Acute β1-Blockade Slows Cardiorespiratory Kinetics And Increases Oxygen Extraction At Onset Of Moderate Intensity Exercise

PURPOSE: Cardio-selective beta-blockade slows pulmonary oxygen uptake kinetics (VO2) at exercise onset; however the contribution of heart rate (HR) and the corresponding impact on active muscle deoxygenation kinetics have not been characterized. The purpose of this study was to determine if β1-blockade slows VO2 kinetics and underlying cardiovascular and active muscle deoxygenation mechanisms during moderate intensity exercise in healthy younger women. METHODS: Thirteen healthy women (age 21-32) were tested on a recumbent cycle ergometer during constant-load cycling transitions from 20 watts to 85% of estimated lactate threshold (81.0 ± 20.7 W) with an intravenous infusion of saline or the fast-acting β1-selective antagonist esmolol in randomized order, separated by 45 minutes. Breath-by-breath VO2 was measured by indirect calorimetry (COSMED, Italy), vastus lateralis deoxygenation (ΔHhb) was measured by near infrared spectroscopy (Artinis, The Netherlands), and heart rate (HR) was measured with EKG. Phase 2 VO2, ∆Hhb, and HR data were modeled with single exponential functions. Comparisons were made using paired t-tests and correlations were tested using Pearson’s correlation coefficient. RESULTS: Esmolol infusion increased the pVO2τ (24.8 ± 8.0 vs. 39.1 ± 11.9 sec, p = 0.006), HR amplitude (26 ± 9 vs. 31 ± 8 bpm, p = 0.004) and ∆Hhb (48.05 ± 46.0 vs. 61.93 ± 53.1, p < 0.008). VO2τ was correlated with HRτ (r = 0.645, p = 0.032) and the reduction in steady state HR (r = 0.738, p = 0.010). pVO2τ was not correlated to ∆Hhbτ (r = -0.133, p = 0.753). CONCLUSIONS: Acute β1-blockade slows pulmonary VO2 kinetics, decreases steady-state heart rate and evokes a larger increase in active muscle oxygen extraction during the onset of moderate intensity leg exercise in young women. These findings expand our understanding of the central and peripheral circulatory effects of esmolol at the onset of exercise and suggest that in healthy women, an acute reduction in oxygen delivery is buffered by a rapid, but lagging, increase in muscle deoxygenation. Supported by NIH Grants R21AG054940, UL1 TR000127, and T32DK120509.