Abstract
It is said that caffeine is the most widely consumed psychoactive substance in the world, with 80% of the adult population consuming sufficient quantities to affect brain function. 1
Chronic caffeine use produces physical, emotional and psychological dependence; acute withdrawal of caffeine may result in the caffeine withdrawal syndrome. Features of the syndrome include headache, fatigue, irritability, nausea and vomiting, constipation, tachycardia and altered blood pressure with 13% of those affected showing distress or functional impairment. The syndrome typically starts 12–24 h from withdrawal and lasts two to nine days. 2
Enteral feeds given to adults in hospital do not contain caffeine (British National Formulary Borderline Substances; https://bnf.nice.org.uk/borderline-substance-taxonomy, accessed 16 July 2020). Thus, many patients will be obliged to experience acute caffeine withdrawal soon after admission to intensive care unit (ICU), with the effects in some lasting for more than a week.
Caffeine is a methylxanthine whose main mechanisms of action are competitive antagonism of adenosine receptors (mainly but not exclusively A1 and A2a), mobilisation of intracellular calcium and non-selective competitive inhibition of phosphodiesterases. Of these, only the adenosine antagonism is likely to be significant at the usual self-administered doses. 3 In chronic users, it can be difficult to separate the direct effects of caffeine, which show inter-individual variation, from the effect of prevention of withdrawal.
It is arguable that patients suffering from acute caffeine withdrawal are at higher risk of agitation and delirium as a result of the mood changes and physiological upset. However, apart from avoidance of acute withdrawal syndrome, administration of caffeine may have beneficial effects in ICU. There is observational evidence of neuroprotection in severe traumatic brain injury: a level over 1 µmol/L in cerebrospinal fluid was associated with higher incidence of favourable Glasgow Outcome Score at 6 months, attributed to either upregulation of A1 or antagonism of A2a adenosine receptors. 4 Aminophylline, a closely related drug, is still used as a bronchodilator and has been used to protect renal function and maintain cardiac output in the ICU. 5
In conclusion, perhaps the tradition of “a nice cup of tea” should not be overlooked for patients (as well as relatives and staff) when things are not going well in the ICU. Just make it a strong one.
