Transplantologiya 2014

Key words: lung transplantation, inhaled nitric oxide, nitrite/nitrate, methemoglobin.

INTRODUCTION

Endogenous nitric oxide (NO) is a vasodilator synthesized from L-arginine by NO-synthase and acting upon the endothelial lining of blood vessels. NO is involved in the regulation of the systemic and pulmonary vascular resistance. Under normal conditions, NO is predominantly produced by endothelial NO-synthase (eNOS) [1, 2]. An alternative NO-synthase isoform, namely inducible NO-synthase (iNOS), is stimulated by cytokines, bacterial endotoxins and can produce excessive amounts of NO during inflammation or stress [2, 3].

Understanding of the role played by the pulmonary vascular endothelial dysfunction and NO synthesis disorders in the pathogenesis of acute and chronic pulmonary circulation disorders contributed to the introduction of inhaled NO (iNO) as an exogenous analogue of a natural regulator of vascular tone in a complex therapy of patients after lung transplantation (LT) [4].

During LT, the main role of iNO is believed to belong to its impact on the pulmonary vascular resistance. Meanwhile, in pulmonary hypertension, iNO may reduce the pulmonary vascular resistance without changes in the systemic vascular resistance, and improve oxygenation by dilating the blood vessels in the ventilated areas of the lungs reducing the shunt fraction [5-7]. Moreover, iNO probably has a beneficial effect after lung transplantation attenuating the reperfusion injury of the endothelium and reducing a primary graft failure.

The administration of iNO in low concentrations seems safe. In high concentrations, however, its toxicity is predominantly related to NO₂ production and methemoglobinemia. Interaction of iNO with oxyhemoglobin results in the production of methemoglobin (MetHb) and nitrite/nitrate (NOx) [8]. Almost 70% of iNO is excreted in urine within 48 hours in the form of nitrates [9]. Excessive concentrations of circulating MetHb can cause a tissue hypoxia [10]. MetHb is not capable of binding oxygen, so MetHb production may induce a hemic hypoxia. Moreover, in the presence of MetHb, the oxyhemoglobin dissociation curve is shifted to the left resulting in a decreased oxygen delivery to tissues. That is why the inhaled NO administration demands a close monitoring of blood MetHb levels. However, there are very few studies to investigate blood NOx levels in the lung transplant recipients in whom the inhaled NO was administered. Therefore, the aim of this study was to investigate the relationship between the iNO administration and the blood MetHb and NOx levels in patients after LT.

MATERIALS AND METHODS

The study included 7 patients (6 females and 1 male) aged between 24 and 55 (36.3 ± 4.0) years after lung transplantation. Lung transplantations were performed for primary pulmonary hypertension (2 cases), cystic fibrosis (2 cases), lymphangioleiomyomatosis, sarcoidosis, and idiopathic pulmonary fibrosis (1 case each).

Mechanical lung ventilation (MLV) was provided in an individually parameter-selected mode using Primus devices (in the Operating Room) and EvitaXL devices (in ICU). Ventilator settings and lung biomechanics were recorded by a monitoring system in a real time.

The inhaled NO therapy was conducted using commercially available gas mixture NO-NO₂ with the NO concentration of 1000 ppm (parts per million). The iNO (5-20 ppm) was delivered into the inspiratory limb of the breathing circuit of the ventilator system at 60-80 cm away from the Y-piece. A low gas flow rate was ensured using the Bedfont Nitric Oxide Inhaled Therapy Flow system. An iNO volumetric flow rate (ml/min) was set up and adjusted according to a required concentration and the readings of the electrochemical NO-NO₂ analyzer. Duration of iNO-therapy ranged from 2 to 4 days, the mean iNO concentration was 30±1 ppm.

Measurements of serum NOx were performed using the reduction reaction of nitrate into nitrite with cadmium in the presence of zink. [11]. Acid-base status, blood gas, and arterial MetHb were assessed using the ABL 800 Flex blood gas analyzer (Radiometer, Denmark). The measurements were made from day 1 to day 10 post-surgery. The control group included 25 healthy volunteers, mean age 32.7 ± 8.6 years; with male/female ratio 17/8.

The statistical analysis was performed using Statistica 10.0 and MS Excel Package Software. The comparisons between the study group and the control group were made using Mann-Whitney U-test. Spearman rank correlation was used to test the association between variables. The data were expressed as medians and interquartile range (25^th and 75^th percentiles).

RESULTS AND DISCUSSION
Studies of the blood NOx and MetHb levels in patients after LT over time demonstrated a statistically significant increase of NOx levels and 2.5 times on days 1, 2, 3, 4, and 5 after surgery, respectively (Table. 1). Graphs of NOx levels changing over time individually for each patient after LT are presented in the Figure demonstrating that blood serum NOx levels exceeded normal range in all the patients for 5 days following surgery, later on gradually decreasing in 6 patients, and nearly reached the normal values at day 10. MetHb levels remained within normal rage (Table. 2).
Table 1. Changes of blood NOx and MetHb levels over time in patients after lung transplantation (data are presented as medians and interquartile ranges).