2008B15 Describe the changes that occur with ageing that can affect
oxygen delivery to the tissues during moderate exercise.

Ageing

·   Gradual time-dependent loss of cellular function and physiological reserve, resulting in death

Moderate exercise

·   3-6 METs = VO₂ 750-1500mL/min

·   e.g. walking up stairs

·   Predominantly aerobic metabolism

·   Dependent upon oxygen delivery

Oxygen delivery

·   DO₂ = CO x CaO₂

·   Where CaO₂ = [Hb] x 1.34 x SaO₂ + 0.03 x PaO₂ (per L)

Summary

·   ↓Cardiac output

o Resting CO: ↓1% per year with age

o Max CO: more substantial decrease

o Decline attenuated with training

·   ↓Oxygen content

·   Hence decreased reserve

↓Preload (PL)

·   ↓Blood volume (↓lean mass)

·   ↓Skeletal muscle pump (sarcopaenia)

·   ↓LV compliance (LVH)

↑Afterload (AL)

·   ↓Aortic compliance, ↓Windkessel effect

o Media thickened, less elastin

o Intima thickened, fibrotic

↓Rate (HR)

·   ≈ 220 – (age x 0.7)

·   Decreased sympathetic response

o ↓β1 Adrenoceptors

o ↓Post-adrenoceptor signalling

o ↓SNS response to exercise, ↓baroreceptor reflex

o Partly offset by ↑circulating noradrenaline and ↓noradrenaline reuptake

·   Impaired conduction

o ↓Conduction velocity

o ↓Pacemaker cells in SA and AV nodes (50% age 20, 10% age 75)

o Fatty, fibrous infiltration of conduction system

↓Contractility (CY)

·   ↓Myocyte number

o 2° to apoptosis, necrosis

·   Partly offset by concentric hypertrophy

o 2° to ↑afterload

Comorbidities

·   Hypertension: ↑SVR -> ↑AL

·   Coronary artery disease ->↓O₂ supply -> ↓work

·   Cardiomyopathy: ↓contractility

·   Severe valvular disease (e.g. aortic stenosis) -> fixed stroke volume

·   Atrial fibrillation -> ↓preload (especially important with poorly compliant LV)

↓Haemoglobin

·   Senile anaemia

o ↓Bone marrow function

·   Increased risk of renal dysfunction

o ↑pO₂ in renal medullary interstitium -> ↓EPO release

↓V_A max

·   ↓Respiratory muscle strength

o Respiratory muscle sarcopaenia

·   ↑Work of breathing

o Senile emphysema -> ↑lung compliance -> ↑dynamic compression

o Ossification of costal cartilage -> ↓chest wall compliance

↑V/Q mismatch

·   Emphysema -> ↓lung compliance -> ↓radial traction -> ↑alveolar collapse

·   Closing capacity exceeds FRC when upright at age 66

↓Diffusion rate

·   ↑Thickness (fibrosis)

·   ↓Area (senile emphysema)