ECG/Blood Pressure Practical

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Practical : ECG/Blood Pressure

15. Explain the physiological basis of the

ECG.

The heart functions as a mechanical pump by

undergoing rhythmic cycles of contraction (systole)

followed by relaxation (diastole).

The co-ordination of this activity is intrinsic to the heart itself, although

the overall heart rate and force of contraction can be influenced by its

innervation by the autonomic nervous system.

Cardiac muscle is an excitable tissue; that

is, individual cells can generate and transmit Action Potentials, and these

large electrical depolarisations of the cell membrane propagate directly (and,

therefore, very rapidly) from one cardiac muscle cell to another via special

junctions between the cells called intercalated discs. So, if one part of the

heart muscle generates an Action Potential, then a ‘wave’ of Action Potentials

spreads rapidly from cell to cell through the rest of the tissue. As each cell

undergoes its own Action Potential, there is an accompanying influx of calcium

ions (Ca++) which activates the contractile machinery within that

cell (via a process called excitation-contraction

coupling); thus, the ‘wave’ of electrical that propagates through cardiac

tissue is accompanied by a wave of contraction. There is, therefore, a

direction correlation between the electrical activity of the heart and its

mechanical function. This means that there is great diagnostic value in being

able to record and interpret the electrical activity of the heart as it can

tell us a lot about the mechanical effectiveness of this most vital pump.

Although the amount of electrical activity

produced by individual cardiac muscle cells is tiny, the fact that millions of

them are undergoing Action Potentials in a co-ordinated fashion means that

there are fairly large electrical changes occurring in the chest cavity. These

can be recorded as voltages on the surface of the body between suitably-placed

electrodes; a recording of these voltages over time is called an Electrocardiogram (ECG).

The ECG records electrical activity

generated by the heart, by recording current from terminals or leads placed on

specific areas of the body. Different leads look at the heart from different

directions. Depending on which direction the current is travelling in respect

to the observing lead (i.e. the ECG lead in question, e.g. lead I, aVF, V1,

etc.), the lead will record a positive wave when the current is negative

electrode.

16. Explain the relationship between the

electrical events of the ECG and the mechanical events of the heart’s pumping

action

The P wave is produced by depolarising

electrical activity which causes the atria to contract. The impulse then enters

the AV node where it is delayed; this generates a flat segment on the ECG,

called the PQ interval. The segment is flat because the small amount of tissue

involved in conduction only generates a small amount of electrical activity

that cannot be perceived by surface ECG recording.

The QRS wave is produced by depolarising

electrical activity in the ventricles which causes their contraction. The

amplitude of the QRS wave is greater than the P wave as there is more

ventricular tissue than there is atrial tissue.

The T wave is produced by repolarising

electrical activity in the ventricles (which accompany their relaxation).

Include diagram.

17. Explain the origin and significance of

systolic and diastolic pressures.

Blood pressure is not constant throughout

the cardiac cycle, it oscillates between two extremes - systolic (the peak

pressure as the ventricles are contracting) and diastolic (the lowest pressure

at the end of the filling phase, just before the ventricles contract again),

with the variation between the two responsible for the pulse.

The heart is intermittent and so isn’t

constantly generating pressure, the pressure comes from the elastic walls of

the arteries. So, when the heart contracts and ejects blood (systole), some of

the energy transmitted through the ejected blood stretches the large arteries

and is stored in their elastic walls. When the heart is relaxed and the

arteries are no longer being stretched, the stretched arteries passively recoil

and so continue to generate the pressure needed to keep blood flowing. As their

stretch and stored energy reduces, the pressure generated reduces – but it

doesn’t drop to zero because by the time it has dropped by about a quarter

(from 120 mmHg to 80 mHg) the heart will be at the end of its diastolic

(filling) phase and will be about to contract again. So, blood pressure oscillates

between two extremes; these two pressures are known as systolic pressure and

diastolic pressure and represent the highest and lowest pressures in the major

arteries during the cardiac cycle - typically, their values are 120 and 80 mmHg

and are reported as 120/80. They are of vital importance to clinicians as they

say a lot about the overall state of the cardiovascular system. The systolic

pressure reflects the amount of work the heart is being asked to perform with

each beat, as it takes work to generate pressure. The diastolic pressure

reflects the state of the blood vessels, both their ‘stretchiness’ and how much

resistance to blood flow they are providing.

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