INTRODUCTION: Thoracic loading increases physiological strain and is accompanied by modified, and sometimes, impeded ventilation. However, the combined effects of the restrictive and inertial forces accompanying thoracic loading on the elastic properties of the entire respiratory system are unknown. Therefore, we assessed that impact by varying both the size and distribution of loads around the chest wall using a body armour and backpack ensemble.
METHODS: Eleven males (age: 27.27 [SD 4.78] y, height: 182.12 [SD 6.74] cm; mass: 79.55 [SD 8.42] kg) participated in four treatments: a control (clothing only), one load with a 50:50 mass distribution (anterior:posterior; 35 kg) and two loads with a 25:75 mass distribution (35 and 54 kg). An oesophageal balloon (10 cm) was positioned behind the right atrium and used to estimate intrapleural pressure. For each treatment, seated subjects performed static respiratory pressure volume relaxation manoeuvres from residual volume to total lung capacity. Transpulmonary, transthoracic and transrespiratory static pressure-volume data were collected. Curve fitting permitted the post hoc attainment of tissue compliance for the lung tissue, chest wall and total respiratory system over end-expiratory to end-inspiratory lung volumes. Values are reported as mean (± SEM).
RESULTS: Increasing thoracic loading with 25:75 mass distribution reduced lung-tissue (control: 4.91 ± 0.68, 35 kg: 3.82 ± 0.26, 54 kg: 3.04 ± 0.31 L.kPa-1, p < 0.05), chest-wall (control: 4.50 ± 0.49, 35 kg: 3.14 ± 0.30, 54 kg: 2.81 ± 0.46 L.kPa-1, p < 0.05) and total respiratory compliance (control: 2.64 ± 0.26, 35 kg: 1.65 ± 0.10, 54 kg: 1.40 ± 0.09 L.kPa-1, p < 0.05). Varying the load distribution of 35 kg did not influence either lung-tissue or chest-wall compliance (p > 0.05). Nonetheless, the 25:75 distribution decreased compliance of the total respiratory system compared to the 50:50 distribution (50:50 loading: 1.92 ± 0.10, 25:75 loading: 1.65 ± 0.10 L.kPa-1, p < 0.05).
CONCLUSIONS: The current observations demonstrate that the compliance of the total respiratory system was firstly reduced in a mass-dependant manner, and further modified by redistributing a greater proportion of a given mass onto the back. Such mechanical perturbations to the respiratory system are likely to elevate the elastic work of breathing under conditions in which thoracic load carriage already elicits significant increases in ventilation.