Biomedical Research Unit (Respiratory)
Southampton has been awarded two Biomedical Research Unit (BRU) grants in Nutrition and Respiratory, by the National Institute for Health Research (NIHR). PICU are co-applicants in the respiratory BRU and we are leading on the SARPS study.
SARPS: The kinetics of pulmonary surfactant phospholipids in children with acute hypoxic respiratory failure
We have previously demonstrated a significant disruption of the normal profile of phospholipids found in broncho-alveolar lavage (BAL) fluid taken from children with respiratory failure. These molecules are the major constituents of surfactant, a substance that is secreted by cells lining the alveoli of healthy children and adults. Normally surfactant lowers surface tension within alveoli (both singular and within lung units) preventing alveolar collapse and minimising ventilation diffusion mismatch and thus hypoxia. In acute hypoxic respiratory failure (AHRF) changes in surfactant composition greatly reduce its activity promoting lung collapse and impaired gas exchange. We have demonstrated changes in surfactant composition(both qualitative and quantitative) and an increase in alveolar concentration s of plasma protein and activated neutrophils and their products in BAL fluid sampled from PICU children with AHRF. What we do not know is whether injury reduces the rate of surfactant production or increases the rate of its hydorolysis or increases surfactant related oxidative stress. We have also not established the role of both plasma and alveolar surfactant protein D in the pathogenesis of this disease process.
Choline is an amine which is a naturally occurring constituent of the vitamin B complex. We have previously demonstrated a significant disruption of the normal profile of phospholipids found in broncho-alveolar lavage (lung) fluid taken from children with respiratory failure. These molecules are the major constituents of surfactant, a substance that is secreted by cells lining the air sacks of healthy children and adults. Normally surfactant acts as a detergent, lowering forces that promote lung collapse and allows the air sacks to remain open and able to participate in gas exchange (oxygen in and carbon dioxide out). In respiratory failure changes in surfactant composition greatly reduce its detergent activity promoting lung collapse and impaired gas exchange. What we do not know is whether injury reduces the rate of surfactant production or increases the rate of breakdown.
Choline is an organic compound, classified as an essential nutrient grouped within the Vitamin B complex. This natural amine is found in the lipids that make up all cell membranes and is also an integral constituent of surfactant. All of the choline seen in pulmonary surfactant is known to be derived exclusively from plasma.
Adequate intakes for this micro nutrient of between 425 to 550 milligrams daily have been established by the Food and Nutrition Board of the Institute of Medicine of the National Academy of Sciences for adults,. This equates to between 6 and 8 mg/kg body weight per day and can thus be extrapolated to children.
Deuteriated choline chloride ([methyl-D9] choline chloride) is a STABLE, NON RADIOACTIVE, NON-TOXIC isotope of naturally occurring choline that has been used successfully in human adult volunteers.
We wish to ask the following questions in children with acute hypoxic respiratory failure:
- Can [methyl-D9] choline chloride be use effectively to study surfactant metabolism in children with AHRF
- Is diminished production a major cause of AHRF in children?
- Is there significant hydrolysis and oxidative damage of surfactant point, that could lead to potential therapeutic interventions in children with AHRF?
This pilot study may provide a useful stepping stone to future investigational or therapeutic trials of investigational medicinal products in paediatric AHRF.
The importance of AHRF in children.
- Paediatric ALI has a high mortality (22%) compared with the overall mortality of pediatric intensive care unit patients.
- Several clinical risk factors contribute independently to an increased risk of death and prolonged mechanical ventilation, including the initial oxygenation defect, as measured by the PaO2/ FiO2 ratio, the presence of nonpulmonary, non-CNS organ system dysfunction (hepatic, renal, hematologic, or gastrointestinal dysfunction), and the presence of CNS dysfunction, all of which are identifiable and interpretable in the clinical and research settings.
- A significant proportion of pediatric patients with acute lung injury do not require mechanical ventilation at the time of diagnosis.
- By identifying patients earlier in the course of ALI/ARDS, therapeutic strategies may be tested and applied earlier, thus potentially decreasing patient morbidity and mortality.
- Mortality is 15% if P/F ratio 200><300mmHg but 28% if P/F ratio <200. This is 3 and 5 times the national average mortality figure for children admitted to PICU in the UK
AHRF therefore has a considerable impact on childhood mortality, UK healthcare services and the public health in general. Targeted use of existing or novel adjunctive therapies to improve outcome is therefore required. One proposed agent is exogenous surfactant. Before embarking upon randomised trial however, more information about the metabolism of native surfactant in children with AHRF is required. If hydrolysis and oxidative damage of surfactant is the main patho-physiological cause of acquired surfactant deficiency in these children then exogenous administration of surfactant is unlikely to affect outcome. If on the other hand type II pneumocyte abnormalities lead to reduced surfactant production then exogenous therapy may well be a target for interventional trials with mortality or ventilator free day outcome measures.
Pathophysiology of AHRF
The primary objective of this patho-physiological pilot study is to measure the rate of absorption, storage, synthesis and incorporation of [methyl-D9] choline chloride into surfactant in alveoli of children with AHRF compared with children without AHRF.
The secondary objectives are to assess both the degree of hydrolysis and the amount of oxidative damage to surfactant that occurs in children with AHRF compared with children without AHRF.
Summary of Study Design
Phospholipids are the major components of lung surfactant, a detergent that keeps the lung expanded. Variability of their molecular composition correlates with differences in respiratory physiology and have been described in a variety of lung diseases, including neonatal and acute respiratory distress syndrome, pneumonia, cystic fibrosis and asthma. However, the extent to which such changes are due to alterations in either synthesis, oxidative damage or hydrolysis of phospholipids has not been established. Methods for analysis of phospholipid synthesis that monitor incorporations of non-radioactive stable isotopes offer obvious advantages for human studies. We have previously demonstrated the feasibility and safety of metabolic analysis of surfactant PC molecular species in humans using an intravenous infusion of [methyl-D9]choline together with mass spectrometry.
[methyl-D9] choline chloride infusion and blood sampling
After informed consent ventilated, sedated children on PICU with pre-existing arterial and venous canulae will be given an intravenous infusion of sterile, pyrogen free [methyl-D9] choline chloride (3.6mg/kg) over a three hour period.
Blood Blood samples (2 millitres) will collected from an indwelling arterial line at 2,4,6,12, 24,48,72,96,120 hours after start of [methyl-D9]choline chloride infusion for measurement of labelled phospholipid, serum phospholipase A2, lysophospholipids and surfactant protein D
Bronchoalveolar lavage specimens
Specimens of lung fluid will be collected by gentle suctioning after instillation of 1-2 teaspoons of sterile salty water down the breathing tube. This will be performed at baseline after enrolment and before the administration of [methyl-D9]choline chloride infusion and at 24 hour intervals thereafter for 4 days or until ventilation is discontinued, whichever is the shorter period. This a technique that we have demonstrated to be well tolerated in such children in two previous 4th year student projects have been approved by the South West Hampshire LREC previously Every student involved will as has been the case in previous studies, be supervised directly by a consultant paediatric intensivist whilst performing the broncho-alveolar lavage. Specimens will be filtered and frozen for storage and subsequent analysis.
This study may provide the pilot data necessary for the design of a definitive trial of surfactant replacement therapy or drug therapy aimed at reducing both hydrolysis and oxidative stress of endogenous surfactant in children with AHRF.