Thermometry methods and norms
The traditional and most widespread standard for measuring body temperature is the axilla (armpit). Normal values in this area range from 36 to 37 °C. Physiological processes dictate the presence of circadian rhythms: throughout the day, the temperature can fluctuate within 1 °C.
In certain clinical situations, rectal measurement is used. This method is indicated in cases of general hypothermia, when superficial tissues do not reflect the true core temperature, as well as in the presence of local lesions in the axillary region. In gynecology, rectal thermometry is used to track basal body temperature and determine ovulation.
A drop in body temperature below the physiological norm is called hypothermia. In clinical practice, this condition is relatively rare. The main causes of hypothermia include severe general exhaustion (cachexia), heart failure, pronounced intoxication, and endocrine disorders, particularly hypothyroidism.
Classification by degree of elevation
To assess the severity of the patient's condition and choose management tactics, fever is classified according to the maximum level of temperature elevation. There are four main grades:
- Subfebrile: temperature stays in the range of 37–38 °C.
- Moderate (febrile): values range from 38 to 39 °C.
- High: from 39 to 41 °C.
- Hyperpyretic: exceeds 41 °C. This is an extreme reaction that poses a direct threat to body systems.
Etiology: why temperature rises
Temperature elevation is a universal symptom triggered by a multitude of factors. The main groups of causes include:
- Infectious processes: The most frequent cause. The response is triggered in response to the introduction of bacteria, viruses, and other pathogens.
- Non-infectious (aseptic) inflammation: Occurs when own tissues are destroyed (e.g., necrosis in myocardial infarction) or against the background of autoimmune pathologies.
- Malignant neoplasms: Fever acts as part of a paraneoplastic syndrome. It is typical for Hodgkin's lymphoma (differs in a prolonged persistent course), hypernephroma (renal cell carcinoma), and bronchogenic carcinoma.
- Drug reaction: Temperature elevation can be a side effect of analgesics, antibiotics, and other medications.
- Endocrine regulation: An excess of certain hormones directly enhances heat production. A classic example is prolonged low-grade fever in hyperthyroidism.
- Neurogenic (central) mechanisms: Occur with direct structural damage to the diencephalon (diencephalon). This is caused by encephalitis, head trauma, and hemorrhages into the cerebral ventricles.
Fever of unknown origin is singled out separately — a complex clinical situation where the true cause of persistent temperature elevation remains unrecognized for a long time.
Pathogenesis: mechanism of thermoregulation restructuring
True fever is based on a cascade of complex biochemical reactions that alter hypothalamic settings. The scheme of the process development is as follows:
- Exposure to primary pyrogens (infectious agents or tissue breakdown products).
- Activation of leukocytes in the focus of inflammation.
- Synthesis of specific molecules — pro-inflammatory cytokines. A key role is played by interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor (TNF).
- Cytokines reach the brain via the bloodstream and stimulate the production of prostaglandin E2.
- Prostaglandin E2 acts on the thermoregulatory center in the hypothalamus, shifting its "set point" (set point) to a higher level. From this moment on, the hypothalamus perceives normal body temperature as hypothermia and forces the body to store heat.
Fever and hyperthermia: clinical significance
A key task in differential diagnosis is distinguishing between true fever and heat-related illnesses (hyperthermia).
The main pathogenetic difference of hyperthermia is that the "set point" of the thermoregulatory center remains normal. The problem lies solely in a balance disruption: either heat production (or external heat input) critically exceeds the heat dissipation capacity, or the heat dissipation mechanisms themselves are impaired. As a result, the temperature rises uncontrollably, despite the fact that the body is trying its best to lower it.
It is important for the physician to remember the main principle: fever does not always signal an infection. It is necessary to scrupulously investigate the cause (tumor, endocrine disorder, central nervous system lesion, overheating), since the unjustified appointment of antimicrobial therapy may not only fail to help, but also worsen the patient's condition.
Briefly: answers to topical questions
What clinical stages are distinguished in the course of fever?
Three main clinical stages are distinguished in the course of fever.
- Onset of symptoms (stadium incrementi) — the initial stage caused by a shift in the ratio between heat production and heat dissipation in favor of temperature elevation.
- Peak severity (stadium fastigii) — the period of the height of the disease with the greatest severity of general nonspecific symptoms and the appearance of specific ones.
- Attenuation of manifestations (stadium decrementi) — the period of temperature drop and fading symptoms.
What types of temperature curves exist depending on daily fluctuations?
Depending on the amplitude and character of daily fluctuations, the following types of temperature curves are distinguished.
- Continuous fever (febris continua) — daily fluctuations do not exceed 1 °C.
- Remittent fever (febris remittens) — daily fluctuations are 1–2 °C, without dropping to normal.
- Hectic fever — daily fluctuations of more than 2–3 °C with rapid drops.
- Irregular fever — complete absence of patterns in fluctuations.
- Inverted fever — morning temperature is higher than evening temperature.
What types of temperature reduction are distinguished in the third stage of fever?
During the subsidence of the febrile period, two variants of temperature reduction are distinguished.
- Lytic decrease (lysis) — a gradual drop in temperature over several days. This variant is more common and more favorable.
- Critical decrease (crisis) — a sudden and rapid drop in temperature within a few hours. It occurs due to the sharp activation of heat dissipation (dilation of arterioles, profuse sweating) and carries the risk of developing acute vascular collapse.
Class and exam questions
How does fever differ from hyperthermia?
In fever, the hypothalamus purposefully raises the thermoregulatory "set point". In hyperthermia, the point remains normal, but the body physically cannot cope with excess heat due to impaired heat dissipation or overheating.
Are antibiotics always necessary when body temperature rises?
No. Fever can be caused by non-infectious factors: myocardial infarction, medication use, endocrine disorders, or the growth of a malignant tumor.
What is fever of unknown origin?
This is a clinical condition in which the patient has a prolonged elevated temperature, but the exact cause (etiology) cannot be established despite examination.
What else to study on this topic
- Detailed mechanism of prostaglandin E2 synthesis and targets for antipyretic drugs
- Features of thermometry in different body areas (axillary, rectal, oral)
- Paraneoplastic syndrome as a cause of prolonged low-grade fever
- Algorithm for searching for causes in fever of unknown origin
- Neurogenic fever in diencephalic lesions
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