In the summer in warm climates, an area of high pressure with little or no rain or clouds, the air and ground easily heats to excess. A static high pressure area can impose a very persistent heat wave.
The position of the jet stream allows air on one side to be considerably warmer than the other side. Heat waves are far more common and more severe on the warm side and at times an unusual position of the jet stream places unusual warmth in an unusual place for hot weather, and imposes a heat wave. El Niño and La Niña (opposite reaction to El Niño) can severely disrupt the positions of the jet streams
Large desert zones and dry areas are more likely to get extreme heat because there is rarely any high cloud cover with very low humidity.
Winds from hot deserts typically push hot, dry air towards areas normally cooler than during a heat wave. During the summer an area that has no geographic features that might cool winds that originate in the hot deserts get little mitigation, especially near the summer solstice when long days and a high sun would create warm conditions even without the transport of hot air from other locations. Should such a hot air mass travel above a large body of water, as a sirocco of Saharan origin crossing the Mediterranean sea, it likely picks up much water vapor with a reduction in temperature but far greater humidity that makes the original desert air little less moderate as demonstrated in a high heat index. Heat waves can also come from air originating over tropical seas penetrating far into the middle latitudes, as often occurs in the eastern United States and southeastern Canada. The heat island effects of large cities only exacerbate heat in large cities that endure heat waves because of the weakness of night-time cooling.
Hyperthermia, also known as heat stroke, becomes commonplace during periods of sustained high temperature and humidity. Sweating is absent from 84%-100% of those affected. Older adults, very young children, and those who are sick or overweight are at a higher risk for heat-related illness. The chronically ill and elderly are often taking prescription medications (e.g., diuretics, anticholinergics, antipsychotics, andantihypertensives) that interfere with the body's ability to dissipate heat.[8]
Heat edema presents as a transient swelling of the hands, feet, and ankles and is generally secondary to increased aldosterone secretion, which enhances water retention. When combined with peripheral vasodilation and venous stasis, the excess fluid accumulates in the dependent areas of the extremities. The heat edema usually resolves within several days after the patient becomes acclimated to the warmer environment. No treatment is required, although wearing support stocking and elevating the affected legs with help minimize the edema.
Heat rash, also known as prickly heat, is a maculopapular rash accompanied by acute inflammation and blocked sweat ducts. The sweat ducts may become dilated and may eventually rupture, producing small pruritic vesicles on an erythematous base. Heat rash affects areas of the body covered by tight clothing. If this continues for a duration of time it can lead to the development of chronic dermatitis or a secondary bacterialinfection. Prevention is the best therapy. It is also advised to wear loose-fitting clothing in the heat. However, once heat rash has developed, the initial treatment involves the application of chlorhexidine lotion to remove any desquamated skin. The associated itching may be treated with topical or systemic antihistamines. If infection occurs a regimen of antibiotics is required.
Heat cramps are painful, often severe, involuntary spasms of the large muscle groups used in strenuous exercise. Heat cramps tend to occur after intense exertion. They usually develop in people performing heavy exercise while sweating profusely and replenishing fluid loss with non-electrolyte containing water. This is believed to lead to hyponatremia that induces cramping in stressed muscles. Rehydration with salt-containing fluids provides rapid relief. Patients with mild cramps can be given oral .2% salt solutions, while those with severe cramps require IV isotonic fluids. The many sport drinks on the market are a good source of electrolytes and are readily accessible.
Heat syncope is related to heat exposure that produces orthostatic hypotension. This hypotension can precipitate a near-syncopal episode. Heat syncope is believed to result from intense sweating, which leads to dehydration, followed by peripheral vasodilation and reduced venous blood return in the face of decreased vasomotor control. Management of heat syncope consists of cooling and rehydration of the patient usingoral rehydration therapy (sport drinks) or isotonic IV fluids. People who experience heat syncope should avoid standing in the heat for long periods of time. They should move to a cooler environment and lie down if they recognize the initial symptoms. Wearing support stockings and engaging in deep knee-bending movements can help promote venous blood return.
Heat exhaustion is considered by experts to be the forerunner of heat stroke (hyperthermia). It may even resemble heat stroke, with the difference being that the neurologic function remains intact. Heat exhaustion is marked by excessive dehydration and electrolyte depletion. Symptoms may include headache, nausea, and vomiting, dizziness, tachycardia, malaise, and myalgia. Definitive therapy includes removing patients from the heat and replenishing their fluids. Most patients will require fluid replacement with IV isotonic fluids at first. The salt content is adjusted as necessary once the electrolyte levels are known. After discharge from the hospital, patients are instructed to rest, drink plenty of fluids for 2 – 3 hours, and avoid the heat for several days. If this advice is not followed it may then lead to heat stroke.
One public health measure taken during heat waves is the setting-up of air-conditioned public cooling centers.
No comments:
Post a Comment