Acid Base Thermoregulation
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What is an acid?
substance that can donate an H+ ion
What is a base?
Substance that can accept an H+ ion
What is it called when a proton (H+ ion) is transferred?
an acid-base reaction
What is a conjugate pair?
- Separated by only an H+ ion, for example
- BH+ and B
- HA and A- are both conjugate pairs
What is a strong acid?
- An acid that easily gives up H+ ion
- Essentially 100% dissociation in water
What is a weak acid?
- An acid that does not give up H+ easily
- Less than 100% dissociation in water
What happens when an acid dissociates?
Acid split apart in water, becomes H+ and an anion (X-)
Why is maintenance of H+ concentration so important?
- Essential for optimal enzyme functioning
- Proper distribution of lytes
- Optimization of myocardial contractility
- Optimal Hgb saturation
What is the concentration of H+ at the normal pH level of 7.4?
What does pH measure?
H+ ion concentration
What is "p," as in pH?
p= - log of, in this case, pH= -log [H+]
What does pH equal?
pH= -log [H+] = log 1/[H+]
If [H+] = 10^-6 moles / L, what is the pH?
What does a change in 1 pH unit mean?
H+ concentration changes by a factor of 10
What is K for HA ⇋ H+ + A-?
What is the name for K?
- K (equilib constant = dissoc constant=ionization constant) =
- [Products] / [Reactants]=
- [H+] [A-] / [HA]
What is -log[H+]?
- = pH
- also expressed as log 1 / [H+]
What does K mean?
- K= equil constant = numerical description of balance btw products and reactants
- K>1 rxn favors products
- K<1 rxn favors reactants
What are acid sources in the body?
CO2 and H+ ion
How do CO2 and H+ ion differ?
- CO2 is volatile (can be eliminated from the lungs)
- H+ is non-volatile or fixed
How is CO2 generated in the body?
biochem pathways such as glycolysis, citric acid cycle, ETC
How much CO2 is produced in the body each day? How does this compare to the amt eliminated by the lungs?
- 15-20 moles CO2 produced qd from aerobic metabolism
- Under normal conditions, amt produced = amt eliminated by lungs
How is H+ ion produced in the body?
- Protein metabolism
- Lactic acid (anaerobic metabolism)
- Ketone bodies (triglycerides)
How is H+ ion eliminated by the body?
Via the kidneys; H+ is secreted from blood into tubular fluid (urine)
What is the Henderson-Hasselbalch equation?
pHa = pKa + log HCO3- / 0.03 x PaCO2
pKa carbonic acid = 6.1 at RT
What is the significance of the value 0.03 in the H-H equation?
0.03 mmol/L/mmHg = solubility coefficient for CO2 in plasma
What is a simplified way of expressing pH?
pH = [HCO3-] / PaCO2 = kidneys / lungs
What is the normal ratio of bicarb to CO2?
How to buffers defend against H+ changes?
- act within fractions of a second
- regulates excessive changes in H+ ion
- local and immediate but incomplete
How do the lungs defend against H+ changes?
- Acts in mins
- Regulates CO2 removal
How do the kidneys defend against H+ changes?
- Acts within hours to days
- Excrete either acidic or basic urine
- Are most powerful to correct imbalance but take the longest
How does the defense against H+ ion changes differ btw buffers and kidneys and lungs?
- Buffers act immediately and locally, but incompletely
- Lungs and kidneys provide more complete and systemic correction but take longer
What does the effectiveness of a buffer depend on?
- 1) amt of buffer
- 2) pKa of buffer system (80% buffering occurs +/- 1 pH unit of pKa of buffer system)
- 3) pH of environment
What is a buffer?
Any substance that can reversibly bind H+ ions
What happens when pH < pKa
acid environment shifts equation to non-ionized form (HA) which makes acid drug more effective and basic drug less effective
What is pKa?
pH at which 50% drug is ionized and 50% nonionized
What are examples of buffer systems?
bicarbonate, phosphate, ammonia, proteins
What are examples of ICF buffers?
phosphate and proteins (proteins are most important ICF buffer)
What is an example of an ECF buffer?
Where does the majority (60-70%) of buffering occur? ICF or ECF
What are examples of protein buffering systems?
Hgb, ammonium ion (from glutamine)
What are the components of the bicarb buffer?
- a water sol'n with 2 ingredients:
- 1) weak acid H2CO3
- 2) bicarb salt NaHCO3
Which is a stronger acid- inorganic or organic?
What is the pKa of the bicarb buffer system?
Why is bicarb such an effective buffer despite that fact that its pKa is not that close to physiologic pH?
It is present in all fluid compartments
How does the effectiveness of the bicarb buffer change in an acidotic environment? Why?
Bicarb becomes a more effective buffer because its pKa (6.1) is closer to that of the body when acidotic
How do the kidneys regulate ECF H+ conc?
- 1) secretion of H+ ions (out)
- 2) reabsorption of bicarb ions (in)
- 3) production of new bicarb ions
What is the net effect of the bicarb buffer in the kidneys?
H+ eliminated in urine, bicarb reabsorbed
What role does the Na/ K / ATPase pump play in the bicarb buffer system?
Active transport of Na out, K in, this creates a gradient for Na concentration, allowing Na to go back in in exchange for H+ ion (which is then eliminated)
What is the pKa of the phosphate buffering system?
How does the pH of the ICF compare to that of ECF?
- pH ICF < ECF, so even less of gap between pH and pKa
What is the net effect of the phosphate buffer in the kidney?
- H+ eliminated with sodium phos
- bicarb is able to be reabsorbed because it is not needed to buffer the H+ ion
How much phosphorous is available / day to buffer?
30-40 meq / day
Where do the bicarb and phos buffers occur?
in the tubular cells of the kidney
Where does the ammonium buffer system occur?
In the proximal, distal, or thick ascending loop of Henle
How is the ammonium ion buffering system a protein buffering system?
Glutamine (an amino acid) is broken down into 2 bicarb and 2 ammonium ions in the kidney
What is the net effect of the ammonium ion buffer system?
- H+ excreted in urine as NH4 + + Cl-
- 2 bicarb are reabsorbed as not needed to buffer H+ ion
Where does the ammonia buffering system occur?
Collecting tubular cells of kidney
What is the net effect of the ammonia buffering system?
- H+ excreted into urine as NH4 + + Cl-
- bicarb reabsorbed as not needed to buffer H+
How do the lungs affect acid / base balance
Lungs control CO2 excretion
If plasma bicarb decreases what happens to ventilation?
Ventilation increases to compensate by decreasing CO2
If pH decreases by 0.1, what would happen to ventilation?
It would increase 2-fold
What is BMR?
- heat produced by body from metabolism
- 1kcal / kg/ hr
Why does heat need to be released by the body?
Metabolism (BMR) produces heat (1 kcal / kg/ hr), if all heat were retained by the body, T would increase at a rate of 1 C / hr!!
What is afferent input?
Thermal info derived from tissues throughout the body (brain, skin, spinal cord, and deep tissues)
What is central processing and where does it occur?
- -Hypothalamus integrates input signals (too warm or too cold based on set point- like a thermostat) and sends out efferent response
- -Occurs in the pre-optic area of the hypothalamus
What are efferent responses?
- Meant to return body back to set point temp
- Ex: behavior changes, BV constriction, shivering, etc.
What percentage to core and skin surface temp contribute to thermoregulation?
- 66% core and deep body
- 34% skin surface
How is heat produced in the body?
- -muscle work
- -hormonal influence (thyroxine, GH, testosterone)
- -metabolic effects of catecholamines (SNS stim)
- -chemical activity within cells
- -thermogenic effect of food
What are the 4 mechanisms of heat loss? How much does each contribute?
- 1) radiation (40-60%)
- 2) evaporation (20%)
- 3) convection (conduction or air) (15%)
- 4) conduction (3%)
How does radiation heat loss occur?
- Heat travels from warm object (or person) to a cooler object via infrared waves
- Ex: being in a cold OR
How does evaporation heat loss occur?
- Loss of moisture on the skin surface and resp tract
- Depends on humidity of enviro
- (More humidity means less of a gradient and so less heat can be lost)
Why is there likely to be more heat loss via evaporation with anesthesia?
Inhalation of dry gases
How much water is lost / hr via evaporation and how much is due to loss from lungs and how much from skin?
- 30 ml / hr
- 1/3 lungs
- 2/3 skin
How does convection heat loss occur?
- Due to moving air or "wind chill"
- Loss is proportional to square root of air velocity
- Ex: wheeling a patient down a hallway in just a johnny
How does conduction heat loss occur?
- Due to direct contact
- Ex: cold OR bed, cold IV sol'n
- Clothes decrease heat loss from convection but only when they are dry!
How does the body compensate for HYPERthermia?
- Vasodilation of cutaneous BV, 8X increase in rate of transfer of heat to skin
- Sweating- increases loss of heat by evaporation
- Decrease in heat production (inhibition of shivering)
What branch of the nervous system controls sweating? Via what NT?
- SNS, neurotransmitter is Ach (cholinergic) (not NE which is the usual NT in the post synaptic SNS)
- With SNS activity you are likely to have increased heat production and a need to sweat
Why is sweating beneficial?
Removes heat at a rate greater than 10x the basal rate of heat production
How does the body compensate for HYPOthermia?
- Increased thermogenesis
What are methods of increased thermogenesis?
- SNS stimulation
- Thyroxine secretion
What are the 2 temperature compartments?
- Core (chest, abd, pelvis, head)
- Periphery (extremities and skin)
What is poikilothermia?
- Body equilibrating with ambient T
- Our T varies based on RT
What % of its will experience hypothermia if nothing is done to maintain body T?
When is T highest?
- Highest early eve
- Lowest am
- due to circadian rhythms
Under GA, what thermoregulatory responses are available?
Vasoconstriction and nonshivering thermogenesis
How does GA interfere with T regulation?
It interferes with all 3 components of T reg (afferent, central processing, and efferent)
How does regional anesthesia interfere with T reg?
- Impairs afferent input (how T is perceived) and efferent output
- Central processing is NOT effected
How do anes agents affect BMR and heat production?
- Decreased BMR and heat production
- Also volatiles lower the threshold for vasoconstriction to occur
What are the most accurate sites for T monitoring?
PA, tympanic membrane, esophagus, oro and nasopharynx
What is the least accurate site for T monitoring?
- Intermediate are : bladder, rectum, axilla
What are the metabolic consequences of HYPOthermia?
- Decreased metabolism, decreased CO2 production, decr O2 consumption, enzyme controlled rxns affected
- (unless you start shivering, then huge incr in O2 consumption)
What are other consequences of HYPOthermia?
- Immune issues (phagocytes can't work as well)
- Coag issues (incr bleeding)
- SNS stimulation
- neuroprotection- cerebral and SC protection
How can we help pts to conserve heat in the OR?
- Start in the holding area
- warm blankets
- fluid warmer
- bair hugger
- pt before staff comfort
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