IUGR, Respiratory Muscle Function, and Exercise Capacity in Childhood

Introduction

Epidemiological evidence supports the existence of a link between intrauterine growth restriction (IUGR) and reduced exercise capacity in young adulthood. Prolonged intrauterine hypoxia results in redistribution of fetal cardiac output towards the vital organs at the expense of skeletal muscles, and experimental data show that the muscles of animals exposed to IUGR may suffer permanent structural and functional changes. Prematurity, which often accompanies IUGR, is also associated with reduced exercise capacity later in life, supposedly due to airflow limitation, air trapping and/or reduced gas-exchange capacity.

The respiratory muscles (RM) play a central role in the development (and perception) of locomotor muscle fatigue, which represents the major determinant of exercise limitation in otherwise healthy subjects. Heavy and/or sustained respiratory work leads to accumulation of metabolites in RM and triggers reflexes that increase the sympathetic vasoconstrictor outflow to the skeletal muscles, thus promoting fatigue. In support of the above concept, targeted RM training has been shown to increase the fatigability threshold and improve exercise performance.

RM function can be assessed by means of strength and endurance, which can be estimated non-invasively by the maximum inspiratory and expiratory pressure (Pimax and Pemax), and the tension-time index (TTImus), respectively. TTImus is a composite parameter that reflects the balance between the capacity of RM and the load imposed upon them; high TTImus values indicate low endurance and increased risk of respiratory fatigue. In adults, impaired exercise tolerance is associated with reduced RM strength and endurance, both in normal and pathological conditions. However, similar data are not available in children.

Hypothesis and objectives

The study hypothesis is that IUGR may have long-term effects on RM function, thus leading to reduced exercise capacity later in life. Other factors, such as the presence and degree of respiratory dysfunction, the performance of the skeletal muscles and the nutritional status, may also be involved. The study objective is to investigate the above hypothesis by comparing RM function and cardiopulmonary exercise testing (CPET) parameters between school-aged children exposed to IUGR and healthy controls, taking also into account the aforementioned confounders.

Methods

Population

In this case-control study, 50 school-aged children (7-10 years old) exposed to IUGR (birth weight \<10th percentile \& fetal ultrasound documentation) will be compared with 100 matched for age and gestational age controls. The study will be performed in the Pediatric CPET laboratory of the University Hospital of Patras, Greece, during a 2-year period (2018-2020). Participants will be recruited from the long-term follow-up program offered to all children who are born preterm or with IUGR, and from the local schools (healthy full-term controls). The protocol will be approved by the hospital Ethics Committee and parental informed consent will be obtained prior to enrollment.

Protocol

After a thorough review of the medical history, participants will undergo the following tests:

1. Nutritional status, body composition and skeletal muscle strength. Initially, weight and height will be measured, and the body mass index (BMI) will be calculated. Body composition (muscular mass, body fat, water) will be determined by the InBody 270 Body Composition Analyzer (Biospace, Seoul, Korea) using bioelectrical impedance analysis. Skeletal muscle strength (grasping power) will be measured using a digital grip dynamometer (Grip-D, TAKEI, Japan).
2. Lung function measurements. Spirometry, measurement of lung volumes (helium dilution technique) and measurement of lung diffusion capacity for carbon monoxide (DLCO) will be performed prior to CPET using the Jaeger MasterScreen PFT device (CareFusion, San Diego, USA). Spirometric measurements will be repeated at 5, 10, and 15 minutes after CPET.
3. RM function. Pimax, Pemax, airway pressure at 100 msec after occlusion (P0.1), and Ti and Ttot will be measured by the Micro 5000 device (Medisoft, Sorinnes, Belgium) according to the guidelines. TTImus will be calculated as (Pimean / Pimax) x (Ti / Ttot), where Pimean is the mean airway pressure resulting from the formula Pimean = 5 x P0.1 x Ti. RM function will be determined a) prior to CPET, b) during CPET when the anaerobic threshold (AT) will be reached, and c) after CPET, when heart rate (HR) and oxygen consumption will be normalized (recovery period).
4. CPET. CPET will be performed by the Ultima CPX system (Medgraphics, St. Paul, USA), using a cycle ergometer and according to a standardized protocol11 and the established guidelines. The following parameters will be recorded: total work in Watts, maximum HR, maximum oxygen consumption (VO2max), AT indices (work, HR, VO2 ) and duration of recovery.

Statistical analysis

Between-group comparisons will be performed with Student's t or Mann-Whitney U test, as appropriate. Linear regression analysis will be used to explore the relationship between RM function and CPET parameters, after adjustment for nutritional status, body composition, lung function, and prematurity. The trend of Pimax, Pemax, and TTImus changes during CPET (baseline - AT - recovery) will be also assessed and compared between groups. The analyses will be performed using the IBM SPSS version 23.0 (IBM Corp., Armonk, NY).

Innovation and implications

The study will be the first to explore whether IUGR is associated with impaired exercise tolerance in childhood due to RM dysfunction, while taking into account the confounding effect of prematurity, impaired lung function, body composition and nutritional status.

Should the relationship IUGR - RM dysfunction - exercise limitation be confirmed, it will provide new insights on the long-term effects of IUGR; impaired exercise tolerance may lead to reduced physical activity, thus enhancing the well-known metabolic and cardiovascular consequences of IUGR later in life. In this regard, the findings of this study may assist in identifying children at risk and planning targeted strategies to improve exercise capacity in this vulnerable population.