Relationship among EMG and contractile responses of the diaphragm elicited by hypotension.
In a canine model, we investigated the effects of severe hypotension on the indexes of diaphragmatic failure. We measured 1) the transdiaphragmatic pressure obtained in response to 20- and 100-Hz stimulation of phrenic nerves (Pdi20 and Pdi100), 2) the power spectrum of diaphragmatic electromyogram (EMG), 3) the ratio of integrated diaphragmatic EMG to Pdi (Edi/Pdi), and 4) the rate of relaxation of Pdi100 and Pdi20. Arterial blood pressure (Pa) was reduced to 40-50 mmHg by a balloon inflated in the inferior vena cava and was maintained at this level until Pdi100 declined to 75% of the control value (100% shock time, ST). A recovery period of 60 min at normal Pa was allowed. During hypotension, Pdi100 and Pdi20 declined only at 100% ST [95.0 +/- 13.0 (SE) min]; however, only Pdi100 recovered within 15 min. The power spectrum shifted to low frequencies early and progressively during shock period. Edi/Pdi rose significantly at 80 and 100% ST and recovered within 15 min. The relaxation rate of Pdi20 and Pdi100 increased significantly at 100% ST only. We conclude that 1) diaphragmatic contractility is depressed during severe hypotension, 2) changes in the power spectrum occurred first in the shock state, followed by alterations in Edi/Pdi, and subsequently both changes in the frequency-pressure curve and relaxation rate occurred last.
Hussain
S N
SN
Department of Medicine, Royal Victoria Hospital, Montreal, Quebec, Canada.
Marcotte
J E
JE
Burnet
H
H
Collett
P
P
Roussos
C
C
D000818:Animals; D001794:Blood Pressure; D003964:Diaphragm / Q000503:physiopathology*; D004285:Dogs; D004576:Electromyography; D007022:Hypotension / Q000503:physiopathology*; D007700:Kinetics; D009119:Muscle Contraction; D009126:Muscle Relaxation; D010791:Phrenic Nerve / Q000502:physiology