How does the heart beat The heart beat

How does the heart beat?

• The heart beat is MYOGENIC i. e. the electrical signals needed to make the muscles contract originate in the muscle itself (rather than via nerves)

• Sino-atrial node, part of muscle in right atrium, is origin of the electrical signals (impulses) • Called the ‘pacemaker’ • Wave of impulses from SAN spread along the muscle cells across the wall of both atria • Causes muscles to contract atrial systole

• Impulses do not travel directly to ventricles because of the ring of non-conducting tissue between the atria and the ventricles • Impulses reach atrioventricular node (AVN) at the beginning of the septum; slight delay • Impulses conducted down specialised muscle cells in wall of septum called the Bundle of His

• Reach muscles at base of heart then travel up wall of ventricles in special fibres (from the Bundle of His) called Purkinje fibres causing contraction • Contraction passes from base of heart upwards so blood is pumped upwards and through semi lunar valves. • Impulses travel from SAN AVN then base of ventricles time delay so atria contract to fill ventricles before ventricles contract.

Diagnosis of coronary heart disease • Physical examination. • Review of medical history. • ECG (=electrocardiogram) • ECG records the electrical activity of the heart

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Activity 1. 8 • 1: Why is there a non-conducting layer between the atria and ventricles? • So that the impulses can only pass to the ventricles via the AVN, thus causing a delay and ensuring coordinated contraction of the ventricles after the atria have completely contracted

• 2: Why are impulses conducted to the base of the heart before moving up through the ventricle walls? • To ensure the ventricles contract form the apex upwards, pushing the blood up and out of the heart (look at where the arteries leave the heart!)

• Q 3: Where are impulses temporarily delayed within the conducting system? • At the AVN/the boundary between the atria and the ventricles

• Q 4: Why are these impulses delayed? • To ensure that the atria have completely finished contracting and emptying, and the ventricles are full before the ventricles contract.

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P Wave impulses spreading across the atrial systole PR interval to ventricles time taken for impulses to be conducted from SAN to AVN QRS complex impulses spreading up through ventricles ventricular systole T wave recovery of ventricles during diastole

Fig 7 (Q 1)

Squared paper passes through ECG at 25 mm per second. 1 min = 300 squares. One square = 0. 2 sec. Heart rate = distance between two QRS complexes. This value/300 = heart rate Fig 2 Qu 1: What is heart rate of person whose ECG trace is above (Fig 2) 300/5 = 60 bpm

Bradycardia = slow heart beat < 60 bpm Fig 3 Q 2: is the person with the ECG trace in Fig 3 bradycardic? Heart rate = 300/7 = 43 bpm = bradycardia

Tachycardia = rapid heart rate. 100 bpm See fig 1. 22 C on page 19 in textbook. Is this patient tachycardic? Heart rate = 300/2 = 150 bpm = tachycardia

Fig 4 Qu 4: Which one of the waves is missing from this ECG trace? Where might the problem lie in the heart? What could this problem be? P waves missing. Erratic QRS complexes. Lack of coordinated electrical activity in atria. Problem might be with SAN • SAN fails to produce impulses • Impulses fail to pass through atria walls

Answer to Q 5 During period of pain the ST segment lies below the normal position and there may be inversion of the T wave.

• Qu 6: in patients with ischaemic heart disease, the PR interval on the ECG trace can be longer than normal, over 0. 2 sec, due to a problem with the conduction in the heart. Where might the impulses be delayed to cause the extended PR interval? • Conduction though the AVN may be delayed.

Qu 7: In patients with pulmonary hypertension, blood pressure in the lungs is much higher than normal. Their ECG trace has a large P wave, similar to the one in Fig 5. Suggest which part of the heart will be enlarged in a person with this condition? Give at least one reason for your answer. Fig 5 Right atrium, because the P wave is larger. Right atrium has to generate a higher pressure to get blood into ventricles because the right ventricle struggles to pump blood to the lungs

Each lead used in an ECG looks at the heart from a different angle. The positions of 6 leads are shown in Fig 6 Qu 8: Mr Wilson. A patient who has had a heart attack, has abnormal ECG traces from leads 1 to 4. From the position of the leads on the chest, suggest whether the damaged heart muscle that caused the abnormal traces is in the anterior (front) or posterior (back) side of the heart. Anterior. Leads 1 to 4 view the front of the heart.

• Qu 9: Explain why the P wave is usually smaller than the QRS complex on a normal ECG trace. • Less muscle in atrium than ventricle so less electrical activity.