In severe hypothermia behaviour may become increasingly erratic and illogical. As the core temperature drops below 35°C, the heart rate begins to progressively slow, leading to bradycardia which is defined as a resting heart rate of 60 bpm or less. This leads to reduced tissue and organ perfusion.
Unless treated, severe hypothermia may progressively lead to cardio-respiratory failure and death. However, clinicians have to be extremely careful when pronouncing someone dead as a result of hypothermia since the heart rate and brain activity can be reduced significantly, leading to a torpid state that closely resembles death. Hypothermia is usually treated by moving the patient to a warm environment, ensuring that they are adequately clothed, ideally in multiple layers of dry clothing, and if necessary re-warming the body using heated blankets, water bottles or devices that blow warm air over the body.
Elderly patients are particularly vulnerable to hypothermia in the winter months and this is explored in Grace’s case study.
Case study: Grace – hypothermia
Grace is 82 and lives on her own in a small two-bedroom terraced house. Her only regular income is her state pension, which she proudly manages very effectively using little boxes to put aside money each week for food, utility bills and a small amount of pocket money for her four grandchildren.
This winter has been cold with several heavy snowfalls and Grace has been worrying about her heating costs. Instead of using her central heating, since the beginning of January she has been spending most days in her living room using a small fan heater for warmth and taking a hot water bottle to bed. Amy is Grace’s youngest daughter and travels from Brighton to visit her mother once a week. During her last visit Amy was horrified to find all of the heating turned off and her mother wandering the house in a very confused state.
From what you have learnt in this chapter it should be clear that Grace is suffering from hypothermia. Nurses are often required to pass on advice to patients and their families, and this is highlighted in Activity 2.2.
Activity 2.2 Evidence-based practice and research
What advice would you give to Grace and her family to reduce the chances of Grace suffering hypothermia again?
Grace’s case study illustrates well the negative impact of a variable such as body temperature straying too far from its ideal physiological set point. However, set points are not always fixed and can be shifted when necessary.
Pyrexia: the fever response
Since the core temperature of the body is maintained at around 37°C, many human pathogens have adapted to replicate fastest at this temperature. During infection their numbers can grow exponentially, placing the body at risk of systemic infection and potentially sepsis and septic shock. Fortunately, the human body can respond by increasing body temperature to help slow down pathogenic replication. When leukocytes (white blood cells) begin to fight infection by trapping and killing pathogens, they release a small protein called interleukin-1 (IL-1). IL-1 can initiate a fever response by binding to receptors on the hypothalamus (Dinarello, 2015).
In response to IL-1 the hypothalamus releases a chemical called prostaglandin E2 (PGE2) which functions to shift the set point of the hypothalamus up from 37°C to 38–9°C. This increases the temperature of the body and takes it outside the favourable range for bacterial and viral replication, and as a result the rate of infection will slow. A fever response also allows more rapid trapping and killing of pathogens by leukocytes, some of which function more efficiently at these higher temperatures. Unfortunately, the other cells within the body are now outside of their optimal temperature range and their enzymatic activity slows; as a result, during a fever response we suffer malaise, feeling very ill and lacking in energy.
A normal fever response of 38–9°C is generally regarded as being healthy and beneficial to the body since it slows pathogen growth and speeds up killing, but when the fever response goes beyond 39°C, e.g. 40°C and beyond, this can be dangerous since cellular enzymes may be denatured and life-threatening convulsions can occur. Most non-steroidal anti-inflammatory drugs, e.g. aspirin, are very effective at reducing fever and function by blocking the production of PGE2 in the hypothalamus, thereby preventing the set point being shifted upwards.
To further your understanding of pyrexia, read through Prisha’s case study.
Case study: Prisha – tonsillitis
Prisha is a 19-year-old undergraduate student and for the last week has been suffering from a sore throat. Initially she thought she was suffering from a common cold, but on looking at her throat in a mirror she was horrified to see that her tonsils were enlarged and purulent. The campus GP recorded her tympanic temperature at 39.1°C and, on noting her inflamed tonsils, took mouth swabs and palpated her cervical lymph nodes (in the neck), and noted they were swollen and tender. She was immediately diagnosed with tonsillitis, put on a course of antibiotics and advised to rest. A few days later Prisha was informed that the throat swabs had revealed she had streptococcal tonsillitis; her GP was happy that this was responding well to the prescribed antibiotics.
Throughout this chapter we have seen that homeostasis is reliant on negative feedback mechanisms to minimise deviations from the physiological set point to constrain a variable within its normal range. However, in some physiological situations it is desirable for the overall health of the body to temporarily deviate dramatically from a physiological set point.
Positive feedback
The term positive feedback is used to describe situations where deviations from the physiological set point are amplified and made larger. In humans the classic example of positive feedback occurs during childbirth (Figure 2.3). As the time of delivery nears at around 9 months, the muscular wall (myometrium) of the uterus is progressively stretched. This activates stretch receptors in the uterine wall and nerve impulses are generated and relayed to the brain. The posterior pituitary gland releases the hormone oxytocin, which circulates in the blood before binding to receptors on the smooth muscle cells of the myometrium, initiating uterine contraction.
This process is then perpetuated with uterine contraction leading to further oxytocin release, which then itself leads to further uterine contraction. This establishes a positive feedback loop during which the uterine contractions get more and more powerful and closer together until eventually the baby is delivered. Levels of oxytocin remain high in the mother’s blood following delivery; this hormone is often referred to as the ‘love hormone’ since it promotes feelings of love and affection which play a key role in the bonding between mother and newborn baby. Oxytocin also stimulates the ‘let-down reflex’ which pushes milk into the milk ducts of the breast towards the nipple to allow the baby to feed.
Figure 2.3 Childbirth (parturition)
Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://openstax.org/books/anatomy-and-physiology/pages/preface
Now that you have completed the chapter, attempt the multiple-choice questions in Activity 2.3 to assess your knowledge.
Activity 2.3 Multiple-choice questions
1 The set point of a variable isa) The value that the variable is always kept
