11/5- Heat and Water Homeostasis with Aging Flashcards
What factors would decrease the older man’s chances of survival compared to the younger man out in the desert?
- How would your thinking be altered if they had just 2 canteens of water?
Old guy is more likely to become dehydrated because:
- Decreased body water to start with
- Decreased ability of kidney to retain salt
- Decreased ability of kidney to retain water and longer time to required to reach maximal capacity
- Decreased recognition and severity of thirst; he will not drink enough to maintain euvolemia
- Decreased ability to maintain BP once dehydrated
- Regulation of kidney by ADH is impaired when stimulus to the CNS is decreased intravascular volume
Total body water decreases ___% in elderly men
Total body water decreases 15-20% in elderly men
In the plane crash scenario, what factors cause the old guy to more likely become hyperthermic?
Older man:
- Requires higher core temperature to start sweating (positive effect on water balance, but negative on temperature balance)
- Has decreased effectiveness of sweating because of less skin arteriole vasodilation
- Has decreased maximal sweat output by sweat glands
What age-related changes increased the probability of an elderly person (tied to his hospital chair in a hospital) not drinking or being thirsty but then falling and found to be dehydrated?
- Old people do not get thirsty; even at Na 160, only 40% were mildly thirsty (100% of young men were thirsty)
- Despite impending dehdyration old guys have impaired maximal urine concentrating ability and they produce more urine
- Due in part to changes in kidney blood flow distribution such that the longest nephrons have died with age and their blood flow now goes to the medulla and washes out the vasa recta (max urine conc decreases from 1200 to 800 in healthy 75 yo)
- Add all the reasons more likely to fall
What are the age related changes that put the elderly at the highest risk for hyperthermia (and dying during heat waves)?
- Impaired temp recognition
- Impaired sweating such that core body temp to induce sweating is degrees higher
- Once sweating starts, the max production of sweat is reduced so the max excretion of heat is reduced
- Impaired skin arteriole vasodilation results in decreased delivery of heat to skin
- Once sweating they may not take in fluids adequately because of the impaired thirst drive
Other factors:
- Fear (didn’t want to open windows)
- Finances (spending more on meds, less on AC)
- Social isolation (can’t depend on friends/family)
- Dementia (not recognize risks in heat wave)
What would contribute to an elderly person going into heart failure (respiratory distress, rales) following usual trauma IV lines after an auto accident?
Young person could pee out enough not to go into heart failure; old people cannot
- Renal perfusion decreases to ~same extent as renal mass (about 1/3). Thus, less delivery to be excreted by kidney
- It takes older person much longer to suppress ADH so that she can excrete maximally dilute urine so it will take her longer to reach maximal dilution and the maximally dilute urine will not be as dilute
- After water loading old people may actually excrete a larger percent of what is filtered at maximal function (but they filter less so they excrete less)
- When her heart senses increases in filling pressures, it will release ANP and BNP. The kidneys of old people are less responsive to both natriuretic peptides
- ALSO venous compliance may be decreased; can sequester less volume in veins
How does creatinine clearance change with age?
- Decreased CrCl by 35% in healthy older men (not HTN, no DM, no drugs)
- Decreased concentrating and diluting capacity
- Increased number of sclerotic glomeruli (30%)
- Dependence on the PGs to maintain filtration
- Decreased RBF and renal mass
- Decreased clearance of renal drugs
How do dietary interventions differ in elderly vs. young (in the case of a low salt diet)?
- As opposed to young people, old people do not rapidly adjust to decreased sodium intake; in this case it is to your advantage
- After instituting a low sodium diet, the elderly continue to excrete Na as if on a high Na diet; thus they lose sodium, water and decrease BP (great HTN treatment)
- Even after 5 days in one study, older group still excreting as much Na as young group after 2 days; old group was in negative sodium balance at 7 days when the study ended
What causes the increased loss of water and salt with decreased intake in the elderly?
- Impaired recognition of thirst and serious dysregulation of thirst
- Impaired retention of salt and water
- Takes much longer to reach maximum retention
- Maximum urine concentration for old people is still poor
- Dropout of longest nephrons in old kidney
Why might an elderly lady who presented as somnolent with elevated Diazepam (Valium) levels following trouble sleeping remain sleepy even after days?
- Valium is highly lipid soluble and will be accumulated in the body fat of this lady
- It is highly likely that 50% of her body weight is fat so she has a lot of volume to store it in; the wash out can take weeks (after stop by mouth, just released from fat)
- Also, the half life of Valium is tripled (at least) in older people (up to 96hrs)
Case
- Elderly woman found after falling in garden
- Looks like she spent the night in her nightgown on the gorund
- Woman is minimally responsive and EMTs make disparaging remarks about her functional state
- Rectal temp is 92’F
- You wrap her in blankets and start to warm her up with improving LOC
- What risk factors did she have for developing hypothermia?
Risk factors for hypothermia:
- Impaired temp recognition; old people are unable to discriminate temperature that are easy for young to perceive a shot or cold
- The old brain is more sensitive to almost any insult including hypothermia; once her consciousness was impaired she did not have the ability to come in from the cold
- Older person can make very little use of shivering, and this is often totally ineffective in generating warmth
- Brown fat mechanism is unavailable to old people (don’t have any)
- Old people have little regulation of skin vascular tone such that they are unable to vasodilate in the heat nor vasoconstrict in the cold. Continued perfusion of the skin leads to larger heat loss and further hypothermia
- Decreased heat production per body weight; though this is in large part due to changes in body composition (this lady is approx 50% fat), aging may slow metabolic rate and heat production
Why are “you not dead until you are warm and dead”
In the setting of hypothermia, it is hard to determine death, this is most commonly invoked after kids fall thorugh the ice into cold water. The hypothermia protects the brain, increases its tolerance to being hypoperfused and forces one to redefine death
How does temperature relate to sepsis and prognosis?
When a septic pt is hypothermic instead of being hyperthermic, it usually means overwhelming sepsis that has a poorer prognosis
- (old rats have less fever after LPS injection)
