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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Rough notes for some of the short answer questions on this subject.