Regulation of Heart Pumping

(1) INTRINSIC cardiac regulation of pumping in response to changes in volume of blood flowing into the heart (Frank-Starling Law)

(2) Control of heart rate and strength of heart pumping by ANS

Frank-Starling Law

• “Volume of blood ejected by the ventricle depends on the volume present in the ventricle at the end of diastole”

• Underlying principle – Length-tension relationship in cardiac

muscle fibers• SV & CO correlate directly with

EDV• EDV correlates with VR• CO = VR (FS Law ensures this)• Cardiac muscle normally operates

only on the ascending limb of the systolic curve

Explanation of FS Law

Concept of Contractility

• Inherent cardiac M Ca++ based ability – INOTROPISM

• Modified by ANS, catecholamines

• Loading situations of the heart• Preload

– Stretch-induced enhancement in contraction» More overlapping of thick & thin filaments» More Ca++ sensitivity of troponin C» More Ca++ release from SR

• After load

Heart Control by ANS

• Sympathetic • NE via action on Beta-1 receptors

– Positive CHRONOTROPIC» Increased HR (increase Phase-4 depolarization)

– Positive IONOTROPIC» Increased force of contraction (increased inward Ca+

+ current during plateau + increases the ability of SR Ca++ pump)

– Positive DROMOTROPIC» Increased conduction velocity through AV node

(increased inward Ca++ current)» Decreased PR interval

– Positive BATHMOTROPIC » Increased excitability of myocardium

Heart Control by ANS

• Parasympathetic– SA node, atria & AV node have supply,

ventricles don’t!– Ach via muscarinic receptors

• Negative chronotropic» Decreasing phase-4 depolarizations

• Negative dromotropic• Negative ionotropic

• Vagal escape

Determinants of Performance of Heart as a Pump

• 4 factors: ‘Loading’ conditions of the cardiac muscle

(1) Preload, or the initial length to which the muscle is stretched prior to contraction

(2) Afterload, or all the forces against which cardiac muscle must contract to generate pressure and shorten

‘Extrinsic’ factors

(3) Contractility, or inotropic state

(4) Inotropic effect of increased heart rate (beats/min)

S-A Nodal Action Potential– IcaL (long-lasting)

– IcaT (transient)

– Firing potential:

-40 mv– Hyperpolarization

Cardiac Impulse• Initiated in SA node• Spreads radially into atrial muscle mostly @ 0.3 m/sec• Atrial conduction is done via bands of fibres

• Anterior• Middle• Posterior

• Arrives at AV node after 0.03 sec• AV delay of 0.13 sec occurs

• 0.09 in bundle• 0.04 in bundle of HIS• Reason for delay?• Benefit of delay?

• Total delay at this point is 0.16 sec

Cardiac Impulse

• After AV node – Velocity is maximum– Bundle of HIS – 1 m/sec– Purkinje system – 4 m/sec

• From the top of septum – via purkinje system – all of ventricle – 0.06 – 0.1 sec

• Total duration: 0.22 sec• Parts that are last depolarized

• Posterobasal portion of left ventricle• Pulmonary conus• Upper most part of septum

Normal ECG

• ECG is produced only when current flows through the heart and this occurs only when the heart is partially depolarized/polarized

Normal ECG• P wave

• Atria depolarize before contraction

• 0.08 - 0.10 sec• QRS complex

• Ventricles depolarize before contraction

• 0.06 – 0.10 sec• T wave

• Ventricles repolarize• Atrial T wave is obscured by

QRS• Duration normally not taken

• U wave• Inconstant finding• Slow repolarization of

papillary muscles

Normal ECG• PR interval – 0.16 sec

• Time b/w beginning of P wave and beginning of QRS complex

– Interval between the beginning of electrical excitation of the atria and the beginning of excitation of the ventricles

» Prolonged: Vagal stimulation, AV block

» Shortened: Accelerated AV conduction, sympathetic stimulation

• ST interval (QT minus QRS) – 0.32 sec• Ventricular repolarization

• Q-T interval – 0.2-0.40 sec • Ventricular depolarization and ventricular repolarization• Corresponds to AP duration

» Prolonged: ventricular extrasystole

Dipole

• The electric dipole consists of two equal and opposite charges, +q and –q, separated by a distance d

• Dipole vector:• Vector whose magnitude is equal to the dipole

moment [voltage] and that points from –ve charge to + one

• Direction of dipole is from –ve towards +ve

1. A wave of depolarization heading toward the +ve electrode is recorded as a +ve voltage

• Represents Atrial & Vent. Depol.2. A wave of repolarization moving away from a +ve electrode

produces a +ve voltage difference• T-wave (Vent. Repol.)

3. A wave of repolarization moving toward a +ve electrode produces a –ve voltage deflection

• Atrial Repol.

Vectors and Mean Electrical Axis

• Individual waves of depol. – electrical vectors

• Summation of electrical vectors at any instance – mean electrical vector