Environmental medicine

Chapter 18 Environmental medicine




Disease and the environment


The incidence and prevalence of disease and causes of death within a community are a reflection of interrelated factors:



Some of these environmental effects have been clearly documented within the last decade, e.g. the massive civilian mortality and morbidity during the current Afghan and previous Iraq wars, and loss of life and disease prevalence following the 2006 tsunami, the earthquakes in Sechuan (2008) and Haiti (2010) and cyclone Nargris in the Irawaddy delta. Flooding caused by El Niño in East Africa not only resulted in an increase in breeding sites for mosquito vectors but a major outbreak of Rift Valley fever due to the enforced close proximity of cattle with humans.


Smoking (active and passive), obesity and excess alcohol consumption also play a significant role in disease. Worldwide health programmes have been established in most countries to reduce their effects.




Heat


Body core temperature (TCore) is maintained at 37°C by the thermoregulator centre in the hypothalamus.


Heat is produced by cellular metabolism and is dissipated through the skin by both vasodilatation and sweating and in expired air via the alveoli. When the environmental temperature (TEnv) is >32.5°C, profuse sweating occurs. Sweat evaporation is the principal mechanism for controlling TCore following exercise or in response to an increase in TEnv.


Heat acclimatization. Acclimatization to heat takes place over several weeks. The sweat volume increases and the sweat salt content falls. Increased evaporation of sweat reduces TCore.


Heat cramps. Painful muscle cramps, usually in the legs, often occur in fit people when they exercise excessively, especially in hot weather. Cramps are probably due to low extracellular sodium caused by excess intake of water over salt. Cramps can be prevented by increasing dietary salt. They respond to combined salt and water replacement, and in the acute stage to stretching and muscle massage. TCore remains normal.


Heat illness. At any environmental temperature (especially with TEnv of >25°C), and with a high humidity, strenuous exercise in clothing that inhibits sweating such as a wetsuit can cause an elevation in TCore in <15 minutes. Weakness/exhaustion, cramps, dizziness and syncope, with TCore >37°C, define heat illness (heat exhaustion). Elevation of TCore is more critical than water and sodium loss. Heat illness may progress to heat injury, a serious emergency.




Heat injury


Heat injury (heat stroke) is an acute life-threatening situation when TCore rises above 41°C. There is headache, nausea, vomiting and weakness, progressing to confusion, coma and death. The skin feels intensely hot to the touch. Sweating is often absent, but not invariably.


Heat injury can develop in unacclimatized people in hot, humid windless conditions, even without exercise. Sweating may be limited by prickly heat (plugging or rupture of the sweat ducts, leading to a pruritic papular erythematous rash). Excessive exercise in inappropriate clothing, e.g. exercising on land in a wetsuit, can lead to heat injury in temperate climates. Diabetes, alcohol and drugs, e.g., antimuscarinics, diuretics and phenothiazines, can contribute. Heat injury can lead to a fall in cardiac output, lactic acidosis and intravascular coagulation.





Cold


Hypothermia is defined as a core temperature of <32°C. It is frequently lethal when TCore falls below 30°C. Survival, with full recovery has however been recorded with TCore of <16°C. Cold injury includes:





Hypothermia


Hypothermia occurs in many settings.


At home. Hypothermia can occur when TEnv is below 8°C, when there is poor heating, inadequate clothing and poor nutrition. Depressant drugs, e.g. hypnotics, as well as alcohol, hypothyroidism or intercurrent illness also contribute. Hypothermia is commonly seen in the poor, frail and elderly. The elderly have diminished ability to sense cold and also have little insulating fat. Neonates and infants become hypothermic rapidly because of a relatively large surface area in proportion to subcutaneous fat.


Outdoors on land. Hypothermia is a prominent cause of death in climbers, skiers, polar travellers and in wartime. Wet, cold conditions with wind chill, physical exhaustion, injuries and inadequate clothing are contributory. Babies and children are at risk because they cannot take action to warm them themselves.


Cold water immersion. Dangerous hypothermia can develop following immersion for more than 30 min to 1 hour in water temperatures of 15–20°C. In TWater below 12°C limbs rapidly become numb and weak. Recovery takes place gradually, over several hours following rescue.






Sequelae


Pulse rate and systemic BP fall. Cardiac output and cerebral blood flow are low in hypothermia and can fall further if the upright position is maintained, the thorax restrained by a harness or by hauling during evacuation. This is why helicopter and lifeboat winch rescues are often carried out with a stretcher, rather than a chest harness.


Respiration becomes shallow and slow. Muscle stiffness develops; tendon reflexes become sluggish, then absent. As coma ensues, pupillary and other brainstem reflexes are lost; pupils are fixed and may be dilated in severe hypothermia. Metabolic changes are variable, with either metabolic acidosis or alkalosis. Arterial PO2 may appear normal, i.e. falsely high.


There is shift of the oxygen dissociation curve to the left because of the reduction in temperature of haemoglobin. Thus, if an arterial blood sample from a hypothermic patient is analysed at 37°C, the PO2 will be falsely high. Within the range 37–33°C this factor is around 7% per °C. Many blood gas machines also calculate the arterial saturation; this too will be falsely high. When a patient is monitored using a pulse oximeter, the level of arterial oxygen saturation (SaO2) will however be correct – but if SaO2 is then converted by calculation to PaO2, a downwards correction must be applied – simply due to hypothermia.


Bradycardia with ‘J’ waves (rounded waves above the isoelectric line at the junction of the QRS complex and ST segment) are pathognomonic of hypothermia. Prolongation of PR and QT intervals and QRS complex also occur. Ventricular dysrhythmias (tachycardia/fibrillation) or asystole are the usual causes of death.







Cold injury





Management


Transport the patient – or if this is impossible, make them walk, even on frostbitten feet – to a place of safety before commencing warming. Warm the frozen part by immersion in hand hot water at 39–42°C, if feasible. Assess hypothermia. Continue warming until obvious thawing occurs; this can be painful. Vasodilator drugs have no part in management. Blisters form within several days and, depending on the depth of frostbite, a blackened shell – the carapace – develops as blisters regress or burst. Dry, non-adherent dressings and aseptic precautions are essential, though hard to achieve. Frostbitten tissues are anaesthetic and at risk from further trauma and infection. Recovery takes place over many weeks, and may be incomplete. Surgery may be needed, but should be avoided in the early stages.


Mar 31, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Environmental medicine
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