adenosine is a naturally occurring endogenous nucleoside
adenosine is rapidly taken up by RBC's & endothelial cells such that almost completely eliminated from circulation after 1st pass → plasma T1/2 < 10secs;
hence can expect exaggerated response in anaemic patients, & reduced response if infusion in peripheral vein rather than IV bolus into central vein;
Precautions and contraindications:
only use with full resuscitative facilities on hand
avoid in severe coronary artery disease as risk of vasodilatation of normal vessels may produce ischaemia in vulnerable territory
give half these doses if administering via a central venous catheter (CVC).
Insert cannulae into a large proximal peripheral vein (the cubital fossa is ideal) with three way tap attached
Draw up starting dose of adenosine 0.1-0.2 mg/kg (6 or 12mg for an adult). If necessary dilute to 1 ml with normal saline.
Draw up 10 ml saline flush & warn patient they will feel awful!
Turn on the ECG trace recorder
Administer adenosine as a rapid IV push followed by the saline flush
Repeat procedure at 2 minutely intervals, until tachycardia terminated, increasing the dose of adenosine by 0.05mg/kg (6mg for an adult) each time up to a maximum of 0.5 mg/kg (max dose 18mg).
adenosine success rates in adults with Rx SVT:
1 dose 43%; 2nd dose 34%; 23% require a 3rd dose although a third of these will fail (ie. 8% of all patients and 23% of those with PH SVT)
failure risk score = (age/heart rate on admission) + number of past paroxysmal SVT episodes
if score > 1.18 then sens. 96% and spec. 71% for failure of adenosine protocol
ref: Am J Emerg Med 2008; 26:304-9.
Adverse Effects:
usually last < 2 min. due to short half life
facial flushing -18% if IV bolus;
SOB -12% if IV bolus;
chest pain/pressure - 11% if IV bolus;
hypotension
occasionally, more prolonged AV block or sinus arrest after IV bolus
dose dependent decreased renal blood flow, GFR & urine flow if infusion
myocardial ischaemia presumably via coronary steal;
AV block - 2nd/3rd degree occurred in 6% with infusion @ 140ug/kg/min for 6min;
adenosine used to Rx SVT is said to result in atrial fibrillation in up to 12% of patients (Strickberger et al. Ann.Intern.Med. 1997)
adenosine used to unmask flutter waves in atrial flutter has been reported to result in 1:1 conduction with haemodynamic compromise.
Pharmacodymanics and physiology:
Effects mediated by adenosine receptors on outer surface of cell membrane:
Type A1:
activate K channels & inhib. c-AMP accumulation;
predominate in myocardium;
Type A2:
stimulate release of EDRF & the accum. of i/cell. c-AMP;
esp. in coronary arteries;
Type A3:
Type A4:
Adenosine may play a key role in the production of, & mediation of anginal PIC;
Electrophysiologic effects (mediated by A1):
-ve chronotropic effect on sinus node automaticity;
-ve dromotropic effect on AV nodal conduction;
In supraventricular tissues (sinus node, AV node):
activates K outward current in a distinct K channel
⇒ tissue hyperpolarisation;
In ventricular tissue:
acts primarily by attenuation of catecholamine-stimulated Ca fluxes;
Summary:
when given as a bolus dose IV, it directly inhibits AV nodal conduction and increases AV nodal refractory period, but has only mild effects on SA nodal function.
Cardioprotective effects:
Adenosine is released from myocardium when O2 supply:demand ratio falls →:
coronary vasodilatation
antiadrenergic via:
decr. release NA from cardiac symp. Ns;
attenuates stimulating effects of catecholamines on heart;
antiarrhythmic:
exog. adenosine → decr. severity of ischaemia-induced vent. arrhthm.
metabolic effects:
delays myocardial ATP depletion;
incr. myocardial glucose uptake;
incr. glycolytic fluxes;
attenuation of microvascular injury:
? prevention of neutrophil/platelet aggreg.
? inhib. of free O2 radical release from activated neutrophils;