Consequences of preterm birth on lung function, physical activity and exercise capacity

Abstract

The incidence of preterm birth is increasing worldwide. Some of the survivors of preterm birth will be affected by varying degrees of disabilities like lower cognitive or respiratory function. Moreover, the survivors will encounter an increased risk of non-communicable diseases like hypertension, coronary heart disease, and diabetes, later in life. More knowledge is needed in order to prevent these adverse outcomes.

Physical activity (PA) and exercise have well-established positive effects on several non-communicable diseases. In addition, there is growing evidence that physical activity has a positive effect on cognitive function. In study I and II, we used information from the conscript register and linked it to birth characteristics in the medical birth register in order to associate preterm birth to later exercise capacity and cognitive function. The results revealed that young men born preterm have lower exercise capacity than men born at term, with a step-wise relation to gestational age. Furthermore, cognitive function was positively associated with increases in exercise capacity, across all gestational ages. Men born extremely preterm (<28 weeks gestational age) with the lowest exercise capacity, exhibited the lowest results on the cognitive function test.

To evaluate if a reduced exercise capacity in young adulthood could be a consequence of less physical activity in childhood, 71 children born extremely preterm and 87 controls born at term, wore an activity monitor on the wrist for seven days at 6.5 years of age. Extremely preterm boys were less active than term boys, which could be linked to severe brain injury during infancy, which was more prevalent in preterm boys. There was no statistically significant difference in physical activity when comparing all preterm born children with controls. From study I-III we conclude that preterm birth and morbidities during infancy are associated with level of PA in childhood. Furthermore, prematurity can be correlated to lower exercise capacity in young adulthood and exercise capacity is related to cognitive function. Further studies need to reveal if increased PA could mitigate these late outcomes of preterm birth.

Exercise capacity and physical activity could be affected by pulmonary function. Children born preterm may develop bronchopulmonary dysplasia (BPD) in infancy, a chronic lung disease which may affect respiratory function through childhood and into adult life. The diagnostic criteria for the disease lack objectivity and may not reflect the level of respiratory function. To test the utility of a physiological definition of BPD severity, 200 infants born very and extremely preterm had a modified oxygen reduction test at 36 weeks postmenstrual age. Values of shift (kPa), ventilation:perfusion ratio, and shunt (%) were derived from analysis of the shape and position of the saturation (SpO2) vs pressure of inspired oxygen curve, using a dedicated computer program. Shift was shown to be the most useful measure, approximately corresponding to the extra supplemental oxygen required for a sick infant to achieve the same SpO2 as a respiratory healthy infant. We conclude that shift could provide a physiological based, continuous outcome measure of BPD severity, with the possibility to increase objectivity of BPD diagnostics. More studies are needed to evaluate short-term repeatability and to understand the prognostic value.