A comparison between the effect of nitrous oxide and nitrogen on arterial PO2

Professor John F. Nunn: scientist, anaesthetist and polymath

John Francis Nunn (1925-2022) was an anaesthetist and clinical scientist who used his incomprehension of the science of anaesthesia in his early career to guide an extensive lifetime of innovative research. His interests outside of medicine led to him developing renowned expertise in such diverse areas as Egyptian hieroglyphs and the origins of the Earth's atmosphere. He was an outstanding communicator, writing four books alongside an impressive number of papers covering diverse topics from cell biology to history. His greatest contribution is in the understanding of respiratory physiology during anaesthesia which continues to underpin current routine anaesthetic practice and patient safety.

Persisting concentrating and second gas effects on oxygenation during N2O anaesthesia

Theoretical modelling predicts that the concentrating effect of nitrous oxide (N2O) uptake on alveolar oxygenation is a persisting phenomenon at typical levels of ventilation - perfusion (V/Q) inhomogeneity under anaesthesia. We sought clinical confirmation of this in 20 anaesthetised patients. Arterial oxygen pressure (P(aO2)) was measured after a minimum of 30 min of relaxant general anaesthesia with an inspired oxygen (F(I O2)) of 30%. Patients were randomly allocated to two groups. The intervention group had N2O introduced following baseline blood gas measurements, and the control group continued breathing an identical F(I O2) in nitrogen (N2). The primary outcome variable was change in P(aO2). Mean (SD) in P(aO2) was increased by 1.80 (1.80) kPa after receiving a mean of 47.5 min of N2O compared with baseline conditions breathing O2/N2 (p = 0.01). This change was significantly greater (p = 0.03) than that in the control group: + 0.09 (1.37) kPa, p = 0.83 and confirms the presence of significant persisting concentrating and second gas effects.

Noninvasive measurement of intrapulmonary shunting

OBJECTIVE

To investigate the accuracy and precision of a noninvasive approach to measurement of pulmonary shunt fraction using simultaneous application of 2 fundamental respiratory mixing equations: the direct Fick equation for oxygen and the shunt equation of Berggren. This can be performed without mixed venous blood sampling and requires measurement of oxygen uptake and pulmonary blood flow.

DESIGN

Comparison with invasive shunt fraction measured using mixed venous blood sampling and with estimated shunt fraction using an assumed arteriovenous O(2) content difference.

SETTING

Major teaching hospital.

PARTICIPANTS

Nine patients undergoing anesthesia for cardiac surgery.

INTERVENTIONS

Pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow) and by bolus thermodilution for comparison.

MEASUREMENTS AND MAIN RESULTS

The mean shunt fraction measured by the invasive method was 0.145 (range 0.057-0.263). When pulmonary blood flow was measured using an indirect Fick technique (nitrous oxide throughflow), the absolute mean bias for noninvasive shunt fraction was -0.005 with a standard deviation of 0.012. Correlation was excellent (r(2) = 0.95, p < 0.001). Agreement was less precise when pulmonary blood flow was measured using thermodilution (mean bias + 0.001 with a standard deviation of 0.018).

CONCLUSIONS

The noninvasive method is an accurate substitute for invasive shunt fraction measurement with mixed venous blood sampling.

Intraoperative changes in arterial oxygenation during volume-controlled mechanical ventilation in modestly obese patients undergoing laparotomies with general anesthesia

BACKGROUND

In obese patients, arterial oxygenation can be greatly impaired during general anesthesia. Both avoidance of denitrogenation and application of positive end-expiratory pressure (PEEP) during mechanical ventilation may be effective in preventing such impairment of arterial oxygenation.

METHODS

We studied 28 obese/overweight and seven non-obese (BMI < 25 kg x m-2) patients who underwent laparotomies with general anesthesia (i.e. isoflurane with or without nitrous oxide). During anesthesia, their lungs were mechanically ventilated at a rate of 10 breaths x min-1 with a constant flow, inspiratory-to-expiratory ratio 1 : 2, and tidal volume approximately 10 ml x kg-1. The obese/overweight patients were allocated to four different groups in terms of denitrogenation and application of PEEP (7 cm H2O) during the ventilation (n = 7 each). In the non-obese patients, their denitrogenated lungs were ventilated without application of PEEP. Arterial gas analyses were performed before induction of anesthesia, and 30, 90, 150 and 210 min after tracheal intubation. The ratio of PaO2 to FiO2 was calculated as an index of arterial oxygenation.

RESULTS

No significant changes in the PaO2/FiO2 ratio were observed throughout the study in the non-obese patients and in the obese/overweight patients whose non-denitrogenated lungs were ventilated with PEEP. In the obese/overweight patients whose lungs were ventilated after denitrogenation or without application of PEEP, significant decreases in the PaO2/FiO2 ratio were observed 30 and 90 min after tracheal intubation.

CONCLUSIONS

In obese or overweight patients under general anesthesia, it may be advisable to avoid denitrogenation and apply PEEP during mechanical ventilation in order to minimize the impairment of arterial oxygenation.

Oxygen in air (FiO2 0.4) improves gas exchange in young healthy patients during general anesthesia

PURPOSE

One hundred percent O(2) is used routinely for preoxygenation and induction of anesthesia. The higher the O(2) concentration the faster is the development of atelectasis, an important cause of impaired pulmonary gas exchange during general anesthesia (GA). We evaluated the effect of ventilation with 0.4 FiO(2) in air, 0.4 FiO(2) in N(2)O and 100% O(2) following intubation on the development of impaired gas exchange.

METHODS

Twenty-seven patients aged 18-40 yr, undergoing elective laparoscopic cholecystectomy were administered 100% O(2) for preoxygenation (three minutes) and ventilation by mask (two minutes). Following intubation these patients were randomly divided into three groups of nine each and ventilated either with 0.4 FiO(2) in air, 0.4 FiO(2) in N(2)O or 100% O(2). Arterial blood gases were obtained before preoxygenation and 30 min following intubation for PaO(2) analysis. Subsequently PaO(2)/FiO(2) ratios were calculated. Results were analyzed with Student's t test and one-way ANOVA. P value of < or = 0.05 was considered significant.

RESULTS

Ventilation of the lungs with O(2) in air (FiO(2) 0.4) significantly improved the PaO(2)/FiO(2) ratio from baseline, while 0.4 FiO(2) in N(2)O or 100% O(2) worsened the ratio (558 +/- 47 vs 472 +/- 28, 365 +/- 34 vs 472 +/- 22 and 351 +/- 23 vs 477 +/- 28 respectively; P < 0.05).

CONCLUSION

Ventilation of lungs with O(2) in air (FiO(2) 0.4) improves gas exchange in young healthy patients during GA.

Speed of collapse of the non-ventilated lung during one-lung anaesthesia: the effects of the use of nitrous oxide in sheep

By enhancing gaseous uptake from the non-ventilated lung during procedures performed thoracoscopically, the rapid diffusion properties of nitrous oxide would be expected to speed lung collapse and so facilitate surgery. To assess the effect of nitrous oxide on the speed of absorptive lung collapse, a study was conducted using 11 anaesthetised sheep. Speed of collapse was assessed in an indirect manner by recording the time required in a closed-chest situation for the airway pressure distal to a single lung airway occlusion to decrease to - 1.0 kPa. The influence of nitrous oxide was assessed by comparing the time taken for this decrease in airway pressure when the animal was being mechanically ventilated with 50% nitrous oxide in oxygen with the time taken when using 100% oxygen. In all assessments, it was found that the decrease in airway pressure to - 1.0 kPa occurred in a shorter time when nitrous oxide was used. The findings lend support to the hypothesis that during thoracoscopic surgery, mechanical lung ventilation with an oxygen/nitrous oxide mixture will increase the rate of gaseous uptake from the non-ventilated lung and so hasten its absorptive collapse.

Subdural air collection: a likely source of radicular pain after lumbar epidural

This case conference reports two cases of epidural anesthesia in which air was used to identify the epidural space during a loss-of-resistance placement technique. Both patients subsequently complained of severe pain and subdural air was demonstrated in case 1 by computed tomography and in case 2 by magnetic resonance imaging. The possible causes of the pain syndrome experienced by both patients are discussed.

Effect of ventilation-perfusion inhomogeneity and N(2)O on oxygenation: physiological modeling of gas exchange

Ventilation-perfusion (VA/Q) inhomogeneity was modeled to measure its effect on arterial oxygenation during maintenance-phase anesthesia involving an inspired mixture of 30% O(2) and either N(2)O or N(2). A multialveolar compartment computer model was constructed based on a log normal distribution of VA/Q inhomogeneity. Increasing the log SD of the distribution of blood flow from 0 to 1.75 produced a progressive fall in arterial PO(2) (Pa(O(2))). The fall was less steep in the presence of N(2)O than when N(2) was present instead. This was due mainly to the concentrating effect of N(2)O uptake on alveolar PO(2) in moderately low VA/Q compartments. The improvement in Pa(O(2)) when N(2)O was present instead of N(2) was greatest when the degree of VA/Q inhomogeneity was in the range typically seen in anesthetized patients. Models based on distributions of expired and inspired alveolar ventilation give quantitatively different results for Pa(O(2)). In the presence of VA/Q inhomogeneity, second-gas and concentrating effects may have clinically significant effects on arterial oxygenation even at "steady-state" levels of N(2)O uptake.

Ambient pressure oxygen reservoir apparatus for use during one-lung anaesthesia

An ambient pressure oxygen reservoir bag apparatus for connecting to the nonventilated lung as soon as single-lung ventilation is initiated is described. The theoretical benefits are the facilitation of collapse of the lung on the side of surgery and a reduced likelihood of arterial desaturation. Although these main benefits are yet to be proven, the authors believe that the weight of theoretical argument and practical observation serves to justify the use of the apparatus while the outcome of suitably designed clinical trials is awaited. It can be used for all one-lung anaesthetics and is especially recommended for thoracoscopic surgery, where temporary re-expansion of the nonventilated lung is either counter-productive or contraindicated, and where there is a possibility that lung collapse may be delayed.

Gas movement in the nonventilated lung at the onset of single-lung ventilation for video-assisted thoracoscopy

To assess the potential for atmospheric nitrogen to enter the nonventilated lung following the initiation of single-lung ventilation, the nonventilated lung of 10 patients undergoing video-assisted thoracoscopy was connected to the air in a water-filled spirometer, and gas movement out of and back into the lung was measured. Airway pressure from both lungs and pleural pressure from the nonventilated side were also measured. With each breath of positive-pressure ventilation to the ventilated lung prior to the thoracic cavity being opened to the atmosphere, the pressure transmitted to the opposite hemithorax generated a mean (range) tidal movement of gas in the nonventilated lung of 134 (65-265) ml. In addition, ongoing gas exchange resulted in a progressive influx of gas from the spirometer over the 110-120 s measurement period of a mean (range) volume of 155 (70-320) ml. This easily preventable influx of atmospheric nitrogen could, in theory, predispose to arterial desaturation and to delayed lung collapse after the parietal pleura is opened.

Kinetics of absorption atelectasis during anesthesia: a mathematical model

Recent computed tomography studies show that inspired gas composition affects the development of anesthesia-related atelectasis. This suggests that gas absorption plays an important role in the genesis of the atelectasis. A mathematical model was developed that combined models of gas exchange from an ideal lung compartment, peripheral gas exchange, and gas uptake from a closed collapsible cavity. It was assumed that, initially, the lung functioned as an ideal lung compartment but that, with induction of anesthesia, the airways to dependent areas of lung closed and these areas of lung behaved as a closed collapsible cavity. The main parameter of interest was the time the unventilated area of lung took to collapse; the effects of preoxygenation and of different inspired gas mixtures during anesthesia were examined. Preoxygenation increased the rate of gas uptake from the unventilated area of lung and was the most important determinant of the time to collapse. Increasing the inspired O2 fraction during anesthesia reduced the time to collapse. Which inert gas (N2 or N2O) was breathed during anesthesia had minimal effect on the time to collapse.

Effects of endotracheal intubation on airway neuropeptide content, arterial oxygenation and lung volumes in anaesthetized rats

BACKGROUND

General anaesthesia affects lung volume and pulmonary gas exchange. What role is played by mechanical stimulation by the endotracheal tube?

METHODS

We investigated the effects of intubation on arterial oxygenation and lung volume in rats.

RESULTS

Endotracheal intubation caused an increase in PA-aO2 and volume of trapped gas in the lung. This was accompanied by a reduction in neuropeptides and calcitonin gene-related peptide (CGRP) in trachea, bronchi and lung, and in vasoactive intestinal peptide (VIP) in the trachea. The increase in PA-aO2 and volume of trapped gas due to intubation was not altered in the animals given capsaicin, in which neuropeptide levels were reduced.

CONCLUSIONS

These data suggest that the decrease in CGRP and VIP content in the airway tissues may be one of the consequences, but not the cause, of impaired gas exchange by endotracheal intubation. The increase in volume of trapped gas in the lung is apparently not mediated by activation of capsaicin-sensitive sensory nerves.

Lung aeration. The effect of pre-oxygenation and hyperoxygenation during total intravenous anaesthesia

We have investigated the effect of pre-oxygenation and hyperoxygenation (an increase in inspired oxygen fraction from 0.4 to 1.0 after induction of general anaesthesia) on aeration and atelectasis formation in the lungs during total intravenous anaesthesia. Twenty-seven consecutive patients were randomly allocated to group 1 (with pre-oxygenation), group 2 (without pre-oxygenation), or group 3 (hyperoxygenation). Lung aeration was investigated by means of spiral computed tomography. The aeration of lung regions identified by computed tomography scans was divided into five categories: over-aeration, normal aeration, reduced aeration, poor aeration, and atelectasis formation. In group 1 larger areas of atelectasis were found in the basal parts of the lungs compared to group 2. In group 3 a significant increase in atelectatic areas with a corresponding reduction in areas with reduced aeration occurred at the bases of the lungs. The considerable increase in atelectasis associated with pre-oxygenation and its rapid appearance during hyperoxygenation suggest that these procedures should be used with caution.

Atelectasis and pulmonary shunting during induction of general anaesthesia--can they be avoided?

BACKGROUND

Gas exchange is regularly impaired during general anaesthesia with mechanical ventilation. A major cause of this disorder appears to be atelectasis and consequently pulmonary shunt. After re-expansion, atelectasis reappears very slowly if 30% oxygen in nitrogen is used, but much faster if 100% oxygen is used. The aim of the present study-was to evaluate if early formation of atelectasis and pulmonary shunt may be avoided if the lungs are ventilated with 30% oxygen in nitrogen instead of 100% oxygen during the induction of general anaesthesia.

METHODS

Twenty-four adult patients with healthy lungs scheduled for elective surgery were investigated. During induction of anaesthesia, the lungs were manually ventilated via a face mask, using either 30% oxygen in nitrogen (group 1, n = 12) or 100% oxygen (group 2, n = 12). Atelectasis was estimated by computed x-ray tomography and ventilation-perfusion distribution with the multiple inert gas elimination technique, both awake and during general anaesthesia with mechanical ventilation.

RESULTS

No atelectasis was present in the awake subjects. After induction of anaesthesia, the mean amount of atelectasis was minor (0.2 +/- 0.4 cm2) in group 1 and considerably greater (8.0 +/- 8.2 cm2) in group 2 (P < 0.001). The pulmonary shunt was 0.3 +/- 0.7% of cardiac output in the awake subjects. This value increased to 2.1 +/- 3.8% in group 1 and to 6.5 +/- 5.2% in group 2 (P < 0.05). The indices of VA/Q mismatch showed no difference between the two groups.

CONCLUSION

During induction of general intravenous anaesthesia in patients with healthy lungs, gas composition plays an important role for atelectasis formation and the establishment of pulmonary shunt. By using a mixture containing 30% oxygen in nitrogen, the early formation of atelectasis and pulmonary shunt may, at least in part, be avoided.

What is the role of absorption atelectasis in the genesis of perioperative pulmonary collapse?

During anaesthesia the combination of breathing at low lung volume, the administration of nitrous oxide and high inspired oxygen concentrations produces conditions that favour absorption atelectasis. Measures such as adding nitrogen to the inspired mixture and avoiding high inspired oxygen concentrations would reduce the amount of perioperative atelectasis if gas absorption was important in the genesis of perioperative pulmonary collapse. Experimental results demonstrate that these measures do not protect against atelectasis. This indicates that absorption atelectasis does not play a significant role in the genesis of perioperative pulmonary collapse. Compression atelectasis may be the underlying mechanism.

Prevention of atelectasis during general anaesthesia

Atelectasis is an important cause of impaired gas exchange during general anaesthesia; it causes pulmonary shunting. We studied the effects of gas composition on the formation of atelectasis and on gas exchange during the induction of general anaesthesia. In 12 adult patients, the lungs were ventilated with 30% oxygen in nitrogen during anaesthesia induction, and in another 12, a conventional technique was used (100% oxygen during induction and 40% oxygen in nitrogen thereafter). Extent of atelectasis was estimated by computed tomography and the ventilation-perfusion relation (VA/Q) by the multiple inert gas elimination technique. After anaesthesia induction, there was little atelectasis in the 30% oxygen group (mean 0.2 [SD 0.4] cm2) and a significantly greater amount (4.2 [5-6] cm2; p < 0.001) in the 100% oxygen group. Patients in the 30% oxygen group were observed for another 40 min. 6 continued to receive 30% oxygen (subgroup A) and 6 were ventilated with 100% oxygen (subgroup B). During this time, the amount of atelectasis increased to 1.6 (1.6) cm2 in subgroup A and to 4.7 (4.5) cm2 in subgroup B (p = 0.047 for difference between groups). In subgroup A, the shunt (VA/Q < 0.005) increased from 1.6 (2.0)% of cardiac output to 3.2 (2.7)%, but the arterial oxygen tension did not change. In subgroup B, the shunt increased from 2.6 (5.2)% to 9.8 (5.7)% of cardiac output. These results suggest that the composition of inspired gas is important in atelectasis formation during general anaesthesia. Use of a lower oxygen concentration than is now standard practice might prevent the early formation of atelectasis.

Reexpansion of atelectasis during general anaesthesia may have a prolonged effect

Pulmonary atelectasis, as found during general anaesthesia, may be reexpanded by hyper-inflation of the lungs. The purpose of this study was to determine whether such a recruitment is maintained and whether this is accompanied by an improved gas exchange. We studied a consecutive sample of twelve lung healthy adults, scheduled for elective surgery. After induction of intravenous anaesthesia, the lungs were hyperinflated manually. The ventilationperfusion relationship (VA/Q) was estimated with the multiple inert gas method, and in six patients atelectasis was assessed by computed x-ray tomography. The mean pulmonary shunt was 7.5% of cardiac output after induction of anaesthesia and this decreased to 1.0% and 2.8% at 20 and 40 min after the recruitment manoeuvre. Perfusion of poorly ventilated lung regions (low VA/Q), however, increased from 3.7% to 10.6% and 7.8% at 20 and 40 min after the recruitment, respectively. The mean alveolar-arterial oxygen tension difference (PA-aO2) was 14.3 kPa after induction of anaesthesia and 11.1 kPa immediately after recruitment. Forty minutes later PA-aO2 was still 2.0 kPa lower than after induction of anaesthesia (95% confidence interval [CI] 0.3 to 3.8 kPa). PA-aO2 decreased more in obese patients. The mean area of atelectasis decreased from 9.0 cm2 after induction of anaesthesia to 0.1 cm2 immediately after recruitment, and there was a slow increase to 1.9 cm2 (95% CI 0.0 to 3.9 cm2) 40 min later. During general anaesthesia in lung healthy patients, most of the reexpanded atelectatic lung tissue remains inflated for at least 40 min. The recruitment manoeuvre decreases pulmonary shunt, but increases low VA/Q. The net effect on gas exchange is a small reduction of PA-aO2.

Atelectasis and gas exchange impairment during enflurane/nitrous oxide anaesthesia

The development of atelectasis and effects on gas exchange during enflurane anaesthesia in nitrogen/oxygen or nitrous oxide/oxygen (inspired oxygen fraction 0.4) were studied in 16 lung-healthy patients (mean age 49 years). Awake, no subject displayed atelectasis as assessed by computed x-ray tomography of the thorax. Pulmonary gas exchange, studied by multiple inert gas elimination technique, and blood gases were normal. After 10 min of enflurane anaesthesia in nitrogen/oxygen, 14 of 16 subjects had developed atelectasis. After 30 min of enflurane anaesthesia in nitrogen/oxygen or nitrous oxide/oxygen, all patients had developed atelectasis, and a further increase was observed after 90 min of anaesthesia to approximately 5% of the intrathoracic area. There was no difference between the two anaesthesia groups. In the nitrogen group, shunt rose to a maximum of 5.8% at 30 min of enflurane anaesthesia, with a significant reduction to the initial anaesthesia level after 90 min of anaesthesia (3.4%). Perfusion of poorly ventilated lung regions (low VA/Q) averaged 4-5% and did not vary significantly during the anaesthesia. In the nitrous oxide group, shunt increased to 6.3% after 90 min of anaesthesia, and there was a parallel decrease in perfusion of low VA/Q regions. The findings suggest that besides prompt collapse of lung tissue during induction of anaesthesia, absorption of gas from closed-off or poorly ventilated regions takes place and further increases the atelectatic area.

Prediction of postoperative hypoxemia in smokers and non-smokers

Age, weight, spirometric variables, peak expiratory flow and airway closure assessments were correlated to postoperative gas exchange in 40 cholecystectomized subjects grouped according to smoking history. Pao2 was significantly lower in the smoking group both pre- and postoperatively, but the decrease in Pao2 following operation was of the same magnitude irrespective of smoking history. In non-smokers, the preoperative relationship between expiratory reserve volume and closing volume (ERV-CV) showed the highest correlation to postoperative alveolar-arterial oxygen difference P(A-a)o2 (r = -0.88, P less than 0.001) and to the increase of P(A-a)o2 following operation (r = -0.67, P less than 0.001), whereas age was found to be the best preoperative predictor of postoperative gas exchange in smokers.

Postoperative pulmonary function. A comparison of ventilation with nitrogen or nitrous oxide during anaesthesia

Arterial blood-gases and lung volumes were measured in 48 patients before and after upper abdominal surgery. There was no significant difference between the results of 25 patients ventilated with oxygen and nitrogen during anaesthesia compared with a comparable group which received oxygen and nitrous oxide.