Physiology
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Cardiovascular
A 42 year old man presents to ED complaining of feeling light-headed. At triage, his heart rate is found to be 40-60 bpm regular rhythm. ECG demonstrates a normal QRS complex with absent P waves. Which part of the conducting system is most likely acting as pacemaker in this patient:
Answer:
The sinoatrial node (SAN) is the primary pacemaker. The SAN will normally discharge at a rate of 60-100 bpm. If the SAN does not function properly or the impulse generated in the SAN is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker. The atrioventricular node (AVN) is the secondary pacemaker and will normally discharge at about 40-60 beats per minute. The left and right branches of the bundle of His, and the Purkinje fibers, will also produce a spontaneous action potential at a rate of 20-40 beats per minute, so if the SAN and AVN both fail to function, these cells can become pacemakers. It is important to realise that these cells will be initiating action potentials and contraction at a much lower rate.Conducting System of Heart
Physiology / Cardiovascular / Cardiac Cycle
Last Updated: 26th July 2024
The cardiac conduction system initiates and coordinates contraction of the heart.
Sinoatrial Node
The heartbeat is initiated by spontaneous depolarisation of the sinoatrial node (SAN), a region of specialised myocytes in the right atrium. The rate is modulated by the autonomic nervous system. Action potentials in the SAN activate adjacent atrial myocytes and a wave of depolarisation and contraction therefore spreads through atrial muscle. This is prevented from reaching the ventricles directly by the annulus fibrosus.
Cardiac Conduction System. (Image by OpenStax College [CC BY 3.0 , via Wikimedia Commons)
Atrioventricular Node
The impulse generated by the SAN is then channelled through the atrioventricular node (AVN), located between the right atrium and ventricle near the atrial septum. The AVN contains small cells and thus conducts slowly and delays the impulse for about 120 ms allowing time for atrial contraction to complete ventricular filling.
Conduction of the Impulse
Once ventricular filling is complete, the impulse is then transmitted by specialised, wide, fast conducting myocytes in the bundle of His, the left and right bundles, and the Purkinje fibres, by which it is distributed over the inner surface of both ventricles. From here a wave of depolarisation and contraction moves from myocyte to myocyte across the endocardium until the whole ventricular mass is activated.
Pathophysiology
Because the SAN is responsible for the rest of the heart's electrical activity, it is called the primary pacemaker. The SAN will normally discharge at a rate of 60-100 bpm. If the SAN does not function properly or the impulse generated in the SAN is blocked before it travels down the electrical conduction system, a group of cells further down the heart will become its pacemaker.
The AVN is the secondary pacemaker and will normally discharge at about 40-60 beats per minute. The left and right branches of the bundle of His, and the Purkinje fibers, will also produce a spontaneous action potential at a rate of 20-40 beats per minute, so if the SAN and AVN both fail to function, these cells can become pacemakers. It is important to realise that these cells will be initiating action potentials and contraction at a much lower rate.
The SAN controls the rate of contraction for the entire heart muscle because its cells have the quickest rate of spontaneous depolarisation, thus they initiate action potentials the quickest. The action potential generated by the SAN passes down the electrical conduction system of the heart, and depolarises the other potential pacemaker cells to initiate action potentials before these other cells have had a chance to generate their own spontaneous action potential, thus they contract and propagate electrical impulses to the pace set by the cells of the SAN. This is the normal conduction of electrical activity in the heart.
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- Biochemistry
- Blood Gases
- Haematology
| Biochemistry | Normal Value |
|---|---|
| Sodium | 135 – 145 mmol/l |
| Potassium | 3.0 – 4.5 mmol/l |
| Urea | 2.5 – 7.5 mmol/l |
| Glucose | 3.5 – 5.0 mmol/l |
| Creatinine | 35 – 135 μmol/l |
| Alanine Aminotransferase (ALT) | 5 – 35 U/l |
| Gamma-glutamyl Transferase (GGT) | < 65 U/l |
| Alkaline Phosphatase (ALP) | 30 – 135 U/l |
| Aspartate Aminotransferase (AST) | < 40 U/l |
| Total Protein | 60 – 80 g/l |
| Albumin | 35 – 50 g/l |
| Globulin | 2.4 – 3.5 g/dl |
| Amylase | < 70 U/l |
| Total Bilirubin | 3 – 17 μmol/l |
| Calcium | 2.1 – 2.5 mmol/l |
| Chloride | 95 – 105 mmol/l |
| Phosphate | 0.8 – 1.4 mmol/l |
| Haematology | Normal Value |
|---|---|
| Haemoglobin | 11.5 – 16.6 g/dl |
| White Blood Cells | 4.0 – 11.0 x 109/l |
| Platelets | 150 – 450 x 109/l |
| MCV | 80 – 96 fl |
| MCHC | 32 – 36 g/dl |
| Neutrophils | 2.0 – 7.5 x 109/l |
| Lymphocytes | 1.5 – 4.0 x 109/l |
| Monocytes | 0.3 – 1.0 x 109/l |
| Eosinophils | 0.1 – 0.5 x 109/l |
| Basophils | < 0.2 x 109/l |
| Reticulocytes | < 2% |
| Haematocrit | 0.35 – 0.49 |
| Red Cell Distribution Width | 11 – 15% |
| Blood Gases | Normal Value |
|---|---|
| pH | 7.35 – 7.45 |
| pO2 | 11 – 14 kPa |
| pCO2 | 4.5 – 6.0 kPa |
| Base Excess | -2 – +2 mmol/l |
| Bicarbonate | 24 – 30 mmol/l |
| Lactate | < 2 mmol/l |
