Unintended inhalation of nitric oxide by contamination of compressed air: physiologic effects and interference with intended nitric oxide inhalation in acute lung injury

Carbonic anhydrase is not a relevant nitrite reductase or nitrous anhydrase in the lung

KEY POINTS

Carbonic anhydrase (CA) inhibitors such as acetazolamide inhibit hypoxic pulmonary vasoconstriction (HPV) in humans and other mammals, but the mechanism of this action remains unknown. It has been postulated that carbonic anhydrase may act as a nitrous anhydrase in vivo to generate nitric oxide (NO) from nitrite and that this formation is increased in the presence of acetazolamide. Acetazolamide reduces HPV in pigs without evidence of any NO generation, whereas nebulized sodium nitrite reduces HPV by NO formation; however; combined infusion of acetazolamide with sodium nitrite inhalation did not further increase exhaled NO concentration over inhaled nitrite alone in pigs exposed to alveolar hypoxia. We conclude that acetazolamide does not function as either a nitrous anhydrase or a nitrite reductase in the lungs of pigs, and probably other mammals, to explain its vasodilating actions in the pulmonary or systemic circulations.

ABSTRACT

The carbonic anhydrase (CA) inhibitors acetazolamide and its structurally similar analogue methazolamide prevent or reduce hypoxic pulmonary vasoconstriction (HPV) in dogs and humans in vivo, by a mechanism unrelated to CA inhibition. In rodent blood and isolated blood vessels, it has been reported that inhibition of CA leads to increased generation of nitric oxide (NO) from nitrite and vascular relaxation in vitro. We tested the physiological relevance of augmented NO generation by CA from nitrite with acetazolamide in anaesthetized pigs during alveolar hypoxia in vivo. We found that acetazolamide prevents HPV in anaesthetized pigs, as in other mammalian species. A single nebulization of sodium nitrite reduces HPV, but this action wanes in the succeeding 3 h of hypoxia as nitrite is metabolized and excreted. Pulmonary artery pressure reduction and NO formation as measured by exhaled gas concentration from inhaled sodium nitrite were not increased by acetazolamide during alveolar hypoxia. Thus, our data argue against a physiological role of carbonic anhydrase as a nitrous anhydrase or nitrite reductase as a mechanism for its inhibition of HPV in the lung and blood in vivo.

Onsite production of medical air: is purity a problem?

Introduction

Medical air (MA) is widely used in hospitals, often manufactured onsite by compressing external ambient air and supplied through a local network piping system. Onsite production gives rise to a risk of impurities that are governed by the same pharmacopoeia purity standards applicable to commercially produced MA. The question to be addressed in this paper is how to assess if a lack of purity poses a medical problem?

Methods

The MA produced onsite at a major Canadian hospital was monitored for carbon dioxide (CO₂) and other impurity gases at high frequency (one per minute) over a two-month period.

Results

The average CO₂ concentration was 255 ppm. The United States Pharmacopeia (USP) threshold of 500 ppm was exceeded during 1% of the total study period, and the average while exceeding the threshold was 526 ppm. The maximum concentration was 634 ppm.

Discussion and conclusion

To our knowledge, there is only one study that evaluated the effects suffered by respiratory patients of elevated nitric oxide in MA; thus, it is not clear what are the medical bases for the thresholds stated in the USP. To perform a Quality Risk Assessment, the threshold and the time above threshold should be considered in determining the frequency of sampling and analysis, and operating methods required to ensure the quality of MA entering the pipeline meets the clinical, regulatory, and patient safety standards. In conclusion, because the USP does not provide impurity thresholds for specific patients nor time above thresholds, there is a need for the medical community to determine these quantities before it can be known if the purity of MA is a problem.

Mechanical ventilation in acute respiratory distress syndrome

The acute respiratory distress syndrome occurs commonly in critical care. There is an increasing volume of clinical and experimental evidence that poor ventilatory technique that is injurious to the lungs can propagate the systemic inflammatory response and adversely affect mortality. Many ventilatory techniques have been hypothesized to 'protect' the lungs during mechanical ventilation, including tidal volume limitation, high positive end-expiratory pressure, pressure-controlled inverse ratio ventilation, and prone positioning. Experimental techniques include liquid ventilation, surfactant administration and extracorporeal gas exchange. Despite excellent rationale for their use, few techniques, apart from tidal volume limitation, have been shown to improve survival in randomized controlled trials.

Workplace NO and NO2 during combined treatment of infants with nasal CPAP and NO

OBJECTIVE

To determine the workplace concentrations of NO and NO(2) in and around a paediatric incubator during inhaled NO (iNO) treatment and during an accidental emptying of NO cylinders into room air.

DESIGN

We simulated iNO-nasal CPAP treatment in order to assess the impact on the occupational environment. Furthermore, two full NO cylinders for therapy, 1,000 ppm, 20 litres, 150 bar and 400 ppm, 10 litres, 150 bar, were emptied as rapidly as possible into an intensive care unit (ICU) room.

SETTING

University hospital ICU.

MEASUREMENTS AND RESULTS

To correctly gauge the contribution from iNO-CPAP we constructed a system measuring breathing zone and room ventilation inlet-outlet values during a 10-ppm iNO treatment of a simulated infant. Maximal breathing zone values were 17.9 +/- 7.0 (mean +/- 95% CI) ppb for NO and 25.2 +/- 4.8 ppb for NO(2). If room inlet values were subtracted, the contributions to breathing zone values emanating from iNO-CPAP were 14.8 +/- 4.6 ppb for NO and 14.6 +/- 4.6 ppb for NO(2). At the ventilation outlet the maximal contributions were 4.2 +/- 2.9 ppb NO and 9.6 +/- 4.3 ppb NO(2). During rapid total release of a gas cylinder in the ICU room, simulating an accident, we found transient NO levels comparable to the high therapeutic dosing range, but only low NO(2) levels.

CONCLUSIONS

Neither 8-h time-weighted average (TWA) nor 15 min short-term exposure limits (STEL) were exceeded during normal operation or during a simulated accident. The contribution of nitrogen oxides from treatment to workplace air were minor compared to those from ambient air.

Effect of Lung Ventilation With 50% Oxygen in Air or Nitrous Oxide Versus 100% Oxygen on Oxygenation Index After Cardiopulmonary Bypass

OBJECTIVE

This study was designed to assess the use of 100% oxygen or 50% oxygen in air or nitrous oxide after cardiopulmonary bypass (CPB) on atelectasis, as evidenced by the oxygenation index (PaO2/F(I)O2), after coronary artery bypass graft (CABG) surgery.

DESIGN

Prospective, randomized clinical study.

SETTING

University teaching hospital.

PARTICIPANT

Thirty-six adult patients undergoing CABG surgery.

INTERVENTIONS

Patients either received 50% O2 in air (50% O2 group), 50% O2 in N2O (50% N2O group), or 100% O2 (100% O2 group) after CPB.

MEASUREMENTS AND MAIN RESULTS

Apart from demographic and perioperative clinical data, extubation time, mediastinal drainage, and pulmonary complications were also recorded. After CPB, arterial blood gases done at various time points until 3 hours postextubation and oxygenation index were calculated. No significant differences were noted in demographic and perioperative data except preoperative hemoglobin and fluid use. Significant deterioration in arterial oxygenation was noted in the 100% O2 group from the baseline value, whereas significant improvement was seen in the 50% O2 group at 4 time points from baseline value and at all time points from the 100% O2 group. After initial deterioration in oxygenation, no further change was evident in the 50% N2O group. Furthermore, there was a greater increase in the oxygenation index as compared with the 100% O2 group. Time to extubation was also longer in the 100% O2 group than the 50% O2 group.

CONCLUSION

Significant deterioration in arterial oxygenation and an increase in the extubation time occurred with the use of 100% O2 after CPB, whereas better oxygenation was evident with the use of 50% O2 in air.

Contamination of anaesthetic gases with nitric oxide and its influence on oxygenation: study in patients undergoing open heart surgery †

BACKGROUND

Nitric oxide is important in vasomotor regulation. Contamination of anaesthetic gases with nitric oxide could affect gas exchange.

METHODS

We measured oxygenation and nitric oxide concentrations in the inspiratory and expiratory limb of the ventilator circuit in patients about to have cardiac surgery. Measurements were made before surgery when the circulation and respiratory conditions were stable. Fi(o(2)) was set at 0.35. The breathing circuit was supplied with a fresh gas flow greater than the minute volume so that exhaled gas was not re-used. Three gas mixtures were given in sequence to each patient: oxygen and compressed air (AIRc), oxygen and nitrous oxide, and oxygen and synthetic air (AIRs) that was free from nitric oxide. All patients were given AIRs as the second gas and the other two gas mixtures (AIRc and nitrous oxide) were given randomly as the first and third gases.

RESULTS

During ventilation with oxygen-AIRc, the median nitric oxide concentration was 5.6 ppb, during ventilation with oxygen-nitrous oxide it was 5.0 ppb and using oxygen-AIRs it was 1.5 ppb. When AIRc and nitrous oxide were used, Pa(o(2)) was greater and venous admixture was less than when AIRs was used. The different gas mixtures did not affect pulmonary vascular pressures or cardiac output.

CONCLUSIONS

Compressed air and nitrous oxide contain very low concentrations of nitric oxide (<10 ppb). This can affect pulmonary oxygen transfer during anaesthesia.

Essentials of Nitric Oxide for the Pediatric (Cardiac) Anesthesiologist

Short- and long-term survival rates for the operative treat ment of congenital heart disease (CHD) have improved significantly in the past 2 decades. The increasing sophisti cation of the pediatric cardiologist's diagnostic armamen tarium has led to more pervasive use of fetal screening with echocardiography. Early diagnosis and pre-emptive care of the neonate with complex CHD have allowed interventional strategies in the catheterization suite or the operating room to be optimized in both the timing and the quality of pallia tive or corrective procedures. Medications such as prosta glandin E and ventilator strategies using hypoxic and hyper carbic inspired gases exemplify therapies benefitting the contemporary neonate with CHD, often allowing stabiliza tion of the patient before surgery. Surgical care of neonates, infants, and children with CHD has also improved. Insights into maturational differences in myocardial and autonomic function have led to more appropriate myocardial protection strategies and pharmacologic support of the circulation. Recognition of those anomalies in which total correction in the neonate is desirable has stimulated improvements in the technical and cognitive skills of pediatric cardiovascular sur geons and pediatric cardiac anesthesiologists to meet these challenges. The goal of this article is to provide the pediatric anesthesiologist with an overview of inhaled nitric oxide and its relevance to clinical practice.