Matching CO to Metabolic Requirements Flashcards
Describe dynamic and static exercise.
Dynamic = muscles shorten, rhythmic contractions
Static = muscle contraction without muscle shortening, sustained contraction
What is the relationship VO2 vs work?
Linear. Once VO2Max is reached, a small further increase in work may be performed anaerobically
What limits VO2 Max in most people?
CO
Changes in SBP and DBP?
Increase in SBP.
DBP seen to rise, fall or not change. Variable.
What happens to SV if HR is paced?
SV decreases if HR is simply paced, due to the decreased diastolic time, shown in 6 anaesthetised dogs.
Wessale 1990
What happens to CO if HR is simply paced?
Stays roughly the same due to the decrease in SV.
Wessale 1990
What is the effect of age on the response to dynamic exercise?
Lower max HR
Larger SV, due to increased diastolic filling time
Same CO reached, that which is needed to meet metabolic demands of a given exercise demands.
Younger subjects were able to reach a higher level of work and CO (exercise to exhaustion) cf older subjects
What does baroreflex control of the response to exercise predict?
Metabolic vasodilation of skeletal muscle -> drop TPR and subsequently BP -> reduced artertial baroreceptor firing -> reflex increase in HR and contractility -> increased CO to match reduced TPR -> BP restored close to normal
Predicts a transient fall in BP, a delay before HR rises and final exercise BP at normal or slightly below
What is the actual response to exercise, contrary to the baroreflex theory? Which theories explain this?
Immediate increase in HR
No transient fall in BP
Exercise BP > resting BP
Central command. Locomotor areas of brain feed in brain stem as well as muscles, eliciting the HR, contractility, TPR and BP responses. Krogh and Linard, 1913. Argued that the response was too fast for reflex or blood-borne mechanisms from exercising muscle.
EPR. Feedback from muscles feeds into brain stem…..Alam and Smirk, 1938.
What experimental evidence supports central command theory?
Supported by experiments using epidural anaesthesia which reduce BP response (McCloskey, 1981) at same absolute force. BP responses mirrored relative force, not absolute force following neuromuscular blockade. The muscle is doing the same amount of work, therefore producing same metabolites, it’s centrally where the efforts appears higher. The mirroring in the BP response would suggest that the BP response is centrally controlled, not due to feedback from muscles.
Goodwin, 1972. Increasing CC needed to produce same triceps force when vibrating is applied to the antagonist muscle (stretch reflex) and decreased CC when vibrating agonist muscle. HR and BP responses were reduced/increased respectively presumably in response to RPE and CC.
Hypnosis of people to imagine doing exercise -> HR and BP responses
Which brain areas are involved in CC?
Insular and anterior cingulate regions
What evidence supports EPR?
Post-exercise cuff ischaemia maintains BP response until cuff is released (Alam and Smirk, 1937).
Alam and Smirk, 1938. Exercising with sensitive leg - occlusion post exercise - maintains BP. Exercising with insensitive leg - occlusion post exercise - no BP response maintained.
Note, during exercise of the insensitive leg, still HR/BP response. CC input?
Stimulation of cut ventral root to cause leg muscle contraction in anaesthetised cats. Graded response BP to contraction intensity. Response disappeared when dorsal root was cut. BP response enahnced via occlusion of circulation to exercising muscle (Coote, 1971)
Compare HR vs BP recovery in isometric exercise.
BP fails to fully recover until occlusion is released (? chemoreflex), some recovery suggests CC input.
HR recovers even when occluded, so ? chemoreflex not involved? Only CC or mechano-R?
Which receptors are involved in the HR response to exercise?
Not chemoreceptors as HR recovers immediately post exercise even if muscle occluded. Passive stretch muscle -> HR response
What happens to arterial baroreflex in exercise?
Shifted to the right, to a higher operating point. Some evidence of a small decrease sensitivity.
PI vs HR! PI work found decrease sensitivty, HR did not.
