Chapter 12. The cardiovascular system and the ECG
Blood
The normal circulating blood volume is variable, but in a 70 kg adult it is about 5 L (equivalent to about 70 mL/kg body weight).
Cardiac cycle
• The adult human heart beats about 70 times/min at rest
• One heartbeat within a single cardiac cycle therefore lasts about 0.8 seconds or 800 milliseconds (ms)
• The phase of contraction is called systole, the phase of relaxation is called diastole.
Cardiac output
Cardiac output is defined as the volume of blood ejected by each ventricle per minute. Cardiac output is calculated by multiplying the heart rate by the stroke volume (volume ejected per heart beat).
Cardiac output = stroke volume × heart rate
Stroke volume
Three factors determine stroke volume:
1. Preload – the filling of the heart during diastole
2. Afterload – the resistance in the arterial circulation against which the heart has to pump
3. Contractility – the intrinsic performance of the heart muscle at a given preload and afterload.
Regulation of heart rate
• Heart rate <60 bradycardia
• Heart rate >100 tachycardia.
The parasympathetic system, via the vagus nerve, slows the heart rate down. Atropine blocks the vagus nerve thereby increasing the heart rate.
The sympathetic nervous system tends to speed heart rate up and β-blocking drugs (e.g. propranolol or atenolol) prevent tachycardia by blocking the sympathetic nerves.
Adrenaline and noradrenaline are released from the adrenal glands and increase both heart rate and contractility.
Blood pressure
Blood pressure is determined by the cardiac output and the peripheral vascular resistance. The vascular resistance will fall in conditions such as severe anaphylaxis.
Electrocardiography
The ECG gives information about the electrical activity of the heart and can predict problems such as myocardial infarction. It does not enable any conclusions to be made about mechanical activity of the heart such as force of contraction or blood pressure.
Electrocardiograms are conventionally recorded from 12 leads. This is achieved by placing electrodes on the right arm, left arm and left leg, as well as six electrodes in predefined locations on the chest and a ground reference on the right leg. The position of these leads is described in degrees clockwise from lead I (0°); thus lead aVF is 90°, lead III is 120° and lead aVR is 210°. It is apparent, therefore, that leads II, III and aVF ‘look at’ the undersurface of the heart – these are the inferior leads. Lead aVL ‘looks at’ the heart from ‘above’ (i.e. from the left shoulder).
ECG recording paper
• The ECG machine, or the printer of a defibrillator or ECG monitor, runs with a paper speed of 25 mm per second
• The graph paper is divided into small squares of side 1 mm. A large square consists of five small squares and measures 5 mm × 5 mm
• At the standard paper speed, each small square (1 mm) represents 0.04 seconds and each large square (5 mm) represents 0.20 seconds
• The amplitude of the electrical impulse is indicated along the vertical axis. A 10 mm deflection (two large squares) represents a 1 mV electrical signal provided that the ECG machine is properly calibrated.
• The horizontal axis of the ECG trace is time
• One small square equals 0.04 seconds
• One large square equals 0.20 seconds.
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| Figure 12.1. |
| Chest and limb lead placement. |
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