Cardiovascular Physiology

Dr. Nicole Burns February 14, 2013

The Heart Aorta

Superior vena cava

Pulmonary veins

Right atrium

Right ventricle

Inferior vena cava

Pulmonary artery

Pulmonary veins

Left atrium

Left ventricle

Interventricular septum

Sherwood Fig. 9-4a, p. 303

The Heart Wall

•  Endocardium (inner) – Layer of endothelial cells

•  Myocardium (middle) – Cardiac muscle

•  Epicardium (outer)

•  Pericardium – Double-walled membranous sac

Cardiac Muscle (myocardium)

•  Aerobic muscle

– 99% contractile cells

– 1% autorhythmicity cells •  Self-excitate

–  Intercalated discs

Anatomy of conduction

system Sinoatrial (SA) node

Internodal pathway

Right branch of bundle of His Right

ventricle Purkinje fibers

Left branch of bundle of His

Atrioventricular (AV) node

Interatrial pathway

Sherwood Fig. 9-8, p. 306

Atrioventricular (AV) bundle

Pacemaker cells •  All cardiac pacemaker cells display a

spontaneous electrical rhythm –  Rate is dependent on location within the heart and

speed of the membrane potential drift to threshold

Sinoatrial (SA) node Cardiac pacemaker Intrinsic rate of 80-100 A.P./min Conduction speed of 0.05m/sec Atrioventricular (AV) node Intrinsic rate of 40-60 A.P./min Conduction speed of 0.05m/sec

Bundle of His Intrinsic rate of 20-40 A.P./min Conduction speed of 1m/sec Purkinje fibres Intrinsic rate of 15-40 A.P./min Conduction speed of 4m/sec

Electrical Activity in Pacemaker Cell

•  Autorhythmic cells are ‘leaky’ to Na+ and therefore have a unstable membrane potential –  Pacemaker potential- membrane potential drifts towards threshold –  Initiates action potential and ultimately cardiac contraction

Electrical Activity in Contractile Cells

Contractile cells have a stable membrane potential and require an electrical stimulus from the autorhythmic cells to contract

Excitation-Contraction Coupling

Electrical Activity in Contractile Cells Refractory period means tetanus of cardiac muscle is impossible.

Cardiac contractile cells APs exhibit a prolonged plateau phase accompanied by a prolonged period of contraction.

Summary: An effective heart All achieved by the electrical properties of the cardiac muscle

•  Regular contractions at appropriate rate for metabolism (ANS control)

•  Guaranteed time for ventricular filling after atrial and ventricular contractions (refractory period)

•  Contraction duration long enough for physical movement of fluid (plateau phase)

•  Contractile strength sufficient to generate appropriate pressures (plateau phase)

•  Ventricular pressure directed towards exit valves (intrinsic conduction system)

•  Coordination of left & right, and atrial & ventricular contractions (intrinsic conduction system)

•  Matched volume of emptying and filling (intrinsic conduction system)

Electrocardiogram •  Recording of the surface electrical activity of

the heart from electrodes placed on skin – Body fluids are conductors – Non-invasive – Comparison of voltages detected by electrodes

at two points

•  Reflects the cardiac cycle – SUM of activity in ALL cardiac muscle – Exact pattern of activity depends on orientation of

electrodes

The ECG

•  Waves reflect depolarization and repolarization events

•  Baseline reflects when there is no overall depolarization or repolarization –  Occurs when muscle is at

rest, and during sustained contraction

Spread of depolarization

•  General direction of spread of depolarization

Cardiac Vector (normally between -10o and +100o)

0o 180o

ECG Timing

P wave 80-100ms

PR interval

120-200ms QRS Complex

80-120ms ST segment

70-80ms T wave ~200ms

RR interval reflects entire duration of each heart beat

Clinical ECG

1 horizontal box= .2s (small box 0.04s), 5 boxes = 1sec

10 small division upward or downward= 1millivolt

http://library.med.utah.edu/kw/ecg/image_index/index.html

Assessment of orientation of the heart

Localisation of areas that do not conduct electrical activity normally

Assessment of myocardial hypertrophy or atrophy

Accurate measurement of heart rate (60/RR interval)

Respiratory Sinus Arrhythmia

Marquette Electronics Copyright 1996 http://library.med.utah.edu/kw/ecg/image_index/index.html

Normal

HR ↑ with inspiration

HR ↓ with expiration

Expressed more in young and fit

Bradycardia & Tachycardia

•  Bradycardia ≤60bts/min –  Chronic exercise training –  Vagal stimulation

•  Tachycardia ≥ 100 bts/min –  Increased body

temperatures –  Sympathetic stimulation –  Exercise

Breakdown of SA node pacemaker authority

Impulse from SA node is blocked before it enters atria

Latent pacemakers pick up authority

No/small p-waves clue: Atrial fibrillation

Heart block 1st degree: delay in conduction, prolonged P-R interval >0.2s, QRS same

2nd degree: incomplete heart block, P-R interval between .25-.45sec, atria beating faster than ventricles- dropped beats

Compete AV block: P-wave regular frequency completely unrelated to ventricular firing, Ventricular QRS followed by T wave normal.

Breakdown of ventricular coupling or refractory period •  Breakdown of left/right

ventricular coupling- –  Same mechanisms that cause

AV block –  QRS may be considerable

abnormal

•  Breakdown of refractory safety period –  Hypertrophy can cause different

refractory periods in epicardium & endocardium

(Ectopic beats)

Left Ventricular Hypertrophy

High blood pressure

In exercise-adaptation to increased preload/afterload

Enhances pumping capacity

http://library.med.utah.edu/kw/ecg/image_index/index.html

Exercise Hyperkalaemia

Elevated potassium- speeds recovery of action potentials

Often seen in athletes

http://library.med.utah.edu/kw/ecg/image_index/index.html

Summary: ECG

•  An ECG tracing records the electrical activity of the heart – Waves reflect depolarization and repolarization events –  Intervals reflect timing – Both have diagnostic value