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Electrocardiogram (ECG) (Electrical activity of the heart can be detected…
Electrocardiogram (ECG)
CARDIAC OUTPUT
Heart Rate
Measured by the rate of depolarisation in autorhythmic cells
Slowed by: Parasympathetic nervous system
Made faster by: sympathetic nervous system and release of epinephrine from adrenal medulla
Stroke volume
Determined by force of contraction in ventricular myocardium
Influenced by: Contractility and muscle fibres
Mechanical system
Depolarisation triggers mechanical activity.
Systole= contraction of the myocardium results in ejection of blood from the ventricles.
Diastole= relaxation of the myocardium allowing for the ventricles to fill.
Cardiac output= amount of blood pumped by each ventricle in 1 minute. Calculated by multiplying the amount of blood ejected from the ventricle with each heartbeat. SV x HR = CO
Electrical activity of the heart can be detected on the body surface using electrodes which is recorded on an ECG.
P WAVE
Represents depolarisation causing contraction beginning in the SA node and spreading to both atria .
If the P wave is abnormal or not present, this means the SA node is not in control
The size of the P wave represents how long it takes for the atria to depolarise, if this is abnormal this can also suggest complications in the atria.
PR SEGMENT
Represents the delay of the impulse through the AV node, this pause ensures there is adequate time for the blood in the atria to move into the ventricles.
The size of this segment should be within 2.5 small squares which suggests the conduction from the SA node through to the AV node is considered normal
QRS COMPLEX
The largest complex on the ECG
Represents depolarisation of the ventricular muscle.
Depolarisation results in the complex consisting of the initial downward defection (Q wave), a tall upward deflection (R wave) followed by a second downward deflection (S wave)
ST SEGMENT
Isoelectic segment, should be at the same level to the P wave along the isoelectric line.
T WAVE
Represents the heart at rest. This rest period is when the heart begins to fill with blood again.
If this period is reduced, this can result from cardiac output falls due to factors such as tachycardia.
Factors to consider when undergoing an ECG
Patient laying still, no talking, even licking the lips can effect the result, and no phone or electricity close to the patient.
Gaining important health information such as history of present illness. Access what symptoms the PT is experiencing.
Ask the PT of past health history, including history of chest pain, shortness of breath, fatigue, alcohol and tobacco use, rheumatic fever, streptococcal throat infections, stroke, palpitations, hypertension and congenital heart disease.
Medications
Past surgeries or alternative treatments.
FACTORS AFFECTING CO
Exercise, stress, hypovolaemia. The ability to respond to these demands by altering CO is termed as cardiac reserve.
Preload: volume of blood in ventricle at the end of diastole, before the next contraction.
Preload can be affected by a number of conditions such as MI, aortic stenosis, and hypervolaemia.
After load: the peripheral resistance which the left ventricle must pump against.
Affected by the size of the ventricle, wall tension, and arterial BP. If the arterial BP is increased, the ventricles meet increased resistance to ejection of blood, thus increasing work demand.
Eventually this increased work demand results in ventricular hypertrophy which is an enlargement of the cardiac muscle tissue without an increase in CO or the size of chambers.