Asystole and pulseless electrical activity

Question 6 from the second paper of 2004 and Question 2d from the first paper of 2000 both asked the candidates to recall the non-shockable arm of the advanced life support algorithm.  PEA is a situation where one is presented with organised electrical activity (i.e. a potentially perfusing rhythm) in the absence of cardiac output. The old term (electro-mechanical dissociation) is no longer in use.

A brief recall of advanced life support basics

In brief:

• Asystole and PEA are heterogeneous in their aetiology
• They may be associated with different aetiologies: eg. asystole may be more associated
• However, the management is the same for both: CPR, adrenaline and reversal of the cause
• For this reason they are discussed together

Causes of cardiac arrest in general are discussed elsewhere. In brief, the same "Four Hs and four Ts" mnemonic applied to non-shockable rhythms, even though some Hs and Ts are more associated with some and less with others. For instance, PEA is more often the consequence of some sort of restriction on eitehr preload or afterload (i.e. an empty heart pumping) - for instance, obstructive shock, or massive hypovolemia. On the other hand, the arrhythmic causes of arrest are more likely to be due to a cardiac ischaemic event or some sort of metabolic disturbance, like hyperkalemia. Either way, the end result of VF or VT is usually still asystole.

• Hypoxia
• Hypovolemia (or distributive shock)
• Hyper/hypokalemia
• Hyper/hypothermia
• Tension pneumothorax
• Tamponade
• Toxins
• Thrombus

The college answer refers to the 10-step zigzag mnemonic introduced into popular thinking by Walter Kloeck in 1995; the more recent trainees will probably be more familiar with the 4Hs and 4 Ts which is the modern "8-step" version. The "zigzag" refers to the pattern of physical examination, following a jagged course from airway to carotid to auscultation of the chest and praecordium, then to the cubital fossa (toxins? meaning trackmarks?) and so on.

Management to address the usual causes of cardiac arrest

• Hypoxia in an arrest is usually well-managed with adequate bag-mask ventilation and high concentration of supplied oxygen
• Hypovolemia and distributive shock can be ameliorated by the administration of fluids
• Potassium disturbances will be discovered whent he first ABG returns from the blood gas machine, and these can be managed routinely
• Tension pneumothorax can be suspected from history, and confirmed by examination. Management consists of decompression.
• Cardiac tamponade is also suggested by history and examination findings. Management consist of emergency pericardicentesis
• Toxins are suggested by history and characteristic examination findings, eg. pinpoint pupils, the rashj of anaphylaxis, etc. Management consists of administering an antidote, if it is available.
• Thrombosis - in the context of arrest-inducing massive PE - can be suspected from history; confirmation relies on the presence of an ultrasound machine and a skilled operator. Management consists of intra-arrest thrombolysis. For coronary thrombosis, this may not be a viable option.

Unusual causes of asystole which have not made it into the 4 Hs and 4Ts:

People keep submitting these case reports. There are thousands of different weird asystoles every year. This selection includes only the reports published in 2015, just as an example.

• Extreme vagal stimulation, eg. during a seizure - "ictal asystole", Benditt et al (2015)
• Trigeminocardiac reflex, eg. during posterior fossa surgery (Kafrouni et al, 2015)
• Direct pressure on the cardiomotor centres of the brain stem (Hirotsugu et al, 2015)
• Intentional transient arrest, eg. by adenosine during SVT termination or during aneurysm surgery (Lee et al, 2015)
• Cardiac contusion from blunt chest trauma (Morsy et al, 2015)

Unusual causes of PEA which have not made it into the 4 Hs and 4Ts:

• Exotic embolic events:
  • Air embolism
  • Amniotic fluid embolism
  • Fat embolism
• Structural cardiovascular catastrophe
  • Ventricular aneurysm rupture
  • Acute aortic root rupture, eg. failure following repair
  • Acute aortic valve prolapse eg failure following repair
• Unusual obstructive shock:
  • Occlusion of tricuspid or mitral valve orifice by atrial myxoma
  • Occlusion of aortic valve orifice by ventricular mural thrombus
  • Massive dynamic hyperinflation in bronchospasm