**1. Introduction**

The International Association for Study of Pain defines pain as an unpleasant experience, with or without tissue damage, which can be related to individual memories, life expectations and

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emotions [1]. The painful experience involves interpretation of biological aspects of pain and its interaction with social and cultural characteristics [2].

In surgical procedures, moderate to severe pain can be observed in up to 40% of cases [3], representing an important source of complications as well as morbidity and mortality in the postoperative period [4]. Postoperative pain can limit mobility and respiratory function, increasing the incidence of atelectasis, pneumonia and thromboembolic events [5, 6].

Moreover, the lack of adequate pain control in acute situations can lead to chronic pain, with deleterious effects for the patient and health‐related quality of life [7]. Despite these findings, between 50% to 75% of those submitted to major surgery do not receive enough analgesic medication, increasing the risk of complications and length of stay and costs for the health system [8].

Morphine was isolated by a German pharmacist Friedrich Wilhem Sertürner in 1806 and, after that, opioids have become widely used in clinical practice for pain control. Later in 1844, parenteral administration of morphine has started after the introduction of glass syringe [2].

In 1963, Roe demonstrated that administration of small doses of intravenous morphine allowed a better pain control compared to intramuscular injections [9]. Sechzer, in 1968, was the first to evaluate the quality of analgesia after administration of small doses of opioids *per* patient request, performing the first patient‐controlled analgesia (PCA). Due to complex logistic to meet the requests of many patients, which would require numerous nursing staff, Sechzer and other doctors began to develop equipment prototypes for anal‐ gesic administration with reduced costs. The first PCA pump available for marketing was named "Cardiff Palliator" and it was developed in the Welsh National School of Medicine in 1973 [10, 11].

Since then, several drugs and routes of administration have been used in PCA, with differences in analgesic efficacy, tolerability profile, adverse effects, and procedure‐related complications as well as patient satisfaction [12].
