Methemoglobin to cumulative nitric oxide ratio and response to inhaled nitric oxide in PPHN

Methemoglobin and the response to inhaled nitric oxide in persistent pulmonary hypertension of the newborn

BACKGROUND

We aimed to investigate whether the change in methemoglobin levels (ΔMHb) predicts oxygenation response to inhaled nitric oxide (iNO) in persistent pulmonary hypertension of the newborn (PPHN) with lung disease, with or without pulmonary hypoplasia.

METHODS

In this prospective observational study, infants were categorized based on ΔMHb and oxygenation response (ΔPaO2/FiO2) following iNO: ΔMHb ≤0 or ΔMHb>0, and ΔPaO2/FiO2 < 20 mmHg (Non-responder) or≥20 mmHg (Responder). ΔMHb levels were compared among infants with or without pulmonary hypoplasia.

RESULTS

Among infants with pulmonary hypoplasia (n = 28), ΔMHb was not associated with an oxygenation response to iNO or survival without ECMO. Among infants without hypoplasia (n = 29), subjects with ΔMHb>0 following iNO (n = 21) had a greater ΔPaO2/FiO2 (median, 64 mmHg; IQR, 127; p < 0.01) and 100% survival without extracorporeal membrane oxygenation (ECMO) when compared to infants with ΔMHb ≤0 (n = 8; median 10 mmHg; IQR, 33).

CONCLUSIONS

PPHN secondary to lung disease without hypoplasia with increased ΔMHb following iNO was associated with better oxygenation response and survival without ECMO compared to subjects without an increase in MHb.

Gasometric gradients between blood obtained from the pulmonary artery wedge and pulmonary artery positions in pulmonary arterial hypertension

INTRODUCTION

Little is known on the pulmonary gradients of oxyhemoglobin, carboxyhemoglobin and methemoglobin in pulmonary arterial hypertension (PAH). We sought to determine these gradients in group 1 PAH and assess their association with disease severity and survival.

METHODS

During right heart catheterization (RHC) we obtained blood from pulmonary artery (PA) and pulmonary artery wedge (PAW) positions and used co-oximetry to test their gasometric differences.

RESULTS

We included a total of 130 patients, 65 had group 1 PAH, 40 had pulmonary hypertension (PH) from groups 2-5 and 25 had no PH during RHC. In all groups, PAW blood had higher pH, carboxyhemoglobin and lactate as well as lower pCO2 than PA blood. In group 1 PAH (age 58 ± 15 years, 72% females), methemoglobin in the PAW was lower than in the PA blood (0.83% ± 0.43 vs 0.95% ± 0.50, p = 0.03) and was directly associated with the degree of change in pulmonary vascular resistance (R = 0.35, p = 0.02) during inhaled nitric oxide test. Oxyhemoglobin in PA (HR (95%CI): 0.90 (0.82-0.99), p = 0.04) and PAW (HR (95%CI): 0.91 (0.84-0.98), p = 0.003) blood was associated with adjusted survival in PAH.

CONCLUSIONS

Marked differences were observed in the gasometric determinations between PAW and PA blood. The pulmonary gradient of methemoglobin was lower in PAH patients compared to controls and a higher PAW blood methemoglobin was associated with a more pronounced pulmonary vascular response to inhaled nitric oxide. Pulmonary artery and PAW oxyhemoglobin tracked with disease severity and survival in PAH.

Update on PPHN: mechanisms and treatment

Persistent pulmonary hypertension of the newborn (PPHN) is a syndrome of failed circulatory adaptation at birth, seen in about 2/1000 live born infants. While it is mostly seen in term and near-term infants, it can be recognized in some premature infants with respiratory distress or bronchopulmonary dysplasia. Most commonly, PPHN is secondary to delayed or impaired relaxation of the pulmonary vasculature associated with diverse neonatal pulmonary pathologies, such as meconium aspiration syndrome, congenital diaphragmatic hernia, and respiratory distress syndrome. Gentle ventilation strategies, lung recruitment, inhaled nitric oxide, and surfactant therapy have improved outcome and reduced the need for extracorporeal membrane oxygenation (ECMO) in PPHN. Newer modalities of treatment discussed in this article include systemic and inhaled vasodilators like sildenafil, prostaglandin E1, prostacyclin, and endothelin antagonists. With prompt recognition/treatment and early referral to ECMO centers, the mortality rate for PPHN has significantly decreased. However, the risk of potential neurodevelopmental impairment warrants close follow-up after discharge for infants with PPHN.

The pulmonary circulation in neonatal respiratory failure

The pulmonary circulation rapidly adapts at birth to establish lungs as the site of gas exchange. Abnormal transition at birth and/or parenchymal lung disease can result in neonatal hypoxemic respiratory failure. This article reviews the functional changes in pulmonary hemodynamics and structural changes in pulmonary vasculature secondary to (1) normal and abnormal transition at birth, and (2) diseases associated with neonatal hypoxemic respiratory failure. Various management strategies to correct respiratory failure are also discussed.