trinity cvs lecture 1-ecg 12.13.ppt (read-only)
TRANSCRIPT
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Cardiovascular Physiology
Dr. Nicole Burns February 14, 2013
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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
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The Heart Wall
• Endocardium (inner) – Layer of endothelial cells
• Myocardium (middle) – Cardiac muscle
• Epicardium (outer)
• Pericardium – Double-walled membranous sac
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Cardiac Muscle (myocardium)
• Aerobic muscle
– 99% contractile cells
– 1% autorhythmicity cells • Self-excitate
– Intercalated discs
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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
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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
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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
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Electrical Activity in Contractile Cells
Contractile cells have a stable membrane potential and require an electrical stimulus from the autorhythmic cells to contract
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Excitation-Contraction Coupling
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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.
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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)
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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
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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
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Spread of depolarization
• General direction of spread of depolarization
Cardiac Vector (normally between -10o and +100o)
0o 180o
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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
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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)
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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
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Bradycardia & Tachycardia
• Bradycardia ≤60bts/min – Chronic exercise training – Vagal stimulation
• Tachycardia ≥ 100 bts/min – Increased body
temperatures – Sympathetic stimulation – Exercise
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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
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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.
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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)
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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
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Exercise Hyperkalaemia
Elevated potassium- speeds recovery of action potentials
Often seen in athletes
http://library.med.utah.edu/kw/ecg/image_index/index.html
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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