lab 3 : defibrillation, cardioversion and pacing

Electrical therapies

Automatedexternal defibrillators - AED, defibrillation, cardioversion

and pacing

Defibrillation is the passage across the myocardiumof an electrical current of sufficient magnitude

to depolarise a critical mass of myocardium andenable restoration of coordinated electrical activity.

Defibrillation is defined as the termination offibrillation or , the absence of ventricularfibrillation/ventricular tachycardia (VF/VT)

at 5 s after shock delivery; however, the goal ofattempted defibrillation is to restore spontaneous

circulation.

Normal sinus rhythm

Normal sinus rhythm

* each P wave is followed by a QRS

* P waves normal for the subject

* P wave rate 60 - 100 bpm with <10% variation

rate <60 = sinus bradycardia

rate >100 = sinus tachycardia

variation >10% = sinus arrhythmia

P wawes

Normal P waves height < 2.5 mm in lead II width duration < 0.11 s in lead II

normal PR interval 0.12 to 0.20 s (3 - 5 small squares)

normal atrial depolarization,

for short PR segment consider Wolff-Parkinson-White syndrome or Lown-Ganong-Levine syndrome (other causes - Duchenne muscular dystrophy, type II glycogen storage disease (Pompe's), HOCM)

for long PR interval see first degree heart block and 'trifasicular' block

normal QRS complex

corresponds to the depolarization of the right and left ventricles.

< 0.12 s duration (3 small squares)

for abnormally wide QRS consider right or left bundle branch block, ventricular rhythm, hyperkalaemia, etc.

no pathological Q waves

no evidence of left or right ventricular hypertrophy

Normal QT interval = 0.42 s.

Causes of long QT interval:

myocardial infarction, myocarditis, diffuse myocardial disease

hypocalcaemia, hypothyrodism

subarachnoid haemorrhage, intracerebral haemorrhage

drugs (e.g. sotalol,amiodarone)

Normal ST segment It has a duration of 0.08 to 0.12 sec, connects the QRS complex and the T wave . No elevation or depression

causes of elevation include acute MI (e.g. anterior, inferior), left bundle branch block, normal variants (e.g. athletic heart, high-take off), acute pericarditis

causes of depression include myocardial ischaemia, digoxin effect, ventricular hypertrophy, acute posterior MI, pulmonary embolus, left bundle branch block

Normal T wave (represents the repolarization of the ventricles)

causes of tall T waves include hyperkalaemia, hyperacute myocardial infarction and left bundle branch block

causes of small, flattened or inverted T waves are numerous and include ischaemia, hyperventilation, anxiety, drinking iced water, drugs (e.g. digoxin), pericarditis, intraventricular conduction delay and electrolyte disturbance

Sinus tachycardia

Ventricular tachycardia

Ventricular fibrilation

Automated external defibrillators

(1) Make sure you, the victim, and any bystanders are safe.

(2) If the victim is unresponsive and not breathing normally, send someone for the AED and to call for an ambulance.

(3) Start CPR according to the guidelines for BLS.

(4) As soon as the defibrillator arrives switch on the defibrillator and attach the electrode pads. If more than one rescuer is present, CPR should be continued while this is carried out

• follow the spoken/visual directions

• ensure that nobody touches the victim while the AED is analysing the rhythm


5a If a shock is indicated

• ensure that nobody touches the victim • push shock button as directed (fully automatic AEDs will deliver the shock automatically)

• continue as directed by the voice/visual prompts

5b If no shock indicated

• immediately resume CPR, using a ratio of 30 compressions to 2 rescue breaths

• continue as directed by the voice/visual prompts

6 Continue to follow the AED prompts until

• qualified help arrives and takes over

• the victim starts to breathe normally

• you become exhausted


Automated external defibrillators are sophisticated, reliable computerised devices that use voice and visual prompts to guide lay rescuers and healthcare professionals to safely attempt defibrillation in cardiac arrest victims .

Automated external defibrillators have microprocessors that analyse several features of the ECG, including frequency and amplitude.

They are extremely accurate in rhythm analysis.


Safe use of oxygen during defibrillation

In an oxygen-enriched atmosphere, sparking from poorly applied defibrillator paddles can cause a fire.

-Take off any oxygen mask or nasal cannulae and place them at least 1m away from the patient’s chest.

-Leave the ventilation bag connected to the tracheal

tube or other airway adjunct OR disconnect any bag-valve device from the tracheal tube and remove it at least 1 m from pacient chest.

If the patient is connected to a ventilator, leave the ventilator tubing (breathing circuit) connected to the tracheal tube

Minimise the risk of sparks during defibrillation.


CPR versus defibrillation as the initial treatment

Recent evidence has suggested that a period of CPR before defibrillation may be beneficial after prolonged collapse.

It is reasonable for EMS personnel to give a period of about 2 min of CPR (i.e., about five cycles at 30:2) before defibrillation in patients with prolonged collapse (>5 min).

Laypeople and first responders using AEDS should deliver the shock as soon as possible.

Energy level

The recommended initial energy level for the first shock using a monophasic defibrillator is 360 J. With monophasic defibrillators, if the initial shock has been unsuccessful at 360 J, second and subsequent shocks should all be delivered at 360 J.

The initial biphasic defibrillator shock energy should be at least 150 J , afther that 200 J .

If a shockable rhythm (recurrent ventricular fibrillation) recurs after successful defibrillation , give the next shock with the

energy level that had previously been successful.

In children case – optimal energy level is 4J/kg (25kg=100J) mono or biphasic defibrillator

Cardioversion

If electrical cardioversion is used to convert atrial or ventricular tachyarrhythmias, the shock must be synchronised to occur with the R wave : VF can be induced if a shock is delivered during the relative refractory portion of the cardiac cycle.

Synchronisation can be difficult in VT because of the wide-complex and variable forms of ventricular arrhythmia. If synchronisation fails, give unsynchronised shocks to the unstable patient in VT to avoid prolonged delay in restoring sinus rhythm. Ventricular fibrillation or pulseless VT requires unsynchronised shocks. Conscious patients must be anaesthetised or sedated before attempting synchronised cardioversion!

Precordial thump and Pacing

A precordial thump is most likely to be successful in converting

VT to sinus rhythm. Successful treatment of VF by precordial thump is much less likely. Consider giving a single precordial thump when cardiac arrest is confirmed rapidly after a witnessed, sudden collapse and a defibrillator is not immediately to hand.Using the ulnar edge of a tightly clenched fist, a sharp impact is delivered to the lower half of the sternum from a height of about 20 cm, followed by immediate retraction of the fist, which createsan impulse-like stimulus.

Consider pacing in patients with symptomatic bradycardia .

lab 3 : defibrillation, cardioversion and pacing

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