Exercise Stress Testing (M/A)

Exercise Testing MA7

Basic Exercise Physiology

Work, Energy, & Power:

Work= Force*Distance unit = Newton-meter (Joule)

-1 Kilocalorie (kcal) = 4.1868 Joues, representing the constant relationship between Energy & Work

Power= work/time, in J/sec = Watt

Types of Exercise

-Isotonic = dynamic

= alternate rhythmic contraction/relaxation of muscles against resistance

-Isometric = static

= muscle contraction agains a fixed resistance, with little/no muscle shortening

Aerobic Exercise

-in light to moderate exercise, O2 is the major contributor to ATP synth

Anaerobic Exercise

-in short sprints, or heavy isotonic and isometric exercise, it can be done without oxygen for a brief period.

-energy via Phosphocreatinine (PCr)-ATP system

In reality, there is a spectrum, where one starts at aerobic and switches to anaerobic as the main source...

Because energy needs oxygen combustion to --> work, there is a relationship between aerobic work adn oxygen consumption (uptake = VO2).

-ppl need 10mL O2/minute per Watt, in a Nl healthy adult

-kids tend to have a higher oxygen cost of work (less mechanical efficiency)

Testing used to be based on steady-state (not changing the intensity of workload), to non-steady state procedures, with either discrete or incremental changes in the workload.

-Note some diffs:

-if you were to suddenly change from walking to running, you will have an instantaneous increase in work, but your VO2 doesn't rise as fast, you haven't yet modified your ventilation and cardiac output to meet the energy demand. Until you do, your muscles must use anaerobic E sources- ATP & PCr (which are replenished by rate-limiting glycolytic pathways) ==> you are in an O2 Debt; this debt is = to the area under the curve of the work increase (see graph). Once the cardiopulm system catches up in vent & CO, it can replenish the debt with more O2 via aerobic respiration...,

-The time course to get to a steady state (where oxygen supply meets demand) is mathematically described by time constant T, such that you need 5-6 time constant T's to get to steady state.

-Exercise Kinetics = describes the process of getting to a steady state after an increase in workload

-there are time constants described for kids for HR and VO2, but not for Cardiac Output

-if you want to use an assumed VO2 for kids when solving Fick, you CANNOT use it if patient isn't in steady state, because we don't have the time constants for CO, just for HR and VO2.

-actually, we often get similar results bn steady and non-steady states, but one must use caution in doing so. Practically speaking though we often use a non-steady state (ramp or incremental) exercise protocols instead of the classic steady state methods.

Four Principal Systems to Transfer O2 fr atm to mitochondria:

-Lungs, Heart, Blood, Muscle

-if any one gets to max fx'l capacity, more exercise will be limited

-Does O2 supply (pulmonary/CV/blood transport), O2 use (substrate availability, ez fx) independent of O2 supply, or converting chemical into mechanical energy (=muscle efficiency) impose the limiting factor on work capacity?

-usually, in a healthy pt, it is bc of O2 supply that is the limiting step, so the circ system, specifically the total O2 transport capacity (which is = to CO times O2 Carrying Capacity) is the limiting factor.

-during a maximal exercise test to exhaustion, most healthy pts stop bc of leg discomfort or fatigue, but some complain of dyspnea. This is bc of leg muscle fatigue, where O2 demand is >O2 supply to the muscle mitochondria

Maximal-Aerobic Power

-Best index to measure exercise capacity is likely the VO2max = the max O2 uptake = max aerobic power, achieved during exercise.

-VO2 max = defined by a plateau in Vo2 that occurs despite continued exercise = proof that work can be done using anaerobic energy production (though this amt of work in anaerobic state is limited)

-but you must ensure the pt continues to exercise to the max in oder to get the right VO2 max

-very hard to do this in kids

-VO2 peak = peak work capacity = symptom or discomfort limited clinical exercise test = voluntary exhaustion

-used instead of VO2 max bc it's hard to get them to VO2 max

-in practice, there's no real diff bn VO2pk and VO2max

-Max CO - closely correlates to VO2 max, but this is debated still

-O2 supply to exercising muscle d/o O2 carrying capacity (d/o Hgb), and rate of transport fr the source to the destination = CO. Mathematically this is Q*O2 (CO * O2 content in bld)

--> decr Hgb will decr VO2max, and vice versa

-exercising a larger muscle group vs a smaller group, will cause a higher VO2 max

--> higher VO2 max achieved w treadmill rather than cycle bc you use more muscles then

-The amt of muscle mass used is important bc the more used dictates the relative contribution of Stroke Volume and HR in determining CO during exercise.

-This accts for diffs bn girls and boys...,

-in teens, boys have a higher VO2max than girls

-How should we index the norms for VO2? much controversy..., index to weight? age?

-we do use wt (mass) but recognize it is limitations (especially w obesity epidemic...)

-our norms likely do not account for stage of puberty at the age they were collected for...

-also they don't acct for how individuals tend to change over time, so comparing to oneself over time is also dangerous...

-race also plays a role- AA kids may have a lower VO2max than Whites in N America, w AA's having slightly smaller lung volumes than Whites of similar height...

Anaerobic Threshold aka Ventilatory Threshold (VT)

-used as a surrogate for VO2max

-Now called VT bc it recognizes that incremental exercise doesn't reflect the onset of anaerobic metab (as we once thought). The VT (also aka ventilatory anaerobic threshold VAT) = VO2 at which there is a disproportionate incr in minute vent (V_E) relative to oxygen uptake

-An incr in mixed expired O2 [ ] is seen at this pt

-there is often a disproportionate rise in lactate production at this pt too, so it was called anaerobic threshold, but an incr in lactate is not necessary for the disprop. rise in V_E

-Methods to check Ventilatory Threshold-

-V-slope method used most in kids- at this pt during incremental exercise, V_E increases out of proportion to VCO2, so that V_E/VCO2 rises,a nd end-tital CO2 partial P starts to fall.

-this is bc of the H+ mediated drive to breathe created by lactic acid accum in the bld (the lactate had outstripped the blood's ability to buffer the H+)

Cardiac Responses to Exercise

-Cardiac output Q rises linearly with increasing VO2 - Q=kVO2+4

-k averages bn 5-7, and the intercept 4 d/o the size of teh subject but is an approx for kids in the testable range. These relationships are taken from steady state exercise studies

-in non-steady state, the linear relationship bn CO and VO2 isn't necessarily maintained- Q levels off as VO2 rises beyond a certain pt, however it is slight enough to ignore

-however there is little data on this in kids...

-CO d/o HR*SV

-SV- d/o PL, AL, and Contractility - can used EDV, BP, SF or EF to check these...

-HR

-HR is the major determinant in increased CO for most ppl, w a ~linear relationship bn HR and work; though not as clearly true for some kids...

-HRmax that can be achieved- important determinant in VO2max, usually is bn 190-205 for 5-20yo pts, and for pts >20yo it is usually 210-(age*0.65). HRmax declines w age- ?why,

-HR Patterns

-Nl curve- gets to HRmax

-Conditioned curve- HR response if pt improves his fitness--> resting HR declines, and training bradycardia seen- HRmax unchanged, but it is at a higher VO2

-Deconditioned curve- resting HR is higher, HRmax at lower VO2

-Submaximal curve- inadequate effort; also seen in pts w chronrotropic insufficiency (low HRmax) seen in many pts w hrt dz.

-Stroke Volume-

-hard to measure, limited peds data so most extrapolated fr adults

-SV changes depend in part on the position in which you do exercise- maximal if supine, w limited rise in SV during exercise;

-in upright position, SV incr mainly early on in exercise, with little afterward

-adults- we say on avg SV plateaus at 30-40% of VO2max