1 Anaesthesia and Intensive Care, Institute of Medical Sciences, University of Aberdeen, Aberdeen AB25 2ZD, UK. 2 Intensive Care Unit, Aberdeen Royal Infirmary, Aberdeen, UK*Corresponding author
Accepted for publication: March 6, 2002
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Abstract |
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Methods. Mixed expired air samples were taken from the patient breathing system and analysed for exhaled NO using chemiluminescence analysis.
Results. Exhaled nitric oxide concentrations in expired gas correlated with the PaO2/FIO2 ratio (r=0.23, P<0.01). There was a non-significant trend towards a reduction in exhaled NO levels from after induction of anaesthesia to post-bypass time points, with the lowest exhaled NO concentrations occurring at this time (P=0.07). There was no correlation between mean arterial pressure (r=0.1, P=0.54) or mean pulmonary artery pressure (r=0.1, P=0.67) and expired NO levels.
Conclusions. Further work is required to test whether exhaled NO concentration may be useful in diagnosing the onset of acute lung injury in patients undergoing coronary artery bypass grafting.
Br J Anaesth 2002; 89: 24750
Keywords: heart, cardiopulmonary bypass; lung, damage; pharmacology, nitric oxide
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Introduction |
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Nitric oxide (NO) is produced in the lungs from differing sources. These include the bronchial epithelium, vascular endothelium, alveolar and lung interstitial macrophages, and also bacteria within the bronchial tree which are known to produce NO in small quantities. As NO is highly diffusable, all of these lung sources could potentially contribute to measureable exhaled concentrations of NO.2
Most patients undergoing coronary artery bypass surgery (CABG) develop a transient gas exchange abnormality, and some go on to develop acute lung injury. Thus these patients offer an excellent opportunity to study the development of acute lung injury. We hypothesized that exhaled NO concentrations would decrease during the development of acute lung injury in proportion to the severity of injury, and that minimum levels of exhaled NO would be observed in the early post-bypass period at the time of maximum lung inflammation.
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Methods |
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A standardized anaesthetic technique avoiding NO (due to the known presence of contaminating trace concentrations of NO) was used. This included etomidate, fentanyl and pancuronium for induction of anaesthesia, enflurane, oxygen and air for maintenance of anaesthesia, and additional doses of fentanyl at rewarming. Interventions were timed as follows: after induction of anaesthesia, before institution of cardiopulmonary bypass, during rewarming, after cardiopulmonary bypass, after completion of rewarming, the end of surgery, and 4 h after cardiopulmonary bypass. These times were chosen to represent the major interventional time points during cardiopulmonary bypass and also to cover the period of initial lung insult during the early postoperative period.
Expiratory gases were sampled at each time point. Ventilation was temporarily disrupted and an occluded catheter mount was placed over the tracheal tube connector. A 12 French gauge catheter, cut to 8 cm longer than the length of the tracheal tube, was advanced down the tracheal tube until it reached its maximum depth. The dead space of the catheter was aspirated and discarded, and a further 150 ml of gas was aspirated and collected into dried vacuumed plastic bags. This technique has been validated, and NO levels remain stable for up to 1 h after sample collection.3 This system was designed to allow sampling of end-expiratory alveolar gas. The breathing system was sealed proximally to prevent entrainment of atmospheric or dead-space gas whilst sampling. The sampling process took a total of 15 s and was not duplicated. The sample bags were transferred directly to the analyser for analysis. Exhaled NO concentrations were determined using a chemiluminescent analyser (Model 42, Thermoelectron, Warrington, Cheshire, UK) with a minimum sensitivity of 0.5 parts per billion (ppb) and noise of 0.25 ppb. Gas is sampled at 80 ml min1 through a silicate column to remove moisture. The sample reacts with ozone, producing excited NO2 which in turn decays, giving off a photon of energy which is converted into an analogue signal.
An electronic transduction technique was used to measure mean arterial pressure via an invasive arterial line, and mean pulmonary artery pressure via a pulmonary artery catheter. The advice of a statistician was sought for data analysis. Data were grouped, paired and analysed using linear regression for correlations, and KruskalWallis analysis of variance and the MannWhitney U test for time trends, using AstuteTM add-in for Microsoft Excel®.
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Results |
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The exhaled NO levels were correlated against PaO2/FIO2 ratio, mean arterial pressure and mean pulmonary pressure. Figure 1 shows the linear regression plot for the exhaled NO level vs PaO2/FIO2 ratio for the cohort and for each individual patient. It demonstrates a statistically significant positive correlation between exhaled NO and PaO2/FIO2 ratio for the cohort (r=0.23, P<0.01). A similar positive correlation was also found in five out of six patients. Figure 2 shows the changes in exhaled NO levels at different time points. It demonstrates a non-significant reduction in exhaled NO levels between pre-bypass (B) and the early post-bypass time points (D) (P=0.07). There was no correlation between mean arterial pressure and expired NO levels (r=0.1, P=0.54), or mean pulmonary artery pressure and expired NO levels (r=0.1, P=0.67). No patients in this cohort went on to develop acute lung injury.
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Discussion |
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Exhaled NO levels have also been shown to increase in a rat model of sepsis, where NO is thought to be an early marker of lung inflammation but could also represent increased systemic inflammation.10 Further animal work has demonstrated a decrease in exhaled NO with hypoxia, which the authors thought corresponded with altered pulmonary mechanics and gas exchange and might reflect bronchial epithelial dysfunction associated with acute lung injury.11 Results vary on the effects of CABG surgery on exhaled NO levels. One study suggests that although surgery induced a demonstrable inflammatory response, this response was not associated with increased exhaled NO production.12 Another study suggests that exhaled NO levels fall only with increased alveolar-arterial gradient in CABG patients and thus exhaled NO levels decrease with worsening oxygenation.13
Our results demonstrate that for a small group of patients undergoing CABG, the exhaled NO level correlates with the PaO2/FIO2 ratio, which is an index of oxygenation. The PaO2/FIO2 ratio decreases with worsening oxygenation and thus with increasing severity of lung injury. Our results support our hypothesis that exhaled NO concentrations will decrease when acute lung injury is developing, in proportion to the severity of the injury. Our results also agree with those of Ishibe and colleagues.13 There is a trend towards exhaled NO levels being lowest at the two time points immediately after cardiopulmonary bypass when it would be expected that maximum lung inflammation would occur; this further supports our hypothesis.
The explanation of the observed effect could relate to ventilation/perfusion disturbance in the lung. In mild asthma where exhaled NO levels were shown to rise in proportion to the severity of lung inflammation, there is no associated ventilation/perfusion mismatch. All lung zones will take part in gas exchange and contribute to total exhaled NO. In contrast, in acute lung injury, where lung inflammation is patchy and significant ventilation/perfusion abnormalities exist, diseased areas of lung may not take part in gas exchange and thus will not contribute to exhaled NO levels. This would lead to a decline in exhaled NO levels during the development of acute lung injury. An analogy could be made to conditions such as pulmonary embolism associated with massive dead-space ventilation where, despite a marked increase in arterial PaCO2, the expired carbon dioxide falls. Animal work has also suggested that bronchial epithelial dysfunction during hypoxia may decrease endothelial NO production and thus contribute to the decline in exhaled NO.11
This work suggests that further investigation is required to study whether exhaled NO could be predictive of the onset or severity of acute lung injury. Other markers of onset and severity of acute lung injury include cytokines and other neutrophil products in bronchoalveolar lavage, but their assessment requires invasive procedures and expensive laboratory assays which are of limited clinical applicability. Exhaled NO offers the potential of a near-patient test with immediate results to alert the clinician to developing lung injury, and to allow early intervention.
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Acknowledgement |
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References |
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