REVIEWS Paracetamol in critical illness: a review Sarah Jefferies, Manoj Saxena and Paul Young Paracetamol is one of the most widely used medicines in ABSTRACT the world, but there is a paucity of guidance on how to use it appropriately for antipyresis and analgesia in the critical Background: Paracetamol is one of the commonest care setting. In this review, we describe the history of medications used worldwide. This review was conceived as paracetamol and its pharmacology, and review the available a consequence of evaluating the literature in the protocol dataCrit to supportCare Resusc its prescription ISSN: 1441-2772 for critically 1 March ill patients. development of two randomised, controlled clinical trials 2012 14 1 74-80 investigating the safety and efficacy of paracetamol in ICU ©Crit Care Resusc 2012 www.jficm.anzca.edu.au/aaccm/journal/publi- patients (the HEAT [Permissive HyperthErmiA Through Thecations.htm history of paracetamol Avoidance of Paracetamol in Known or Suspected Infection Reviews The synthesis of paracetamol arose from major advances in in the Intensive Care Unit] study; the Paracetamol After chemistry occurring in the 19th century.1,2 With the develop- traumatic Brain Injury [PARITY] Study). ment of analytical chemistry came the ability to isolate and Objective: To provide a historical perspective on the purify medicinal chemicals from natural sources, including introduction of paracetamol into clinical practice, to present morphine from opium and quinine from cinchona bark. the pharmacology of paracetamol in critical illness, and However, chemists in continental Europe were hindered by evaluate the current evidence for its use as an antipyretic wars, which increased demands and restrictions on the and analgesic in intensive care. supply of foreign medicinal plants. Their attention moved to Design: Literature searches were performed using keywords: the development of synthetic alternatives from local “paracetamol”, “acetaminophen”, “critical illness”, resources, and the evolution of organic chemistry enabled “intensive care”, “history”, “pharmacology”, “antipyre*”, the development of new compounds for testing against “analgesi*”, “adverse effect*”, “administration and human disease. Quinine had been shown to reduce the fever dosage”, “toxicity”, “animals” and “humans”. and the disease course of malaria, and in the preantibiotic Data sources: Embase, MEDLINE, PubMed (1947/1950 era, antipyretics were a major focus of medicinal chemistry. to July 2011). The aniline derivatives (acetanilide, phenacetin and para- Review methods: The authors examined each article’s title cetamol) were identified as a class of antipyretic analgesics and abstract, fully reviewing relevant articles, with searching in the late 19th century. Paracetamol was first synthesised of reference lists and additional hand-searching. The most in 1878 by Harmon N Morse of Johns Hopkins University.3 recent and highest quality available evidence was included. However, it was many decades before the clinical utility of Results: Limited data are available on the pharmacology of paracetamol and its advantages over other drugs of the paracetamol in the critically ill. Among patients with sepsis, class were recognised. paracetamol may inhibit the immunological response. The first report of the administration of an aniline Among patients with neurological injury paracetamol can derivative to a patient was published in 1886 when acetani- reduce temperature but appears not to improve outcome. lide was mistakenly dispensed at a clinic in Strasbourg.4,5 When administered with opioids after major surgery, Subsequently, acetanilide’s antipyretic and analgesic effects paracetamol does not reduce the incidence of pain or opioid- were described and it was marketed for clinical use by Kalle related side-effects. and Company under the name antifebrin. The next year, Conclusion: Despite the widespread use of paracetamol in chemists at the German dye manufacturer Friedrich Bayer critical illness, there is a paucity of data supporting its utility in and Company synthesised phenacetin (acetophenetidin).5,6 this setting. Further research is required to determine how Less toxic than antifebrin, phenacetin became a popular paracetamol should be used in the critically ill. antipyretic analgesic and established Bayer as a leading 5 pharmaceutical company. Crit Care Resusc 2012; 14: 74–80 In 1893, Joseph von Mering, a prominent German physiologist, reported on his work with Bayer and Com- pany evaluating a derivative of phenacetin, which was uted methaemoglobinaemia, a side effect of aniline later named paracetamol.5,7 von Mering found the drug to derivatives, to this new drug. In 1894, Hinsberg and be an effective analgesic and antipyretic; however, possi- Treupel published work showing that paracetamol was bly using contaminated preparations, he mistakenly attrib- comparable to acetanilide and phenacetin in antipyretic 74 Critical Care and Resuscitation Volume 14 Number 1 March 2012 REVIEWS effect, but in view of the report by von Mering, paraceta- nolamine (AM404), which acts directly on vanilloid subtype 8 mol was not widely used. In 1899, Bayer produced 1 receptors and indirectly on cannabinoid type 1 (CB1) aspirin, which commonly appeared in combination with receptors in central nervous system thermoregulatory and phenacetin and caffeine in over-the-counter analgesic nociceptive pathways.23 In the central nervous system, mixtures for decades to follow.9 AM404 is formed from the conjugation of p-aminophenol It was not until the late 1940s that two teams of (deacetylated paracetamol) with arachidonic acid in the biochemists in the United States identified paracetamol as a presence of a catalyst, fatty acid amide hydrolase.34 AM404 major metabolite of acetanilide in human blood.10-12 Para- has been shown to inhibit in-vitro COX activity and prosta- 34 cetamol’s role in causing methaemoglobinaemia was dis- glandin E2 formation, and to increase the levels of an puted,13 and it was established as an efficacious endogenous cannabinoid neurotransmitter, N-arachi- analgesic.11,14 In 1953, paracetamol was marketed in the US donoylethanolamide (anandamide), via inhibition of the as a safer alternative to aspirin.5 In the United Kingdom, anandamide reuptake.35 Cannabinoids act on cannabinoid paracetamol first entered the market in 1956 as 500 mg receptors to lower temperature and modify nociceptive Panadol tablets available on prescription.15 signals.36,37 Ottani and colleagues demonstrated that block- Phenacetin was shown to be metabolised to paraceta- ade of CB1 receptors in rats prevented the analgesic effect mol,16 and, in the 1960s, the chronic use of phenacetin was of paracetamol.38 Furthermore, clinical effects, including associated with “analgesic nephropathy”, which was char- euphoria and relaxation, are noted to be shared by canna- acterised by renal papillary necrosis.17-19 Phenacetin was binoids and alanine derivatives.23 eventually identified as a carcinogen20 and subsequently There is evidence that a relationship exists between withdrawn in many countries, including Australia in 1977.21 paracetamol and endogenous opioid analgesic pathways. In the 1980s, paracetamol overtook aspirin as the most An animal study showed that paracetamol interacts at spinal popular over-the-counter analgesic.22 An intravenous for- and supraspinal levels to produce a synergistic analgesic mulation of paracetamol, which was developed in 2004 by effect.39 Three subtype selective opioid receptor antagonists Bristol–Myers Squibb, has further increased its use in the (, and ) have been demonstrated to attenuate this perioperative period. synergy.40 The clinical relevance of a potentiating effect of exogenous opioids on paracetamol analgesia in ICU Pharmacology of paracetamol in critical illness patients is discussed later in this review. Pharmacodynamics Pharmacokinetics Although paracetamol appears to act centrally, the details of The variability in pharmacokinetics as a result of different its pharmacodynamics remain in question. Research to date pathological processes among critically ill patients is well has described two main mechanisms to account for para- established.41,42 In the case of paracetamol, both the peak cetamol’s antipyretic and analgesic effects: selective inhibi- and the area under the curve of serum paracetamol levels tion of cyclo-oxygenase (COX) in the central nervous system; measured over 60 minutes after nasogastric administration and an indirect effect on cannabinoid and vanilloid tone.23,24 are reduced among critically ill patients who are intolerant In the 1970s, paracetamol’s antipyretic effect was shown of nasogastric feeding.43 However, postpyloric administra- to be associated with the inhibition of prostaglandin syn- tion leads to rapid absorption,44 and, based on a study of thetase (since named COX) in the brain.25 In contrast to patients admitted to ICU following cardiac surgery, it non-steroidal anti-inflammatory drugs (NSAIDs), which appears that when paracetamol is administered nasogastri- inhibit COX enzyme activity in central and peripheral cally, absorption is delayed rather than suppressed by critical tissues,26 paracetamol has been found to exert a tissue- illness.45 Rectal absorption of paracetamol is probably specific inhibitory effect on COX via a reduction reaction on poor.46 Although there are very few data examining the the COX enzyme in the presence of a low peroxide pharmacokinetics of paracetamol among critically ill environment,