Chapter 19 EKGs and beyond

ECG/EKG

used to measure heart’s electrical activity

How long is one cardiac cycle?

800 ms

Standard placement of ECG leads

12-lead: measures 12 different electrical activity points of heart

6 on chest

4 on limbs

P wave

depolarization of atria in response to SA node triggering

PR interval

delay of AV node to allow ventricles to fill

QRS complex

depolarization of ventricles; triggers main pumping contraction

ST segment

beginning of ventricular repolarization

T wave

ventricular repolarization

external automatic defibrillator

reestablishes normal sinus rhythm (normal ECG rhythm)

Systole

contraction of a heart chamber

Diastole

relaxation of a heart chamber

Quiescent period

when both the atria and the ventricles are in diastole

What part of the EKG shows atrial diastole?

beginning of cycle until peak of P wave

What part of the EKG shows atrial systole?

Peak of P wave until R wave

What part of the EKG shows ventricular diastole?

beginning of cycle until R wave

What part of the EKG shows atrial diastole (after atrial systole)?

R wave until end of cycle

What part of the EKG shows ventricular systole?

R wave until end of T wave

What part of the EKG shows ventricular diastole (after systole)?

End of T wave until end of cycle

When does the quiescent period end?

Peak of P wave

What are the three heart sounds?

S1, S2, S3

S1 heart sound

AV valves close as ventricular pressure > atrial pressure

S2 heart sound

Semilunar valves shut at beginning of ventricular diastole

S3

heard only sometimes;

filling of ventricles

**May indicate enlarged heart

Normal resting heart rate

75 bpm

When would you expect for heart rate to be higher?

younger children and elderly

When would you expect for heart rate to be lower?

young adults and athletes

Tachycardia

adult resting heart rate >100bpm

Bradycardia

adult resting heart rate <60bpm

First-degree heart block

electrical signals are slowed, yet all signals still reach the ventricles

Second-degree heart block

slower, irregular heart rhythm

not all signals reach ventricles; some beats dropped

Third-degree heart block (complete atrioventricular block)

no electrical signals reach the ventricles → ventricles take over with their own rhythm

Ventricular rhythm is slower and more irregular

What are the 2 major factors that influence cardiac output?

Heart rate and Stroke volume

Definition of cardiac output

amount of blood pumped by ventricles in 1 minute

Cardiac output (CO) calculation

75bpm x 70mL/beat = 5 L/min

Definition of stroke volume

amount of blood the heart pumps in one beat

Stroke volume calculation

End diastolic volume — End systolic volume

End diastolic volume (EDV)

how much blood the ventricles can hold

End systolic volume (ESV)

how much blood is left after ventricular contraction

Factors affecting heart rate

Autonomic innervation

Hormones

Fitness levels

Age

Factors affecting stroke volume

Heart size

Fitness levels

Gender

Contractility

Duration of contraction

Preload (EDV)

Afterload (resistance)

Ejection fraction

Main way to assess left ventricular systolic function

Ejection fraction calculation

Stroke Volume / End Diastolic Volume

EF % = [SV / EDV] x 100

Methods to calculate ejection fraction

Echocardiogram (ultrasound of heart)

MRI (radio waves)

Nuclear stress test

Multi-gated Acquisition Scan

MUGA (radioactive dye)

Normal range of ejection fraction

55-70%

Autonomic innervation of heart

Medulla oblongata

Sympathetic cardiac nerves

Vagus (parasympathetic) nerves

Medulla oblongata

sends signals to regulate heart rate

Sympathetic cardiac nerves

increase cardiac activity

Vagus (parasympathetic) nerves

slow cardiac activity

How does the sympathetic pathway (catecholamines) affect heart rate?

Increases heart rate

What are the hormones of the sympathetic pathway?

Epinephrine and Norepinephrine

How does the parasympathetic pathway affect heart rate?

Decreases heart rate

What is the hormone of the parasympathetic pathway?

Acetylcholine

Regulation of heart rate at the start of exercise

Parasympathetic stimulation is shut down; HR increases

Regulation of heart rate with more strenuous exercise

Sympathetic nervous system causes HR to further increase

Relationship between coronary heart disease and heart attack

Both exhibit the following:

coronary arteries blocked → heart tissue can’t get oxygenated blood → blood flow is cut off → impaired pumping of blood out of heart

Onset and progression of atherosclerosis

1. Normal artery wall

2. Fatty streak stage (macrophage foam cell formation)

3. Atherosclerotic plaque stage (formation of necrotic core)

4. Rupture of endothelium and occlusive blood clot formation