthesia in the cat: Development of cuff pressure when using nitrous oxide]

OBJECTIVE

During inhalation anesthesia with nitrous oxide in oxygen the pressure in the cuff of the endotracheal tube may increase due to diffusion of nitrous oxide into the cuff. The aim of the study was to investigate the development of cuff pressure during nitrous oxide anesthesia under clinical conditions in feline patients and to identify possible influencing factors such as tube size and gas flow rate.

MATERIAL AND METHODS

The prospective study included cats scheduled for inhalation anesthesia with nitrous oxide for a minimum duration of 60 minutes at the Department for Small Animals of the University of Leipzig. Cuff pressure was adjusted with a cuff manometer and its development was recorded.

RESULTS

In total, the cuff pressure values of 24 cats were recorded. Animals were allocated into groups by tube size (ID 4.0 mm and ID 4.5 mm) and by fresh gas flow rate: low flow rate (0.6 l/min) and high flow rate (3 l/min). During anesthesia, cuff pressure increased over time, with statistical significance occurring from 45 minutes onwards in comparison to the initial cuff pressure (p=0.005). After 60 minutes, there was a mean cuff pressure increase of 3 cmH2O. Despite this moderate mean increase, highly variable pressure values up to 48 cmH2O in individual animals were recorded. No cat reached the termination criterion of 60 cmH2O cuff pressure. Effects of tube size (p=0.63) and flow rate (p=0.334) on the cuff pressure were not evident.

CONCLUSION

After a period of 45 minutes of nitrous oxide administration, a significant increase in cuff pressure occurs in the cat. However, tube size and total gas flow rate do not seem to influence the cuff pressure development.

CLINICAL RELEVANCE

When using nitrous oxide during inhalation anesthesia, regular cuff pressure evaluation and correction are necessary and hence recommended in feline patients. As individual pressure changes may be highly variable, no fixed recommendations for optimal management are possible.

Past, present, and future of nitrous oxide

INTRODUCTION

For a drug that has been omnipresent for nearly 200 years, nitrous oxide's (N2O) future seems less certain than its illustrious past. Environmental concerns are coming to the fore and may yet outweigh important clinical benefits.

SOURCES OF DATA

After determining the scope of the review, the authors used PubMed with select phrases encompassing the words in the scope. Both preclinical and clinical reports were considered.

AREAS OF AGREEMENT

The analgesic and anaesthetic advantages of N2O remain despite a plethora of newer agents.

AREAS OF CONTROVERSY

N2O greenhouse gas effect and its inhibition of key enzymes involved in protein and DNA synthesis have provided further fuel for those intent on eliminating its further clinical use.

GROWING POINTS

The use of N2O for treatment-resistant depression has gained traction.

AREAS TIMELY FOR DEVELOPING RESEARCH

Comparative studies for N2O role in combatting the prescription opioid analgesic epidemic may well provide further clinical impetus.

Applications of nitrous oxide for procedural sedation in the pediatric population

OBJECTIVE

The objective of this review was to provide a general descriptive account of the physical properties, end-organ effects, therapeutic applications, and delivery techniques of nitrous oxide (N2O) as used in the arena of procedural sedation.

DATA SOURCE

A computerized bibliographic search regarding the applications of nitrous for provision of sedation and analgesia during procedures with an emphasis on the pediatric population was performed.

RESULTS

The end-organ effects of N2O have been well described in the operating room setting. Aside from its effects on the central nervous system of sedation and analgesia, N2O may alter intracerebral dynamics and alter cerebral blood flow and intracranial pressure especially in patients with altered intracranial compliance. Effects on ventilation include a dose-related depression of ventilatory function and control of upper airway patency. These effects are generally limited in the absence of comorbid diseases and potentiated by other sedative and analgesic agents. The more clinically significant respiratory effect of N2O on ventilatory function is a dose-dependent depression of the ventilatory response to hypoxemia. Hemodynamic effects include a mild direct depressant effect on myocardial function, which in the absence of comorbid cardiac disease is generally compensated by stimulation of the sympathetic nervous system. Nitrous oxide may potentially aggravate pulmonary hypertension. Additional physiologic effects on neurologic and hematologic function may result in inactivation of the enzyme, methionine synthetase. Recent concern has also been raised regarding the potential effects of N2O on immune function and its relationship to perioperative surgical site infections. Given differences in the solubility, N2O will diffuse into and significantly expand gas-filled cavities. Chronic exposure of health care works to N2O is also a concern. Although there are limited data in the literature to clearly substantiate concerns regarding the reproductive toxicity of occupational exposure to N2O, appropriate scavenging and use of other techniques are mandatory. Nitrous oxide has been shown to be effective for a variety of minor surgical procedures such as venipuncture, intravenous cannula placement, lumbar puncture, bone marrow aspiration, laceration repair, dental care, and minor dermatologic procedures. It is generally as effective as midazolam, with several studies demonstrating it to be more effective. However, its utility is not as great in severely painful procedures such as fracture reduction. Demonstrated advantages to parenteral sedation include a more rapid onset and a shorter recovery time with the majority of patients preferring it to over other agents or agreeing to its use for subsequent procedures. The literature also suggests increased success rates with simple procedures such as intravenous cannula placement when compared with placebo. In general, life-threatening adverse events have not been reported. Most common adverse effects include dysphoria and vomiting. For more painful procedures, combination with another agent may be used, and in all cases, topical or infiltrative local anesthesia is recommended.

CONCLUSIONS

In general, N2O is a useful adjunct for procedural sedation. Given the variety of procedures performed in the pediatric patient, ongoing research is required to identify the most appropriate and effective use of this agent. This may be particularly relevant when evaluating its use for procedures associated with significant pain. In these scenarios, the combination of N2O with other agents needs to be evaluated. Given the potential for adverse effects, strict adherence to published guidelines regarding procedural sedation and monitoring is suggested.

Hemodynamic effects of nitrous oxide in isoflurane-anesthetized cats

OBJECTIVE

To determine the hemodynamic effects of nitrous oxide in isoflurane-anesthetized cats.

ANIMALS

12 healthy adult domestic shorthair cats.

PROCEDURE

Cats were anesthetized by administration of isoflurane in oxygen. After instruments were inserted, end-tidal isoflurane concentration was set at 1.25 times the individual minimum alveolar concentration, and nitrous oxide was administered in a Latin-square design at 0, 30, 50, and 70%. Each concentration was administered for 25 minutes before measurements were obtained to allow for stabilization. Heart rate; systemic and pulmonary arterial pressures; central venous pressure; pulmonary artery occlusion pressure; cardiac output; body temperature; arterial and mixed-venous pH, PCO2, PO2, and hemoglobin concentrations; PCV; and total protein and lactate concentrations were measured before and during noxious stimulation for each nitrous oxide concentration. Arterial and mixed-venous bicarbonate concentrations and oxygen saturation, cardiac index, stroke index, rate-pressure product, systemic and pulmonary vascular resistance indices, left and right ventricular stroke work indices, arterial and mixed-venous oxygen contents, oxygen delivery, oxygen consumption, oxygen extraction ratio, alveolar-to-arterial oxygen difference, and venous admixture were calculated.

RESULTS

Arterial pressure, central venous pressure, pulmonary arterial pressure, rate-pressure product, systemic and pulmonary vascular resistance indices, arterial PCO2, and PCV increased during administration of 70% nitrous oxide. Arterial and mixed-venous pH, mixed-venous PO2, and alveolar-to-arterial oxygen difference decreased during administration of 70% nitrous oxide. Results before and during noxious stimulation were similar.

CONCLUSIONS AND CLINICAL RELEVANCE

Administration of 70% nitrous oxide to isoflurane-anesthetized cats resulted in improved arterial pressure, which was related to a vasoconstrictive effect.

Cognitive properties of sedation agents: comparison of the effects of nitrous oxide and midazolam on memory and mood

OBJECTIVES

To compare the effects of nitrous oxide and midazolam on cognition and mood.

DESIGN

A three-way, counterbalanced, cross-over study, using patients receiving conscious sedation for routine dental treatment.

METHODS

On each of three separate visits, patients performed a computerised test battery to determine baseline cognitive performance. Then, following administration of either midazolam, nitrous oxide, or no drug, patients re-performed the test battery. Finally, patients completed visual analogue scales assessing their subjective mood state.

RESULTS

Relative to baseline performance, midazolam administration produced significantly slower reaction times compared with nitrous oxide and no-drug conditions. Furthermore, patients receiving midazolam were impaired in accuracy relative to the other conditions on many of the cognitive tasks, particularly those assessing the recall of information. Patient performance in nitrous oxide and control conditions did not significantly differ. These results could not be explained by differences in mood between the conditions, as subjective mood ratings during midazolam or nitrous oxide administration were very similar.

CONCLUSIONS

It is important for clinicians to be aware that peri-operative recall of information is reduced in patients who have undergone midazolam sedation. This is an advantage for patients who are anxious, and do not wish to be aware of the operative treatment being performed. However, as the cognitive impairment is enduring, an adult escort and written post-operative instructions should be mandatory for midazolam sedation patients. In contrast, the use of nitrous oxide sedation does not significantly impair higher cognitive tasks and thus patients receiving nitrous oxide sedation can resume normal activities in the post-operative period.

Cardiopulmonary compromise after use of topical and submucosal α-agonists: Possible added complication by the use of β-blocker therapy

We report the specifics of 12 cases of severe hypertension after the intraoperative use of topical phenylephrine, submucosal epinephrine, or both. Ten of these 12 patients also developed severe pulmonary edema. Seven of the twelve were treated with β blockers; 3 of whom suffered cardiac arrest. We propose a common mechanism: the vasoconstrictors caused systemic hypertension, increased left ventricular afterload, decreased left ventricular compliance, and decreased cardiac output. In those patients treated with β blockers, decreased contractility and inability to increase heart rate further compromised cardiopulmonary function. (Otolaryngol Head Neck Surg 1997;117:56–61.)

Awareness in anaesthesia: incidence, consequences and prevention

Awareness during anaesthesia is a state of consciousness that is revealed by explicit or implicit memory of intraoperative events. Although large clinical surveys indicate an incidence of explicit awareness of < 0.3% during anaesthesia for general surgery, this adverse effect should be a great concern, because patients may be permanently disabled by the experience of being awake during surgery. Prevention of awareness during anaesthesia starts with an appropriate preoperative visit to the patient. The anaesthetic delivery machines must be properly checked before and during anaesthesia. The anaesthetic depth should be assessed by observation of movement responses, and consequently a minimum of muscle relaxants used. Because the anaesthetic depth can be controlled by determination of endtidal drug concentration, volatile inhaled anaesthesia may be associated with a lower frequency of awareness than other anaesthetic regimens.

Peripheral blood flow in the elderly during inhalational anaesthesia

We investigated whether aging altered the peripheral vascular effects of inhaled anaesthetic agents. Forearm blood flow (FBF) was measured in 20 young (18-34 yrs) and 21 healthy elderly (60-79) patients receiving isoflurane or halothane with 66% nitrous oxide (N2O) in oxygen (O2). After etomidate 0.3 mg/kg and vecuronium 0.1 mg/kg, the trachea was intubated and controlled ventilation instituted with 66% N2O in O2. Halothane or isoflurane were administered to achieve end-tidal concentrations of 0.5% halothane or 0.9% isoflurane after 20 min. FBF was measured by venous occlusion plethysmography during the 20 min study period. Induction of anaesthesia with etomidate decreased FBF below baseline (awake) values in both elderly and young; intubation returned FBF to baseline values in the young but not in the elderly. FBF decreased below baseline values in young and elderly patients receiving halothane and in elderly patients receiving isoflurane but not in young patients receiving isoflurane. FBF was significantly greater in young patients receiving isoflurane than halothane after 20 min administration. We conclude that perfusion of forearm muscle and skin is maintained in the young but not in the elderly during anaesthesia with isoflurane/N2O. Perfusion of forearm muscle and skin decreases in both young and elderly patients during anaesthesia with halothane/N2O. The cardiovascular effects of isoflurane/N2O and halothane/N2O did not differ significantly in healthy elderly patients.

Cardiac electrophysiology and conduction pathway ablation

Invasive cardiac electrophysiological (EP) testing and transcatheter ablation are new methods available for the diagnosis and treatment of complex dysrhythmias. The purpose of this review is to familiarize anaesthetists with these procedures. The information presented combines a literature review with the authors' experience. This article reviews normal cardiac conduction, tachycardia pathogenesis, principles of cardiac EP study and techniques of conduction pathway ablation. The anaesthetic considerations, including the choice of anaesthetic agent, monitoring problems, drug interactions, special methods of dysrhythmia termination in the EP lab, and complications specific to these procedures, are detailed. Balanced general anaesthesia or monitored anaesthesia care (MAC) sedation with benzodiazepines, propofol and narcotics are acceptable. Several conclusions can be drawn: transcatheter ablation is an effective treatment for many reentry tachycardias; anaesthetic assistance for this procedure will increasingly be needed; anaesthesia can easily be provided without influencing accurate EP testing; overdrive pacing is the method of choice for terminating tachydysrhythmias in the EP lab.

Nitrous oxide does not worsen myocardial ischaemia following beta-receptor blockade in isoflurane anaesthetized dogs

The effect of nitrous oxide (N2O) on ischaemic myocardium was investigated in the presence of beta-receptor blockade. Three anaesthetics were compared in each of six dogs: isoflurane 1.8% alone, isoflurane 1.4% with 50% N2O, and isoflurane 1.8% with 50% N2O. Heart rate (HR), systolic aortic blood pressure (SBP), and left atrial pressure (LAP) were held constant during the three treatments. The left anterior descending coronary artery (LAD) was cannulated and perfused with an autoperfusion circuit. Systolic segment length was measured with a sonomicrometer in the LAD and circumflex regions. Regional myocardial blood flow was measured using radioactive microspheres. Propranolol was administered intravenously and then measurements were made during imposition of a stenosis on the perfusion circuit sufficient to decrease systolic shortening by 30%. The substitution of 50% N2O for 0.4% isoflurane had no effect on systolic shortening or transmural myocardial blood flow in the ischaemic or normal region. When N2O was added to 1.8% isoflurane, systolic shortening decreased by 34.6% in the ischaemic and 57.3% in the normally perfused region, while transmural myocardial blood flow distribution did not change significantly. The decrease in shortening was therefore not due to increased ischaemia. These results were similar to those of a previous experiment which was identical except that beta-blockade was absent. It is concluded that beta-receptor blockade does not markedly alter the response of normal or ischaemic myocardium to N2O.

Hemodynamic responses to nitrous oxide during inhalation anesthesia in pediatric patients

STUDY OBJECTIVE

To measure the hemodynamic changes produced by nitrous oxide (N2O) during halothane and isoflurane anesthesia in infants and children.

DESIGN

A repeated measures design in two groups of infants and small children.

SETTING

Operating rooms at a university hospital.

PATIENTS

Nineteen healthy unmedicated infants and small children (mean age 12 months) who required elective surgery.

INTERVENTIONS

Prior to anesthesia induction, cardiovascular measurements were recorded using pulsed Doppler and two-dimensional echocardiography. Following anesthesia induction with halothane (n = 10) or isoflurane (n = 9) in oxygen (O2) and air, anesthetic measures were stabilized at 1.0 minimum alveolar concentration (MAC) and cardiovascular measures were repeated. After 30% N2O was added to the 1.0 MAC anesthetic concentration, a third set of cardiovascular measurements was recorded. A final cardiovascular data set was measured 5 minutes following an increase in N2O concentration to 60%.

MEASUREMENTS AND MAIN RESULTS

Mean arterial pressure (MAP), cardiac index (CI), stroke volume (SV), and ejection fraction (EF) decreased similarly and significantly at 1.0 MAC halothane and isoflurane. Heart rate (HR) increased during isoflurane anesthesia but decreased during halothane anesthesia. The addition of N2O resulted in a decrease in HR, CI, and MAP when compared to 1.0 MAC levels of halothane or isoflurane; however, SV and EF were not significantly changed from levels measured during 1.0 MAC halothane or isoflurane.

CONCLUSIONS

The addition of N2O to halothane and isoflurane anesthesia in infants and children decreased HR. This decrease led to a decrease in cardiac output (CO). Unlike with adults, N2O did not produce cardiovascular signs of sympathetic stimulation in infants and children.

Attenuated sympathetic tone augments nitrous oxide-induced myocardial depression during high-dose fentanyl anaesthesia in dogs

The purpose of this study was to determine whether centrally mediated sympathetic tone exerts compensatory effects on nitrous oxide-induced myocardial depression during high-dose fentanyl anaesthesia. The cardiovascular responses to 60 minutes of nitrous oxide administration after fentanyl, 100 micrograms.kg-1, were examined in 18 dogs with or without autonomic blockade induced by hexamethonium. Decreases of cardiac output and left ventricular stroke work were significantly greater in dogs with autonomic blockade (n = 9) by about 20 per cent of baseline values than in dogs without autonomic blockade (n = 9) at 30, 45, and 60 min. Similarly, left ventricular dP/dt was reduced more in dogs with autonomic blockade than in dogs without autonomic blockade, but this difference was only significant at the 60-min period. It is concluded that the sympathetic nervous system provides partial compensation for myocardial depression induced by nitrous oxide in the presence of high-dose fentanyl, suggesting that large doses of fentanyl do not ablate the centrally mediated sympathomimetic effects of nitrous oxide.

[Changes in renal function induced by anesthesia]

The rate of urine formation and its composition are influenced by the different drugs used during surgery. Anaesthetics act on renal function, not only directly, but also by producing changes in cardiovascular function and in neuroendocrine activity. Many factors may be incriminated: lowered blood pressure and cardiac output, increased sympathetic outflow (renal nerve stimulation and increased plasma catecholamines), increased release of renin, angiotensin and vasopressin. The effects of anaesthetics on the kidney go beyond a simple change in basal haemodynamics and include, for some drugs, an alteration in the ability for the kidney to autoregulate its blood flow and glomerular filtration rate. Studies on toad bladders showed a decrease in transport of water, sodium and organic anions. But, in fact, renal effects of anaesthetics in man and animals depend on the species, the anaesthetic and the method used to study the effect. Most barbiturates and inhalational anaesthetics tend to decrease renal blood flow (RBF) and glomerular filtration rate (GFR). These trends are gradually reversed during recovery. The effects of ketamine and diazepam are not clearly defined. Morphine and fentanyl decrease urine flow and GFR, whilst RBF increases or decreases, depending on whether a direct or indirect measurement technique was used. Muscle relaxants have little effect on renal function. Spinal and epidural anaesthesia only slightly decrease GFR and RBF in proportion to the decrease in mean arterial pressure. Obviously, the preexisting intravascular volume and the quantity of intravenous fluids given strongly influence the renal response to spinal and epidural anaesthesia. Some studies have shown that urine flow rate, creatinine clearance, urinary sodium excretion and RBF are reduced during mechanical ventilation with positive end-expiratory pressure. Surgery itself influences renal function by inducing alterations in prerenal haemodynamics. Operative stress leads to an increase in circulating catecholamines and angiotensin. Significant fluid shifts, excessive blood loss and redistribution of a third space may lead to a prerenal oliguric state, increasing secretion of vasopressin. Acute renal failure (ARF) is a frequently lethal complication of critical surgical illness, due to a variety of factors which interfere with glomerular filtration and tubular reabsorption, such as renal hypoperfusion or nephrotoxic insults. In fact, the initiating aggression ultimately culminates in the development of one or more of the maintenance factors (decreased tubular function, tubular obstruction, decreased GFR and RBF) that reduce urine flow and osmolar excretion. Good management during the perioperative period tends to minimize the risk of developing ARF.

The mechanism of nitrous oxide-induced changes in pulmonary vascular resistance in a dog model of left atrial outflow obstruction

Nitrous oxide has been reported to increase pulmonary vascular resistance (PVR) in patients with pulmonary hypertension secondary to mitral stenosis. Additional data suggest this response involves sympathetic stimulation because the increase in PVR can be prevented by alpha-adrenergic and ganglionic blockade. Whether or not active pulmonary vasoconstriction occurs remains unclear. This study was designed to more fully characterize the influence of N2O on pulmonary hemodynamics during left atrial outflow obstruction (LAO). Responses in an in situ blood-perfused lung lobe were compared with those in the remaining intact lung of six dogs anesthetized with pentobarbital, 30 mg/kg, and morphine, 1.5 mg/kg, and prepared for measurement of peak left ventricular (LV) pressure, LV end-diastolic pressure (LVEDP), LV dP/dt, systemic arterial and pulmonary arterial (PA) pressures, and cardiac output (CO). The pulmonary artery branch supplying the left middle lung lobe was cannulated and perfused at a constant rate with warmed blood. LAO was produced by filling the balloon of a Foley catheter positioned in the left atrium (LA) with enough saline to increase PA pressure by 50%; the balloon was subsequently left filled for the entire protocol. Measurements were first obtained during ventilation with 67% N2, and 33% O2. The inspired gas was then changed to 67% N2O and 33% O2 for 10 minutes, and then returned to the N2/O2 mixture. Once baselines had been reestablished (about 10 minutes), phentolamine, 0.75 mg/kg, was administered, and the response to 10 minutes of N2O again observed. N2O did not change vascular resistance in the isolated lobe, but increased intact-lung PVR.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sevoflurane with or without nitrous oxide on cardiac contractility and sinoatrial node rate

Cardiac effects of sevoflurane (SE) with or without nitrous oxide were examined in the canine blood-perfused papillary muscle and sinoatrial node preparations. Although SE depressed developed tension (DT), mean arterial pressure (MAP) and heart rate of the donor dog (DHR) dose dependently, sinoatrial rate (SAR) was not changed significantly. No significant changes in MAP, DHR and SAR were observed with the addition of nitrous oxide to SE. However, the addition of nitrous oxide to SE resulted in significant decrease of DT. These results suggest that SE depresses BP and cardiac contractility dose dependently, but dose not change heart rate. The combination of nitrous oxide and SE may produce less cardiovascular depressant effect at a given MAC level than SE given alone.

Fifty percent nitrous oxide depresses recovery from anoxic heart failure induced by 100% nitrous oxide

In experiments on an isolated rate heart lung preparation, the effects of 100% oxygen, 50% nitrous oxide or air on myocardial metabolism during recovery from anoxic heart failure were evaluated with intramyocardial high energy phosphates, lactate and glycogen. A hundred percent nitrous oxide was administered until the cardiac output decreased from 30 to 20 ml/min, and then 50% nitrous oxide, air or 100% oxygen was administered. Fifty percent nitrous oxide reduced the cardiac output and caused heart failure again. The ATP level and energy charge in hearts with 50% nitrous oxide were significantly lower than those in the others. These data indicate that 50% nitrous oxide during recovery from anoxic heart failure had deleterious effects on myocardial function and metabolism.

Opiate anaesthesia

Current use of opioids in anaesthesia is reviewed with particular emphasis on the use of opioids in anaesthetic doses, techniques that recently have become popular in cardiovascular anaesthesia. A major benefit of opioid anaesthesia (particularly fentanyl) is the cardiovascular stability which obtains during induction and throughout operation, even in patients with severely impaired cardiac function. Anaesthetic doses of morphine are associated with a higher incidence of cardiovascular disturbances and other problems. Pethidine is unsuitable for cardiovascular surgery because of severe haemodynamic disturbances when high doses are given. Sufentanil and alfentanil may prove more suitable alternatives. High doses of opioids can reduce or prevent hormonal and metabolic responses to the stress of surgery. Even very large doses of fentanyl or its new analogues do not prevent marked increases in plasma catecholamine concentrations in response to cardiopulmonary bypass. The reduction in hormonal and metabolic stress response does not appear to continue postoperatively.

Adverse effects of nitrous oxide

Although once considered completely devoid of complications, it is now recognised that the misuse or inappropriate use of nitrous oxide (N2O) often results in adverse side effects. Hypoxia, particularly the entity 'diffusion hypoxia', can occur with the administration of inadequate amounts of oxygen during or immediately after a N2O anaesthetic. N2O will diffuse into air-containing cavities within the body faster than nitrogen diffuses out. This results in a temporary increase in either the pressure and/or volume of the cavity depending upon the distensibility of its walls. The magnitude of the effect is proportional to the blood supply of the cavity, the concentration of N2O inhaled and the length of time the patient is exposed to N2O. Significant morbidity or even death can result from this phenomenon. A property unique to N2O is its ability to oxidise and inactivate the vitamin B12 components of certain enzymes in both animals and man. One such enzyme, methionine synthetase is essential for normal DNA production. Animal and human studies have demonstrated that the haematological, immune, neurological and reproductive systems are each affected. These adverse effects of N2O can occur after both acute (surgical) or long term (occupational) exposure to the gas. Because of its effects on the pressure and volume characteristics of air-containing spaces, N2O should not be used for patients with bowel obstruction, pneumothorax, middle ear and sinus disease, and following cerebral air-contrast studies. Many anaesthesiologists feel that use of N2O should be restricted during the first two trimesters of pregnancy because of its effects on DNA production and the experimental and epidemiological evidence that N2O causes undesirable reproductive outcomes. Since N2O affects white blood cell production and function, it has been recommended that N2O not be administered to immunosuppressed patients or to patients requiring multiple general anaesthetics. Many anaesthesiologists believe that the potential dangers of N2O are so great that it should no longer be used at all for routine clinical anaesthesia. However, the continued use of N2O remains a controversial topic since, at present, a suitable substitute gas is not available.

Halothane anesthesia in infancy: identification and correlation of preoperative risk factors with intraoperative arterial hypotension and postoperative recovery

Infants are well known to be sensitive to the myocardial depressant effects of halothane. In this study of more than 300 infants, 0 to 24 weeks of age, who received halothane anesthesia, preoperative risk factors were correlated with intraoperative outcome and postoperative recovery. Possible risk factors considered were age, history of respiratory distress syndrome (RDS), preoperative fasting time, and anemia. Intraoperative morbidity was evaluated by occurrence of significant hypotension, slow heart rates, or serious bradycardias. Parameters of postoperative recovery included postanesthetic recovery room admission score, duration of recovery room stay, and time to first feeding. Arterial hypotension occurred with an unexpectedly high frequency of 49.2%. Infants younger than 8 weeks of age (particularly newborns), infants 9-16 weeks of age (fasting eight or more hours), and RDS survivors 0-16 weeks of age were found to be most susceptible to hypotension. Only age and history of RDS, however, proved to be statistically significant risk factors in predicting the probability of intraoperative circulatory depression. Though circulatory depression during halothane inhalation was common, it was not associated with significant intraoperative or postoperative morbidity in terms of cardiorespiratory arrests, serious dysrhythmias, prolonged postanesthetic recovery, or delayed feeding times.

Halothane-relaxant anaesthesia in elderly patients

Twenty-three elderly patients, scheduled for elective cholecystectomy, were studied during halothane-relaxant anaesthesia. Anaesthesia was induced with thiopentone and maintained with halothane in 12 patients, six of whom had also received premedication. Eleven patients were anaesthetized with halothane, without thiopentone induction and with no premedication. Measurements of central haemodynamics were performed awake and during anaesthesia at end-tidal halothane concentrations of 0.5 and 1.0%; at the lower concentration, measurements were also made after addition of nitrous oxide. Premedication and thiopentone had no influence on the subsequent halothane anaesthesia. Halothane caused reductions of cardiac index, mean arterial blood pressure and oxygen uptake. However, neither right atrial nor pulmonary capillary venous pressure increased and the arterio-venous oxygen content difference decreased. These findings differ from those made by others in younger subjects and are probably attributable to a dose-dependent reduction in systemic vascular resistance. The addition of nitrous oxide had only minor effects on central circulation. The results suggest that the age of the patients influences their reaction to halothane anaesthesia.

Haemodynamic response to nitrous oxide during high-dose fentanyl pancuronium anaesthesia

Ten patients subjected to coronary by-pass surgery were studied to determine the haemodynamic effects of replacing Fio2 1.0 normoventilation with nitrous oxide in oxygen (Fio2 0.3) after induction of anaesthesia with fentanyl (50 micrograms/kg), flunitrazepam and pancuronium. In all patients the application of N2O decreased systemic arterial pressures by an average of 10% (P less than 0.001), but left pulmonary arterial pressures and systemic vascular resistance unchanged. The slight bradycardia induced was associated with moderate depression of the cardiac index and the left ventricular stroke work index (P less than 0.001) at the time when the rate-pressure product was decreased by 20% (P less than 0.001). The cardiac depression produced by N2O was most prominent in patients with left ventricular wall hypokinesia and an ejection fraction below 55%, in whom the cardiac work index was diminished by 29%. The replacement of oxygen with nitrous oxide in oxygen during high-dose fentanyl-pancuronium anaesthesia seems not to be associated with sympathetic stimulation, and the myocardial depressant effect of N2O should be weighed against the possible reduction in myocardial oxygen consumption with special care in patients with compromised myocardial function.

Nitrous oxide augments the systemic and coronary haemodynamic effects of isoflurane in patients with ischaemic heart disease

The effects of 70% nitrous oxide, added to 1% end-tidal isoflurane and administered by intermittent positive pressure ventilation (IPPV), on coronary haemodynamics and myocardial oxygenation were investigated in 10 patients with ischaemic heart disease. Standard methods were used for determination of their central haemodynamic effects. Coronary blood flow was measured by the retrograde thermodilution technique and coronary sinus blood sampled for measurement of myocardial oxygen consumption and lactate extraction. One per cent end-tidal isoflurane decreased systemic blood pressure (-39%) by a combination of systemic vasodilation and reduction in cardiac performance. Coronary blood flow remained unaltered despite the fall in coronary perfusion pressure and myocardial oxygen consumption (-30%) and extraction (-30%) fell significantly. Ischaemic ECG changes parallelled by decreased myocardial lactate extraction or lactate production were recorded in 6 of the 10 patients during steady state isoflurane anaesthesia. When nitrous oxide was added to isoflurane there was a fall in heart rate (-13%), a further reduction in systemic blood pressure (-18%) and myocardial oxygen consumption (-31%) and extraction (-17%) whereas all other variables including coronary blood flow remained unaltered. The myocardial ischaemia was worsened in three of the six patients with ECG and metabolic signs of impaired oxygenation during isoflurane alone. It is concluded that nitrous oxide potentiates the systemic and coronary haemodynamic effects of isoflurane in patients with coronary artery disease. The mechanisms for myocardial ischaemia seem to be decreased coronary perfusion pressure and/or redistribution of coronary blood flow by direct coronary vasodilation.

Central haemodynamics and regional blood flow during halothane-nitrous oxide anaesthesia and controlled ventilation. An experimental study in the rat

The haemodynamic effects of halothane-N2O/O2 anaesthesia with controlled ventilation were studied in rats, using the microsphere method. Mean arterial blood pressure was significantly reduced but only minor effects on cardiac output (CO), heart rate, and systemic vascular resistance were seen. During anaesthesia, there were significantly increased fractions of CO delivered to brain, lungs, small intestine and liver (hepatic artery), while the fractions to spleen, stomach and carcass were decreased. Fractional distribution and regional blood flow to heart, kidneys, adrenals and preportal area remained unchanged. When anaesthesia was prolonged from 60 to 90 min, no further changes in central or regional haemodynamics were seen. Considering the minor effects on central haemodynamics and the absence of changes in central and regional haemodynamics at 60 and 90 min, this anaesthesia model should be useful in experimental research.

Attenuation of the cardiovascular intubation response with N2O, halothane or enflurane

The circulatory intubation response was studied in 75 normotensive, otolaryngological patients after a thiopentone-suxamethonium induction followed by 2 min artificial ventilation with 100% oxygen (control), 70% nitrous oxide in oxygen (N2O), halothane 2% with N2O, enflurane 3% with N2O or enflurane 5% in oxygen. The above study groups (n = 15) were chosen after preliminary experiments performed in 25 different patients with halothane 2% (n = 8) or enflurane 3% (n = 6) in oxygen, which did not prevent the increase of arterial pressure after intubation, or with halothane 3% (n = 11) which attenuated the pressor response but caused cardiac arrhythmias in 55% of patients. Enflurane 5% in oxygen attenuated the increase of systolic arterial pressure by 53%, enflurane 3% with N2O by 34% and halothane 2% with N2O by 31%. The increase in heart rate after intubation was lowest in the halothane 2% with N2O group, but there were no statistically significant differences between the groups. Cardiac arrhythmias were commonest in the enflurane 3% with N2O group (20%) and they did not occur in the halothane 2% with N2O group. Considering the total effect on arterial pressure, heart rate and rate-pressure product, we recommend the combination of halothane 2% with N2O.

Effect of combinations of topical anaesthesia, fentanyl, halothane or N2O on circulatory intubation response in normo- and hypertensive patients

Attenuation of the circulatory intubation response was studied using the following combinations: oropharyngeal topical anaesthesia (OTA) + fentanyl 2 micrograms/kg (F2), OTA + 2% halothane with 70% nitrous oxide (H2N2O) or F2 + H2N2O. Firstly, it was observed in 48 normotensive patients that the combinations of OTA + F2 or F2 + H2N2O totally prevented the intubation response; OTA + H2N2O, on the other hand, was less effective. Secondly, the effect of OTA + F2 was studied in 26 hypertensive patients and their 26 normotensive controls of the same age group. The combination prevented the circulatory intubation response also in the hypertensive patients, whose circulatory reactions did not differ from those of the normotensive patients. Nitrous oxide had no beneficial effect on the intubation response.

Comparison of haemodynamic effects of morphine and fentanyl in patients with coronary artery disease

The haemodynamic effects of morphine (2.5 mg kg-1) and fentanyl (16.7 and 25 micrograms kg-1) were compared in patients undergoing coronary artery bypass surgery. Morphine or fentanyl in combination with pancuronium, nitrous oxide and a small dose of thiopentone produced some deterioration of myocardial performance. Both analgesics failed to block haemodynamic responses to noxious stimulation including tracheal intubation and sternotomy. Heart rate increased following tracheal intubation and systemic vascular resistance increased after sternotomy. However, the larger dose of fentanyl (25 micrograms kg-1) was accompanied by the smallest haemodynamic changes. Supplementary anaesthetic agents were often required to maintain haemodynamic stability during sternotomy. It is suggested here that larger doses of fentanyl, or the addition of other intravenous or inhalation anaesthetics might be employed for patients with severe coronary artery disease and good left ventricular function. On the other hand, in patients with poor left ventricular function, the myocardial depressant properties of thiopentone and nitrous oxide may be detrimental.

Nitrous oxide added to halothane reduces coronary flow and myocardial oxygen consumption in patients with coronary disease

The haemodynamic and myocardial metabolic effects of adding 50 per cent nitrous oxide to 0.5 per cent halothane were studied in 13 patients, before the surgical incision for coronary artery vein grafts. Cardiac output and coronary sinus blood flow were determined by thermodilution, along with haemodynamic measurements. Measurements 15 minutes after addition of nitrous oxide revealed a significant decrease in heart rate, arterial pressure, cardiac index, coronary sinus blood flow and myocardial oxygen consumption. There was a significant increase in coronary sinus lactate content, and a significant decrease, from 27 to 11 per cent, in myocardial lactate extraction. We conclude that these circulatory changes were likely to be due to a depression of ventricular function by the nitrous oxide. The myocardia of these patients with severe coronary disease were becoming globally ischaemic while they were receiving 50 per cent oxygen, in the presence of hypotension. Nitrous oxide should be turned off when hypotension occurs in coronary patients.

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.

Nitrous oxide and the baroreceptor reflexes in the dog

The effects were studied of 67% nitrous oxide on the baroreceptor control of systemic haemodynamics in the dog. Nitrous oxide was added to end-tidal halothane concentrations of about 0.9% (H2) together with halothane levels approximately 25% above (H3) and below (H1) the H2 concentration. Baroreceptor function was assessed by brachiocephalic artery occlusion (BCO) or perfusion of the isolated carotid sinuses. In the intact animal, nitrous oxide significantly increased mean arterial pressure only at the H2 level (P less than 0.001). At the closed loop operating point of the carotid sinus reflex (CSR), for all halothane levels, nitrous oxide decreased cardiac output and stroke volume, increased systemic vascular resistance and central venous pressure with no effect upon mean arterial pressure. The operating point gain of the CSR was significantly depressed by nitrous oxide at the H1 and H3 levels (P less than 0.05). The CRS operating characteristics were significantly modified by N2O in a manner which depended upon the underlying concentration of halothane.

Central haemodynamics during halothane and enflurane anaesthesia in vascular surgery

Central haemodynamics were studied before and during reconstructive vascular surgery in 18 patients receiving halothane or enflurane anaesthesia (nine patients in each group). In both groups cardiac output (CO), stroke volume (SV), oxygen uptake (VO2) and arterial blood pressure fell significantly after induction of anaesthesia, whereas the arterio-venous oxygen difference (AVD) and the total peripheral resistance (TPR) remained unchanged. In spite of an average intraoperative blood loss of 600 ml, which had not been substituted for, surgery brought about an increase in CO, SV and VO2, while the AVD and TPR remained unchanged. Following transfusion of 900 ml of blood, CO and SV showed a further significant increase in both groups, whereas VO2 was unchanged and the AVD and TPR fell significantly. The pulmonary capillary venous pressure (PPCV) was stable during induction of anaesthesia and surgery but increased after blood transfusion. The changes in SV and PPCV were analyzed in relation to the concept of ventricular function curves. It is concluded that there were no fundamental differences in central haemodynamics between halothane and enflurane anaesthesia either before or during surgery. In both groups the changes in CO and SV were due mainly to a reduced demand of oxygen transport and expected changes in sympatho-adrenergic tone rather than to myocardial depression.

The effect of halothane with nitrous oxide on baroreflex control of heart rate in man

The assess the effect of halothane with 70 per cent nitrous oxide on the human baroreflex, we observed the heart rate response to pharmacological elevation of blood pressure in subjects anaesthetized to the 1 MAC and 1.25 MAC levels. We observed, in comparison to the awake control, that at 1 MAC halothane with 70 per cent nitrous oxide and oxygen, the baroreflex was depressed and this became more significant at 1.25 MAC anaesthesia. In comparison to previous similar studies on the effect of halothane-oxygen anaesthesia, there was significantly less baroreflex depression at equianaesthetic doses with halothane with 70 per cent nitrous oxide.

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.