Acute Cardiac Responses to Respiratory Muscle Unloading at Different Exercise Intensities

Abstract

Respiration is accomplished by alterations in intrathoracic pressure (ITP) and has physiological implications on the heart. For example, the negative ITP during inspiration is transmitted to the right atrium, which augments venous return, preload, stroke volume (SV), and cardiac output (Q). We sought to determine the impact of respiration on Q during semi-supine cycle exercise by using a proportional assist ventilator to attenuate ITP changes and the work of breathing (Wb). Thirteen healthy participants (6 females) completed three discrete exercise trials at 30%, 60% and 80% peak power (Wmax) with unloaded and spontaneous breathing. Intrathoracic and intraabdominal pressure were measured with balloon catheters placed in the esophagus and stomach. Stroke volume was determined via echocardiography and the Simpson’s biplane method. An electrocardiogram measured heart rate (HR) and a customized metabolic cart measured ventilatory and mixed expired variables such as ventilation and oxygen consumption (V̇O2). Mean esophageal pressure was greater during unloaded relative to spontaneous breathing at all exercise intensities (p<0.0001). Esophageal pressure swings per breath (between spontaneous and unloaded breathing) were different at 30%, 60% and 80%; (-3.5±3.4 vs. -6.8±6.4 vs. -11.9±7.9 cmH2O, respectively (p=0.01). However, the decrease in Wb was not different between exercise intensities (39±22 vs. 46±14 vs. 51±14% from spontaneous breathing for 30%, 60%, and 80%Wmax, respectively, all p>0.05). Cardiac output decreased during unloaded breathing by -1.2±1.3 vs. -1.7±1.4 vs. -1.8±2.0 L min-1 from spontaneous breathing at 30%, 60% and 80%Wmax, respectively (all p<0.05). Heart rate decreased during unloaded breathing by -2±3 vs. -6±4 bpm at 60% and 80%Wmax, respectively (both p<0.05), with no change at 30%Wmax (p=0.2). Stroke volume decreased during unloaded breathing by -10.7±11.2 vs. -10.1±10.2 vs. -8.0±12.3 mL from spontaneous breathing at 30%, 60% and 80%Wmax, respectively (all p<0.05). Oxygen consumption decreased during unloaded compared to spontaneous breathing at 80%Wmax (2.5±0.6 vs. 2.6±0.7 L min-1, p=0.002) with no change at 30% and 60%Wmax (both p>0.05). In summary, attenuating ITP swings resulted in a reduction in Q at all exercise intensities. At 30%Wmax, the decrease in Q may be due to a reduction in SV. At 60%Wmax, Q decreases likely because of a reduction in SV and HR. At 80%Wmax, Q may decrease due reductions in SV, HR and V̇O2. In conclusion, the normally occurring swings in ITP developed during spontaneous breathing is helpful for maintaining cardiac function during exercise.