Heartbeat, Cardiac Physiology, BP & Exercise

Heartbeat, Cardiac Physiology, BP & Exercise

1. Control of rate

   • Act Pot of ventricular myocyte
     • Refractory period - when cell is depolarised, it is inexcitable. Only generates single twitch - tetany = fatal
   • Pacemaker Potential sets heart rate
     • (slope from most -ve value to threshold)
     • +ve/-ve CHRONOtropism (natural - whereas tachy/bradycardia not))
     • Chronotropism = heart rate altered Inotropism = force (contractility) altered Dromotropism = conduction speed altered - AV node conduction) Lusitropy = relaxation rate altered (filling time)
   • Conduction through heart
   • ECG
     • Act pot sets up electrical field around heart (depolarisation moves along membrane) :) non-invasive
     • Einthoven's triangle > LEADS I, II, III
     • 12-lead ECG
       • Waves & Intervals
     • Cardiac arrhythmias
     • Electrical axis wave (II) lies between I & aVF. Allows understanding of electrical axis of heart. Altered by eg.hypertrophy of L ven > shift apex L

2. Cardiac Physiology

   • CARDIAC CYCLE
     • https://en.wikipedia.org/wiki/Cardiac_cycle#/media/File:Cardiac-Cycle-Animated.gif
     • Stroke Volume
       • This represents the amount of blood pumped at each beat (EDV-ESV = SV)
       • Affected by intrinsic (TPR, Filling pressure) & Extrinsic factors (Angiotensin II, NA)
     • CO (l.min-1) = SV (l.beat-1) x HR (bpm)
   • CONTRACTILITY
     • Sarcomere strength & tension RELATED
       • Sarcomere over/understretched - myosin heads less likely to bind to actin - needs to be OPTIMAL RESTING LENGTH
       • AFTERLOAD: wall stress opposing shortening of muscle fibres ie. Tension that needs to be developed by muscle for systole.
         • AKA arterial pressure
       • PRE-LOAD: wall stress in resting myocardium ie. Tension that has been developed during diastole (ven filling) *volume changes this.
         • AKA atrial pressure, ‘filling pressure’ or end diastolic pressure (EDP)
       • LAPLACE's law links: Pressure, Tension, Stress, Wall thickness, Radius
     • STARLING CURVE - describes contractility
       • The ENERGY of CONTRACTION of cardiac muscle is proportional to INITIAL FIBRE LENGTH AT REST
         • Showed SV linked to CVP as CVP returns blood to heart - links systemic & pulmonary circ
     • ^ by Sympathetic: Accelerate HR & conduction, strengthen contraction
       • Postganglionic neurotransmitter: NORADRENALINE. Acts on β-adrenoceptors (mostly β1)
         • NA mechanism
       • +ve inotropes: noradrenaline, adrenaline, digoxin
       • -ve inotropes: Ca channel blockers, β-blockers
   • v by Parasymp: Slow HR & conduction. NOT contractility.
     • Postganglionic neurotransmitter: ACETYLCHOLINE. Acts on muscarinic receptors.

3. Blood Pressure

   • D'Arcy's Law (know already!) is FLOW Q = Change in PRESSURE / RESISTANCE
     • Apply to CSV by Q = MAP - CVS(usually 0) / TPR
     • Combine with Poiseuille’s Law - brings in VISCOSITY, LENGTH, RADIUS
       • Doubling the radius of a vessel will increase flow 16 times!
         • Laminar / Turbulent / Single-file flow
   • How to record
     • Arterial BP (systolic/diastolic) - catheter into peripheral artery
       • Pulmonary arterial BP (systolic/diastolic)- catheter into venous system, through R of heart, into pulmonary artery
         • CVP - catheter into venous system at junction of SVC and R atr. NB. CVP oscillates = interpleural pressure changes during resp cycle. CVP also changes with cardiac cycle (A, C, V waves - not discernable)
           • CO - inject cold saline into R atr. Changes in temp of pulmonary artery - recorded. Large CO 'dilutes' cold saline more effectively.
     • [Use these to calculate TPR, PP, SV, MAP[
   • Ultrafiltration
     • Starling Equilibrium
       • Plasma oncotic pressure = proteins (albumen) in blood vH20 conc, so water moves into capillary. (NB. This opposes hydrostatic blood pressure which pushes H20 and small molecules out of blood, into interstitia)
         • SUMMARY OF STARLINGS EQUILUBRIUM
       • Reflection coeffient = permeability to plasma proteins
       • Oedema = excess of tissue fluid (affects metabolite & gas exchange)
         • Lymphoedema = damage to nodes (eg. testicular cancer surgery)
           • Elephantiasis = parasitic nematode worm blocks drainage
         • Local oedema = Venous or lymphatic obstruction. ^capillary permeability > whealing/ inflammation
         • Generalised oedema = heart failure (^CVP) OR Hypo-proteinaemia (vOncotic pressure from malnutrition/glomerulonephritis/liver disease)
       • Pc & Pi (Capillary & Interstitial Pressure)
   • Regulating BP
     • Baroreceptor Reflex
       • High pressure ^firing rate. Sympathetic output from medulla ^HR & ^TPR = MAP recovers
       • Orthostasis. Supine = Blood evenly distributed. Standing = venous blood pools in legs
         • Stand-up = vCVP & vSV (Frank-Starling mechanism). This vCO & vTPR (see equations). "Transient hypotension"
           • Baroreceptors -ve feedback
             • VasoCONSTRICTION / VasoDILATION
               • Metabolic / Reactive Hyperaemia - increased blood flow to remove metablytes
         • WHAT ELSE AFFECTS BP?
           • Emotive effects
             • Vasovagal syncope - Increase parasymp > bradycardia & vasodilation
             • White-coat hypertension - anxiety > Increase symp ^HR ^TPR = ^BP
             • MAP=CO×TPR CO=HR×SV
           • Circadian Rhythms - BP higher in morning. ^with psychogenic factors/posture/sex/pain
           • Exericse - see above
           • Acute & Long-term affectors

4. Effects of Exercise

   • THE FICK PRINCIPLE - O2 uptake = flow of oxyblood - flow of deoxblood
     • As 'flow' is related to 'CO', this links CO & O2 uptake
   • ACUTE effects
     • Metabolic hyperaemia
       • BP should return to normal within a few hours after exercise. Shorter after aerobic exercise (10-20mins), longer after anaerobic exercise (few hours) as the heart continues to remove lactate and CO2.
     • Dynamic vs Static exercise - diff in diastolic
     • ALSO Central command response = ^HR in anticipation of exercise
   • LONG-TERM effects (aerobic training)

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