The effects of β1-adrenergic blockade on cardiovascular oxygen flow in normoxic and hypoxic humans at exercise
Ferretti, Guido ; Licker, Marc ; Anchisi, Sara ; Moia, Christian ; Susta, Davide ; Morel, Denis
In: European Journal of Applied Physiology, 2005, vol. 95, no. 2-3, p. 250-259
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- At exercise steady state, the lower the arterial oxygen saturation (SaO2), the lower the O2 return $$(\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2}).$$ A linear relationship between these variables was demonstrated. Our conjecture is that this relationship describes a condition of predominant sympathetic activation, from which it is hypothesized that selective β1-adrenergic blockade (BB) would reduce O2 delivery $$(\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}{\text{aO}}_{2} )$$ and $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} .$$ To test this hypothesis, we studied the effects of BB on $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}{\text{aO}}_{2}$$ and $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ in exercising humans in normoxia and hypoxia. O2 consumption $$(\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{2} ),$$ cardiac output $$(\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}, CO_{2}\; \hbox{rebreathing}),$$ heart rate, SaO2 and haemoglobin concentration were measured on six subjects (age 25.5±2.4years, mass 78.1±9.0kg) in normoxia and hypoxia (inspired O2 fraction of 0.11) at rest and steady-state exercises of 50, 100, and 150W without (C) and with BB with metoprolol. Arterial O2 concentration (CaO2), $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}{\text{aO}}_{2},$$ and $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ were then computed. Heart rate, higher in hypoxia than in normoxia, decreased with BB. At each $$\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{2} ,$$ $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}$$ was higher in hypoxia than in normoxia. With BB, it decreased during intense exercise in normoxia, at rest, and during light exercise in hypoxia. SaO2 and CaO2 were unaffected by BB. The $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}{\text{aO}}_{2} $$ changes under BB were parallel to those in $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}.$$ $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ was unaffected by exercise in normoxia. In hypoxia the slope of the relationship between $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}{\text{aO}}_{2} $$ and $$\ifmmode\expandafter\dot\else\expandafter\.\fi{V}{\text{O}}_{2} $$ was lower than 1, indicating a reduction of $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ with increasing workload. $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ was a linear function of SaO2 both in C and in BB. The line for BB was flatter than and below that for C. The resting $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ in normoxia, lower than the corresponding exercise values, lied on the BB line. These results agree with the tested hypothesis. The two observed relationships between $$\ifmmode\expandafter\dot\else\expandafter\.\fi{Q}\bar{{\text{v}}} {\text{O}}_{2} $$ and SaO2 apply to conditions of predominant sympathetic or vagal activation, respectively. Moving from one line to the other implies resetting of the cardiovascular regulation