Background
Every year 17 million people experience their first stroke and 7 million will die of stroke worldwide. There are 1.2 million stroke survivors in the UK alone.1 Most stroke deaths are caused by complications, usually chest infections. Giving prophylactic antibiotics to stroke patients with unsafe swallow did not reduce incidence of poststroke pneumonia in a recently published, large multicentre study. Understanding impaired cough physiology post stroke is very important as we seek to reduce respiratory morbidity and mortality in this large patient group.2
Cough can help clear aspirated material from the lungs; weak (or absent) reflex and voluntary cough in stroke patients are both significantly associated with development of chest infections.3–5 Acute hemiparetic stroke patients have impaired voluntary and reflex cough flows4 6–8 and the more severe the patient’s stroke, the worse the cough flow.6 Reduced functional residual capacity (FRC) could cause impaired cough flows due to impaired flow volume characteristics of the deflating lungs and due to the effect of reduced lung volume on the length and thus the pressure-generating capacity of the expiratory muscles.9
In a previously published study of acute stroke patients, we showed that three predictors (stroke severity score, height and FEV1/FVC ratio) accounted for two-thirds of the variability in voluntary cough flow.6 We concluded that there must be additional factors implicated in variability of cough flow after stroke and one factor could be FRC. It is known that the volume of air in the lungs, immediately prior to an expulsive man oeuvre, has a linear relationship with the flow.10
Previous data on FRC in acute stroke are not available. There are very few published data on FRC in stroke patients, probably due to the difficulty of making measurements in this group. FRC could become reduced as a consequence of previously described chest abnormalities in stroke patients such as elevation of the diaphragm on the paretic side11 12; reduced movement of the diaphragm during inspiration13; weak respiratory muscles8 14 15; impaired corticofugal pathways to the diaphragm16 17 or flattening or reduced movement of the chest wall on the weak side.18–20 Stroke patients can spend a lot of time in reclined positions so the effect of posture on lung volumes is also relevant. FRC has been shown to fall by between 20% and 29% when healthy participants move from the seated to supine position.21 22
In this study, we evaluated FRC in acute hemiparetic stroke patients, with the objective to measure FRC and cough flow in acute stroke patients and compare them with healthy controls, as well as investigate the relationship between FRC and cough flow.
The null hypothesis was that FRC, measured by helium dilution in the reclined position, would be the same for patients with stroke as for a group of healthy controls. As a secondary hypothesis we sought to confirm that stroke patients have impaired voluntary cough flow and reduced air volume inspired prior to cough (cough inspired volume) compared with controls.6 These measurements would enable exploration of the relationship between FRC, cough flow and cough inspired volume. We also decided to measure upright FRC in a subgroup, to look at the effect of position change.