Optoelectronic plethysmography to evaluate the effect of posture on breathing kinematics in spinal cord injury: a cross sectional study

BACKGROUND: Spinal cord injured patients often suffer from respiratory muscles impairment. Spirometry studies showed that in supine position vital capacity increases in such patients since diaphragm increases its inspiratory excursion. To our opinion, however, respiratory kinematics in spinal cord injured patients is disadvantaged in supine position. AIM: To evaluate the effect of posture (sitting and supine) on respiratory kinematics in chronic spinal cord injured patients using optoelectronic plethysmography. DESIGN: Cross-sectional study. SETTING: Outpatients referring to the Movement Analysis Laboratory of a Physical and Rehabilitation Medicine Unit. POPULATION: Twenty chronic spinal cord injured patients (9 tetraplegics, with lesional level ranging from C3 to C7 and 11 paraplegics with lesional level ranging from T1 to T8) and twenty healthy subjects matched for gender, age and smoking habits. METHODS: All subjects underwent optoelectronic evaluation in sitting and supine position during quiet breathing and hyperventilation. Additional trials were performed to derive respiratory functional parameters (vital capacity and forced expiratory volume in the first second) in sitting and in supine position. Compartmental volumes and respiratory functional parameters were analyzed by means of analysis of variance. Post-hoc comparisons by means of t-tests were performed to analyze differences within and between study groups (spinal cord injured patients and healthy subjects, paraplegics and tetraplegics). Phase angle analysis and Konno and Mead diagrams were performed to evaluate if thoracic and abdominal compartments were moving in synchrony during breathing and the results were compared by paired t-tests. RESULTS: Supine position increases vital capacity and forced expiratory volume in the first second. This could be due to the more favorable length of the diaphragm in supine than in sitting position. However in such posture the phase shift between thorax and abdomen during breathing increases. CONCLUSION: Optoelectronic plethysmography measurements showed that even if in supine position there is an improvement in respiratory functional parameters, the respiratory kinematics of the chest wall is disadvantaged. CLINICAL REHABILITATION IMPACT: Our study suggests that the use of abdominal binders could reproduce in sitting position the positive effect of supine position on diaphragm, that could work at a more favorable point of its length tension curve.