Perioperative thirst management

Case history

A 70-year-old woman who underwent cystectomy and a urinary diversion procedure for bladder cancer 12 hours earlier was being cared for in the post-anaesthesia care unit (PACU). Her procedure was uncomplicated, and she had a seemingly stable postoperative course. At this stage, the surgeons advised her to drink small sips of clear fluids.

Clinical problem

A member of the PACU medical team assessed the patient as part of a daily comprehensive review and, on direct questioning, she reported feeling very thirsty.

Question to be answered

Does this feeling of thirst mean the patient is dehydrated?

Discussion

Thirst is a perception that provokes the urge to drink fluids [1]. This sensation is a signal of bodily disequilibrium, triggering a behavioural change to restore fluid balance [2, 3]. When not quenched in a timely manner, the symptom of thirst can become distressing. In the perioperative setting, patients commonly experience thirst. A national cross-sectional observational study in the United Kingdom suggested that thirst is the most prevalent type of severe discomfort within 24 hours after surgery [4]. However, in clinical practice, this symptom is often under-recognised, under-measured and under-treated [5]. This article summarises the physiology relating to thirst, discusses some of the possible factors contributing to this symptom during the perioperative period and, finally, explores thirst assessment and management.

Physiology of thirst

The perception of thirst occurs in higher-order centres of the brain and is controlled by complex integrated neural and hormonal mechanisms that act to regulate blood volume and composition by responding to hypernatraemia, hyperosmolality and hypovolaemia [6].

Whilst the neural pathways involved in thirst generation are complex, central to this process are brain structures called the circumventricular organs (CVOs), which are located in a region where the blood–brain barrier is absent [7]. This region is important because it allows effective communication between the blood and the brain parenchyma. Stimulation of the CVOs results in thirst, mainly mediated through negative feedback loops to maintain sodium concentration, plasma osmolality and blood volume within predetermined set ranges. All three variables are controlled independently of each other (Fig. 1) [6, 8].

Figure 1

Figure 1

Diagram illustrating the physiology of thirst.

Plasma osmolality is the main homeostatic parameter for the regulation of intracellular and extracellular hydration and is tightly controlled at approximately 285 mOsm/kg. Specialised osmoreceptors present in the CVOs are sensitive to increases in osmolality, and as little as an increase of 1–2% in plasma osmolality can stimulate thirst [9]. Extracellular Na⁺ concentration is also tightly regulated within the range 135–145 mmol/L, and an increase in Na⁺ concentration typically indicates water deficit [9]. Blood volume is less tightly controlled, as even small changes in extracellular fluid (ECF) volume are well tolerated due to dynamic compensatory changes in vasomotor tone that modulate the compliance and ability of the vascular system. Consequently, hypovolemic thirst mechanisms are less sensitive than those associated with changes in osmolality and natraemia, and thirst is initiated only after a decrease of approximately 10% of the total plasma volume [9]. Blood volume is monitored by stretch forces within the vasculature walls. The neural link to these receptors is unknown, but appears to involve the CVOs [8]. Additionally, angiotensin II, the product of the hormonal cascade resulting from hypovolaemia, acts directly on the CVOs to stimulate thirst [8]. Interestingly, as hypovolaemia typically reflects the deficit of both free water and solutes, the resultant thirst is sometimes referred to as the ‘sodium appetite’, which describes a feeling of thirst that cannot be relieved solely by water intake without also solute replacement [9]. Once blood osmolality, natraemia and volume are restored, thirst is inhibited through feedback mechanisms, such as the release of atrial natriuretic peptide [8].

In addition to the central homeostatic regulation of ECF volume, natraemia and osmolality near the desired set points, which aim to correct the delayed consequences of ongoing physiological abnormalities, additional mechanisms regulate thirst before changes in blood composition manifest. These ‘pre-absorptive’ pathways account for responses that drive water intake in anticipation of impending systemic solute loads or water deficits associated with various behavioural and environmental conditions [6]. The pre-absorptive pathways include prandial thirst, which complements food ingestion with water intake to maintain isotonicity, and thermic thirst, which occurs in response to hyperthermia (e.g. during exercise), when water intake would counteract the dehydrating effect of thermoregulatory cooling [8].

Aetiology of perioperative thirst

Perioperative thirst occurs commonly, especially during the immediate postoperative period, and has several potential aetiologies. Abnormalities in hydration status reflect relative or absolute disturbances in total body water and serum or plasma osmolality. An elevated osmolality typically, though not invariably, indicates dehydration [10]. In this setting, thirst is an appropriate response, and therapy should be directed towards correcting an existing or induced fluid or electrolyte disturbance occurring during the perioperative period [9, 11].

Thirst that is attributed to fluid deficits, with or without salt deficit, may either be long-standing or result from perioperative events. Chronic dehydration or hypovolaemia are more prevalent in older people because of reduced renal concentrating ability, decisions to drink less fluid due to issues around continence and access to toilets, and the side effects of medications such as diuretics [10]. Preoperative iatrogenic fluid deficits mainly occur due to interventions, such as bowel preparation or prolonged starvation before surgery [12, 13]. Causes for intraoperative fluid deficits include blood loss, inflammation-related fluid shifts, evaporation of water from exposed membranes and inadequate fluid replacement. Postoperative fluid deficits can occur from surgical restrictions on oral intake or from enteral losses from nasogastric tubes, vomiting or high stoma outputs [9].

Thirst may also be described in terms of xerostomia, also called dry mouth, which can occur postoperatively independently of fluid deficit [9]. This may be due to fear or anxiety and is mediated by the sympathetic nervous system [14]. Additionally, dry mouth is a feature of opioid therapy that is ubiquitous after surgery and can modulate angiotensin II function [15]. Opioids are one of the many medications that are commonly used perioperatively and that have anticholinergic properties, leading to dry mouth and thirst [9].

Assessment of thirst

Most patients do not spontaneously report their thirst, even when it causes intense distress [14]. Consequently, healthcare professionals should actively look for associated symptoms. The assessment should evaluate not only the intensity of thirst using a visual analogue scale and verbal, numerical or face scale, but also the patient’s resulting discomfort [14]. An expert group in Brazil has developed the Perioperative Thirst Discomfort Scale for the latter (Fig. 2) [13].

Figure 2

Figure 2

Perioperative Thirst Discomfort Scale. A three-level clinical assessment scale measuring perioperative thirst from ‘not uncomfortable’ to ‘very uncomfortable’ for associated symptoms. This adapted scale was reused with permission from Nascimento et al. [14]. The original scale was developed by Martins et al. [13].

Management strategies for perioperative thirst

Strategies to alleviate thirst can be divided into pre-absorptive and post-absorptive methods [14]. The latter relates to the correction of hypovolaemia, and osmolar and electrolyte imbalances. With regard to the prevention of thirst, although clinical practice guidelines recommend that ‘clear liquids’ may be ingested up to 2 hours before procedures requiring anaesthesia, patients may view this as a a minimum time and fast for a longer period [16]. Emphasis is now being shifted from merely ensuring that a minimal fasting interval has been achieved to actively encouraging patients to drink fluids until 2 hours before elective surgery, or even until transfer to the operating theatre [17, 18, 19]. The intricacies of perioperative fluid management are extremely complicated, hotly debated and beyond the scope of this case study. However, interestingly, innovative strategies are being evaluated by researchers, such as patient-controlled thirst-driven fluid administration [20].

Concerning pre-absorptive strategies for thirst management, the practice of moistening a patient’s mouth with a wet sponge filled with room temperature water is common in clinical practice; however, this is not considered to be particularly effective [3]. By contrast, one may relate to their own experiences of being able to almost immediately quench thirst with a cold drink. Pre-absorptive satiety can occur before any change in blood osmolality, natraemia or restoration of blood volume and can be triggered by cold temperature, in which specialised receptors present in the oropharynx are stimulated. Sensory afferent fibres from the trigeminal and glossopharyngeal nerves conduct signals to the somatosensory cortex, thus inhibiting the thirst reflex [3]. A randomised controlled trial demonstrated that ice popsicles had greater efficacy than water at room temperature for thirst management during the immediate postoperative period. Furthermore, evidence suggests that ice popsicles can reduce the occurrence of postoperative nausea and vomiting and the use of antiemetics [21]. In view of the potential benefits and minimal perceived risk, patients admitted to our PACU are offered ice popsicles, affectionately referred to as ‘POMpops’, on a daily basis [22]. Additionally, on the same physiological premise, strategies involving the use of menthol in lip balm, popsicles and chewing gum can decrease both the intensity of thirst and associated discomfort [11, 23, 24].

Revisiting our clinical vignette, the patient’s thirst could be an indication of dehydration, and it is important to address this and any other reversible physiological factors. However, in keeping with the ethos of patient centred care, we should also acknowledge thirst as an unpleasant experience and consider pre-absorptive strategies to promptly alleviate this symptom. Furthermore, much like pain management, patient–clinician discussions are likely to be important for thirst management. Communication with the patient should include informing them that thirst is a common experience after surgery. It could be argued that this information is more appropriately delivered in the preoperative setting, as an unexpected onset of symptoms may cause unnecessary anxiety and heighten their adverse experience. We should also involve the patient in understanding why they are experiencing thirst and how the thirst symptom can be alleviated.

Disclaimer

This Narrative article is adapted with permission from Case Studies in Perioperative Medicine, a UCL Press open access educational resource, available from https://doi.org/10.14324/111.444.9781787356917.17

Declarations and conflicts of interest