Relationship between maximal sniff nasal inspiratory or maximal inspiratory pressure and arterial pressure of carbon dioxide

Relationship between sniff nasal inspiratory pressure (SNIP) and maximal inspiratory pressure (PI max; percent predicted) and ear lobe partial pressure of carbon dioxide (PCO₂ in kilopascals [kPa]) in amyotrophic lateral sclerosis patients. Open symbols represent patients with bulbar weakness and closed symbols represent patients with limb weakness. The lines represent the lower and upper limits of normal for ear lobe PCO₂ (4.8 to 6 kPa).