Diaphragm electromyogram root mean square response to hypercapnia and its intersubject and day-to-day variation

Diaphragm activation can be quantified by measuring the root mean square of crural EMG (RMSdi) (Beck J, Sinderby C, Lindstrom L, and Grassino A, J Appl Physiol 85: 1123-1134, 1998). To examine intersubject and day-to-day variation in the RMSdi-Pco(2) relationship, end-tidal Pco(2), minute ventilation (Ve), respiratory frequency (f(B)), and RMSdi were measured in seven healthy subjects on two occasions during steady-state ventilation at seven levels of inspired O(2) fraction (Fi(CO(2))) from 0 to 0.08 in random order. RMSdi was measured with a multielectrode esophageal catheter and controlled for signal contamination and diaphragm position. RMSdi was normalized for values obtained during quiet breathing at functional residual capacity, at Fi(CO(2)) of 0.04, and during an inspiratory capacity maneuver (RMSdi%max) as well as ECG R-wave amplitude at functional residual capacity (RMSdi/ECG(R)), f(B), and thickness of the costal diaphragm measured by ultrasound. RMSdi increased linearly with Pco(2) (mean r(2) = 0.83 +/- 0.10); at the highest Fi(CO(2)), RMSdi%max was 40.2 +/- 11.6%. Relative to the intersubject variation in the Ve-Pco(2) relationship, intersubject variations in the slopes and intercepts of the RMSdi-Pco(2) relationships were 1.7 and 1.8 times, respectively, and RMSdi%max-Pco(2) relationships 0.9 and 1.3 times, respectively, and were unrelated to f(B) and diaphragm thickness. Relative to the day-to-day variation in the Ve-Pco(2) relationship, day-to-day variation in the slopes and intercepts of the RMSdi-Pco(2) relationships were 2.8 and 4.4 times, respectively, and RMSdi/ECG(R)-Pco(2) relationships 1.3 and 2.2 times, respectively. It was concluded that the RMSdi-Pco(2) relationship measures chemosensitivity and is best compared between subjects via RMSdi%max and on separate occasions in the same subject via RMSdi/ECG(R).