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Procedural Skills (SLO6)
Question 70 of 127
A 74 year old patient presents to ED feeling unwell with palpitations. She is pale and sweaty and her blood pressure is 80/60 mmHg. Her ECG shows a ventricular tachycardia with a rate of 140 bpm. You proceed to perform DC cardioversion. The shock should be synchronised to which of the following ECG waves?
Answer:
The DC shock must be synchronised to the QRS complex because a shock that falls during the vulnerable period (near the peak of the T wave) can induce VF. Most manual defibrillators incorporate a switch that enables the shock to be triggered by the R wave on the electrocardiogram. After synchronisation to the QRS complex is confirmed on the monitor, a shock is given. When delivering the shock, press the shock button and keep it pressed until after the shock has occurred - there may be a slight delay before the shock is delivered.DC Cardioversion
Procedural Skills (SLO6)
Last Updated: 1st September 2022
Indications
- Treatment of a tachyarrhythmia that has been present for less than 24 hours with an aim to revert to sinus rhythm.
- Treatment of a tachyarrhythmia that has been present for less than 24 hours when pharmacological measures have failed.
- Treatment of a tachyarrhythmia when the patient shows adverse features (shock, syncope, heart failure, myocardial ischaemia).
Pathophysiology
- A transthoracic direct current (DC) shock of sufficient magnitude depolarises the entire myocardium, rendering the entire heart momentarily refractory to repeat depolarisation. Thereafter, the most rapid intrinsic pacemaker, usually the sinoatrial (SA) node, reassumes control of heart rhythm.
- Thus, DC cardioversion very effectively terminates tachyarrhythmias that result from reentry. However, it is less effective for terminating tachyarrhythmias that result from automaticity because the return rhythm is likely to be the automatic tachyarrhythmia.
- For tachyarrhythmias other than ventricular fibrillation (VF) and pulseless ventricular tachycardia (VT), the DC shock must be synchronised to the QRS complex because a shock that falls during the vulnerable period (near the peak of the T wave) can induce VF.
- DC cardioversion can be delivered as monophasic or biphasic current. Monophasic current travels in one direction between the two electrodes. In biphasic devices, the current reverses direction part way through the shock waveform. The biphasic device requires lower energy and has been shown to lead to higher rates of return of spontaneous circulation (ROSC). However, survival outcomes are similar in both device. Most manual and automated external defibrillators (AEDs) are now biphasic because of increased efficiency at restoring sinus rhythm. Biphasic devices are also smaller in size (making the devices more portable).
Procedure
- Because the procedure is frightening and painful, brief general anaesthesia or IV analgesia and sedation (e.g. fentanyl 1 mcg/kg, then midazolam 1 to 2 mg every 2 minutes to a maximum of 5 mg) is necessary.
- Equipment and personnel to maintain the airways must be present.
- The defibrillator pads used for cardioversion may be placed anteroposteriorly (along the left sternal border over the 3rd and 4th intercostal spaces and in the left infrascapular region) or anterolaterally (between the clavicle and the 2nd intercostal space along the right sternal border and over the 5th and 6th intercostal spaces at the apex of the heart). For atrial fibrillation and flutter, use anteroposterior pad positions where practical.
- Most manual defibrillators incorporate a switch that enables the shock to be triggered by the R wave on the electrocardiogram. After synchronisation to the QRS complex is confirmed on the monitor, a shock is given. When delivering the shock, press the shock button and keep it pressed until after the shock has occurred - there may be a slight delay before the shock is delivered.
- The most appropriate energy level varies with the tachyarrhythmia being treated. For synchronised cardioversion of a broad-complex tachycardia start with 120-150J and increase in increments if this fails. For atrial fibrillation start at the maximum defibrillator output. Atrial flutter and regular narrow-complex tachycardia will often be terminated by lower-energy shocks: start with 70 - 120J.
- If synchronisation fails, choose another lead and/or adjust the amplitude. With some defibrillators, the synchronised mode has to be reset if a second or subsequent shocks are required. Other machines remain in the synchronised mode; be careful not to leave the synchronised switch on following use as this will inhibit discharge of the defibrillator when it is next used in cardiac arrest.
- DC cardioversion-defibrillation can also be applied directly to the heart during a thoracotomy or through use of an intracardiac electrode catheter; then, much lower energy levels are required.
Complications
- Complications are usually minor and include atrial and ventricular premature beats and muscle soreness.
- Less commonly, but more likely if patients have marginal left ventricular function or multiple shocks are used, cardioversion precipitates myocyte damage and electromechanical dissociation.
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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 |
