Inhaled nitric oxide reverses cell-free hemoglobin-induced pulmonary hypertension and decreased lung compliance. Preliminary results

Abstract

In order to test the hypothesis that inhaled nitric oxide (NO) reverses the pulmonary hypertension induced by αα-diaspirin crosslinked hemoglobin (ααHb), were studied anesthetized pigs that were administered with a total dose of 200 mg/kg of 10% ααHb. Inhaled NO (5 ppm) was administered for 10 min, and then discontinued for 10 min. This cycle was then repeated with 10 ppm inhaled NO. < 0.05). We conclude that cell-free hemoglobin-induced pulmonary hypertension and decreased lung compliance can be selectively counteracted by inhaled NO.

Background:

In order to test the hypothesis that inhaled nitric oxide (NO) reverses the pulmonary hypertension induced by αα-diaspirin crosslinked hemoglobin (ααHb), were studied anesthetized pigs that were administered with a total dose of 200 mg/kg of 10% ααHb. Inhaled NO (5 ppm) was administered for 10 min, and then discontinued for 10 min. This cycle was then repeated with 10 ppm inhaled NO.

Results:

< 0.05).

Conclusion:

We conclude that cell-free hemoglobin-induced pulmonary hypertension and decreased lung compliance can be selectively counteracted by inhaled NO.

Introduction

].

], a key mediator responsible for the physiological regulation of the vasodilatory tone.

]. We are unaware of any evaluation of the effects of cell-free hemoglobin on lung compliance and airway resistance.

We hypothesized that inhaled NO will selectively counteract the pulmonary hypertension induced by cell-free hemoglobin blood substitutes. To test our hypothesis we performed experiments in pigs administered with inhaled NO after αα-diaspirin crosslinked hemoglobin (ααHb) infusion. We also evaluated the effects of ααHb on dynamic lung compliance and airway resistance, and the response of these parameters to inhaled NO. Our preliminary results demonstrated that cell-free hemoglobin-induced pulmonary hypertension and decreased lung compliance can be selectively counteracted by inhaled NO.

Animal preparation

tension at 35⌓40 mmHg, using the assist control mode an I:E ratio maintained at 1:3. An inspired oxygen fraction of 0.95 was used throughout the experiment, which maintained an arterial oxygen tension of between 400 and 450 mmHg and an arterial oxygen saturation > 97%. Core body temperature was maintained with a heating pad and warming lights.

/CO computer, Abbott, Chicago, IL, USA). Each catheter was connected to a pressures transducer (Transpac Disposable Transducer, Abbott) and to a Biopac Data Acquistion System (Model MP100, Biopac Systems, Goleta, CA, USA) for continuous recording of heart rate, systemic and pulmonary arterial pressures, and waveforms. Blood samples and methemoglobin levels were analyzed by a pH/Blood Gas Analyzer 1303 and CO-Oximeter 482 (Instrumentation Laboratory, Lexington, MA, USA).

at which 50% of hemoglobin is saturated with oxygen) of 29 mmHg and had ≤ 4% of its hemoglobin in the form of methemoglobin. It was provided through a Cooperative Research & Development Agreement with the Blood Research Detachment of the Walter Reed Army Institute of Research.

analyzer, Pulmonox Medical Corp, Tofield, Alberta, Canada).

Experimental protocol

After a 30-min period of stabilization following surgical preparation, baseline data were obtained. αα Hb was administered in cumulative doses of 0.1, 0.5, 1.0 and 2.0 ml/kg, in 5-min intervals to a total dose of 2ml/kg (=200 mg/kg ααHb); the data were collected 10 min after the final ααHb infusion. Inhaled NO, in concentration of 5 ppm, was then administered for 10 min and data were recorded. The inhaled NO was discontinued for 10 min, after which data were again recorded. This cycle was repeated with 10 ppm inhaled NO. After the final measurements the animals were killed with an anesthetic overdose and saturated potassium chloride solution.

Experimental measurements

were continuously monitored; pulmonary artery occlusion pressure (PAOP) was measured in 5 min intervals. Cardiac output was determined by the thermodilution technique and is presented as cardiac index determined using calculated body surface area. Systemic and pulmonary vascular resistance indices (SVRI and PVRI, respectively) were calculated using standard formulae.

All measurements relating to lung volume, pressures, airway resistance and dynamic lung compliance were continuously monitored and recorded with a Ventrak Model 1500 (Novametrix Medical Systems, Wallingford, CT, USA). This system determines dynamic lung compliance by measuring the peak pressure at zero flow [minus any positive end-expiratory pressure (PEEP)] and tidal volume delivered and then calculating the compliance using the following formula:

O) = change in volume/(peak pressure-PEEP).

For airway resistance, the system measures the pressure at the end of inspiration and the peak expiratory flow, then applies the following formula:

O/I/s) = alveolar pressure/(peak expiratory flow/60).

End expiratory pressure (minus PEEP) is used as the alveolar pressure and is measured at the proximal end of the endotracheal tube.

Firstly, data were recorded at baseline (BL) and 10 min after 200 mg/kg ααHb infusion (ααHb). Data were then recorded at the end of each of the following 10-min periods: inhaled NO at 5 ppm (NO 5 ppm), NO discontinued (OFF), inhaled NO at 10 ppm (NO 10 ppm), NO discontinued (OFF).

Statistical analysis

< 0.05 was considered significant.

Results

).

).

Discussion

We demonstrated that inhaled NO can selectively reverse the pulmonary hypertension and decreased lung compliance induced by cell-free hemoglobin. This suggests that its is possible to effectively control potentially deleterious side-effects associated with the clinical use of cell-free hemoglobin-based blood substitutes.

].

].

].

].

]. Therefore, it is tempting to speculate that inhibition of NO by cell-free hemoglobin with subsequent venoconstriction and increased capillary pressure may increase extravascular lung water, contributing to the decreased lung compliance.

], in which a very small dose of cell-free hemoglobin (50 mg/kg) was infused to 11 anesthetized patients showed that mean PAP increased from 21 to 27 mmHg, measured 30 min after infusion. This finding illustrates the potential for adverse effects in humans, particularly in patients with pre-existing diseases and limited cardiac and pulmonary function. On the other hand, hundreds of patients have been tested and safety is claimed with most hemoglobin-based blood substitutes. Unfortunately only limited data are available in the peer-reviewed literature, making it difficult to correlate the concerns raised in this study suggests that it will be an effective approach to selectively counteract the undesirable side-effects of hemoglobin solutions in the pulmonary circulation.

Although caution should be exercised when drawing clinical implications from animal studies, the pig is usually considered an appropriate animal model because of its anatomical and physiological similarities to humans, particularly regarding the heart and lungs. Prospective clinical studies addressing pulmonary pressures and right ventricle performance are needed; complete hemodynamic evaluation should be performed in the ongoing blood substitute trials, as this is the only means to determine whether concerns raised by animal studies are clinically relevant. The limitations of our study, which include a small sample size, no control group and short experimental period, resulted from a small supply of the hemoglobin. However, were clearly demonstrated the potential for inhaled NO to modulate the increased PAP and decreased lung compliance without major effects in the systemic circulation.

We conclude that inhaled No selectively reverses pulmonary hypertension and decreased lung compliance induced by cell-free hemoglobin blood substitutes.

Acknowledgements

The authors thank Tatsuo Uchida for statistical analysis and the US Army for providing the αα -hemoglobin used in this study. The study was performed at the Department of Anesthesiology, University of Texas Medical Branch, Galveston, TX, USA. Luiz F Poli de Figueiredo was a Visiting Assistant Professor at University of Texas Medical Branch during these experiments, with a sponsorship by Fundação de Apoio a Pesquisa Estado de São Paulo, FAPESP-Brazil, Grant 93/3796-5.

Figures and Tables

< 0.05 compared to OFF.

#60; 0.05 compared to ααHb.

Hemodynamic data (mean ± SEM)

<0.05 compared to baseline.

Keywords

• blood substitutes
• hemoglobin
• nitric oxide
• toxicity
• vasoconstriction