Effects of respiratory muscle work on blood flow distribution during exercise in heart failure

Abstract

Heart failure (HF) patients have a reduced cardiac reserve and increased work of breathing. Increased locomotor muscle blood flow demand may result in competition between respiratory and locomotor vascular beds. We hypothesized that HF patients would demonstrate improved locomotor blood flow with respiratory muscle unloading during activity. Ten patients (ejection fraction = 31 +/- 3%) and 10 controls (CTL) underwent two cycling sessions (60% peak work). Session 1 (S1): 5 min of normal breathing (NB), 5 min respiratory muscle unloading with a ventilator, and 5 min of NB. Session 2 (S2): 5 min NB, 5 min of respiratory muscle loading with inspiratory resistance, and 5 min of NB. Measurements included: leg blood flow (LBF, thermodilution), cardiac output (Q), and oesophageal pressure (P(pl), index of pleural pressure). S1: P(pl) was reduced in both groups (HF: 73 +/- 8%; CTL: 60 +/- 13%, P < 0.01). HF: Q increased (9.6 +/- 0.4 vs. 11.3 +/- 0.8 l min(-1), P < 0.05) and LBF increased (4.8 +/- 0.8 vs. 7.3 +/- 1.1 l min(-1), P < 0.01); CTL: no changes in Q (14.7 +/- 1.0 vs. 14.8 +/- 1.6 l min(-1)) or LBF (10.9 +/- 1.8 vs. 10.3 +/- 1.7 l min(-1)). S2: P(pl) increased in both groups (HF: 172 +/- 16%, CTL: 220 +/- 40%, P < 0.01). HF: no change was observed in Q(10.0 +/- 0.4 vs. 10.3 +/- 0.8 l min(-1)) or LBF (5.0 +/- 0.6 vs. 4.7 +/- 0.5 l min(-1)); CTL: increased (15.4 +/- 1.4 vs. 16.9 +/- 1.5 l min(-1), P < 0.01) and LBF remained unchanged (10.7 +/- 1.5 vs. 10.3 +/- 1.8 l min(-1)). These data suggest HF patients preferentially steal blood flow from locomotor muscles to accommodate the work of breathing during activity. Further, HF patients are unable to vasoconstrict locomotor vascular beds beyond NB when presented with a respiratory load.

Figure 1. Influence of inspiratory pressure assistance and inspiratory loading on the pleural pressure volume response during steady-state exercise

A, inspiratory assistance in CTL participants. B, inspiratory assistance in HF patients. C, inspiratory loading in CTL participants. D, inspiratory loading in HF patients. Baseline measurements represent an average of the normal breathing under room air conditions before and after the experimental condition.

Figure 2. Influence of inspiratory pressure assistance and inspiratory loading on pulmonary pressures during steady-state exercise

Figures represent absolute values over time for all exercise conditions. * indicates a significant difference between the HF and CTL groups (P < 0.05), # indicates a significant difference from the preceding exercise condition within the CTL group (P < 0.05), and † indicates a significant difference from the preceding exercise condition within the HF group (P < 0.05).

Figure 4. Influence of inspiratory pressure assistance and inspiratory loading on leg blood flow and haemodynamics during steady-state exercise

Figures represent absolute values over time for all exercise conditions. * indicates a significant difference between the HF and CTL groups (P < 0.05), # indicates a significant difference from the preceding exercise condition within the CTL group (P < 0.05), and † indicates a significant difference from the preceding exercise condition within the HF group (P < 0.05).

Figure 3. Influence of inspiratory pressure assistance and inspiratory loading on cardiovascular haemodynamics during steady-state exercise

Figures represent absolute values over time for all exercise conditions. * indicates a significant difference between the HF and CTL groups (P < 0.05), # indicates a significant difference from the preceding exercise condition within the CTL group (P < 0.05), and † indicates a significant difference from the preceding exercise condition within the HF group (P < 0.05).

Figure 5. Relationship between the percentage change in mean inspiratory P_pl and the percentage change in LBF/ during the transition from baseline exercise to inspiratory assistance in the HF patients

This relationship demonstrates the impact of a reduction in the work of breathing on LBF in HF patients.