Heart Physiology
Electricity of the Heart
Normal sinus rhythm
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Rhythm: regular
HR: 60-100 bpm
P wave: present, round, and upright
PR interval: present; 0.12-0.20 seconds
QRS complex: narrow; 1 P wave for each
QRS
Ø RATE
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1500 methods (regular) count boxes between 2 QRS then divide 1500 with it (1500/number of
small boxes)
• 6 secs second (for irregular rate) You measure how much PQRSTU in 6 seconds x 10
Before calculating the rate you need to check if it’s regular or irregular: take a piece of paper and put a line on 2
QRS, when you move the paper, is it the same size everywhere
Ø P WAVE
• SA node firing
• Present or absent
• Shape: upright and rounded or inverted and
biphasic
Ø PR INTERVAL
• PR: measure of the time required for the impulse to spread from SA node to ventricles
• Normal: 012-0.20 seconds
• When slower, it means it takes more time for the ventricles to contract after atrial contraction
Ø QRS complex:
• Depolarization from AV node through ventricles
• Normal QRS is narrow, 0.06-0.10 seconds (narrow=normal or large=abnormal)
Ø T WAVE
• Repolarization of the ventricles
Ø Diseases:
• Sinus bradycardia
HR below 60
QRS narrow, P wave present, round upright, PR normal range
• Sinus tachycardia
HR above 100
QRS narrow, P wave present, round upright, PR normal range
• Atrial flutter
Rhythm regular
PR absent, QRS narrow, P wave pointy
• Atrial fibrillation
Irregular rhythm (count with 6 sec)
P wave hard to find
PR absent
QRS narrow
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Rate = 0
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Asystole
Ventricular tachycardia
Rate above 100, QRS wide, P absent, no PR, regular
• Ventricular fibrillation
Irregular, no rate, P absent, QRS absent
Vocabulary
Preload: Filling and stretching = preload determines the amount of stretch placed on myocardial fibres, it’s the
volume of blood in the ventricles at the end of diastole before the next contraction (systole). Volume of blood
received by the heart.
Afterload: Pressure to pump the blood against = the peripheral resistance against which the left ventricle must
pump. Affected by the the size of the ventricle, the wall tension and arterial blood pressure. If arterial blood
pressure is elevated, the ventricles meet increased resistance to ejection of blood which increases the work
demand.
BP: the pressure exerted by blood against the wall of the arterial system
Systolic BP: The pressure on arteries when heart contracts
Diastolic BP: the pressure on arteries when heart relaxes
CO: the volume of blood ejected from the heart per minute
SVR: the force opposing the movement of
blood, it is created in small arteries and
arterioles.
SV: amount of blood pumped out of the left
ventricle per beat (about 70mL)
Cardiac Lab values meanings
CK-MB: elevation indicates myocardial
damage (not heart specific)
Troponin: High affinity for myocardial injury (MI)
Myoglobin: oxygen-binding protein in cardiac and
skeletal muscle, elevates after cell deaths
RBC: decreases in infections and rheumatic heart
diseases, increases in inadequate tissue oxygenation
WBC: increases in infections or inflammation to
dispose the necrotic tissue
HCT: high means vascular volume depletion,
decreased can mean anemia
Blood coagulation factors: increase after a MI which
increases the risk for DVT
CRP: elevated means risk for heart disease
Potassium: if low = cardiac electrical instability, risk
of digoxin toxicity, if high = asystole and
dysrhythmias
Sodium: decreases when using diuretics and HF
BP = CO x
SVR
CO = HR x SV