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

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Abstract
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-Pco2 relationship, end-tidal Pco2, minute ventilation (V̇e), respiratory frequency (fB), and RMSdi were measured in seven healthy subjects on two occasions during steady-state ventilation at seven levels of inspired O2 fraction (FiCO2) 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 FiCO2 of 0.04, and during an inspiratory capacity maneuver (RMSdi%max) as well as ECG R-wave amplitude at functional residual capacity (RMSdi/ECGR), fB, and thickness of the costal diaphragm measured by ultrasound. RMSdi increased linearly with Pco2 (mean r2 = 0.83 ± 0.10); at the highest FiCO2, RMSdi%max was 40.2 ± 11.6%. Relative to the intersubject variation in the V̇e-Pco2 relationship, intersubject variations in the slopes and intercepts of the RMSdi-Pco2 relationships were 1.7 and 1.8 times, respectively, and RMSdi%max-Pco2 relationships 0.9 and 1.3 times, respectively, and were unrelated to fB and diaphragm thickness. Relative to the day-to-day variation in the V̇e-Pco2 relationship, day-to-day variation in the slopes and intercepts of the RMSdi-Pco2 relationships were 2.8 and 4.4 times, respectively, and RMSdi/ECGR-Pco2 relationships 1.3 and 2.2 times, respectively. It was concluded that the RMSdi-Pco2 relationship measures chemosensitivity and is best compared between subjects via RMSdi%max and on separate occasions in the same subject via RMSdi/ECGR.