Acid/Base Physiology Flashcards
what is the pH compatible with life
6.8-7.8
What does Ka describe
HA (weak acid) H+ + A- (conjugate base)
Ka = [H+][A-] / [HA]
what is henderson-hasselbalch equation
pH = pKa + log ([A-] / [HA])
pKa
pH where concentration of conjugate base and weak acid are equal
we want pKa near pH (best buffer to accept H+ and anions)
what are intracellular buffers (3)
1) organic phsophates
2) proteins
3) Hb
what are extracellular buffers (4)
1) proteins
2) albumin
3) phosphate
4) bicarbonate
what is the bicarbonate buffering system equation
H2O + CO2 H2CO3 H+ + HCO3-
what does K1 of bicarbonate buffering system describe
what enzyme catalyzes the reaction
H2O + CO2 H2CO3
catalyzed by carbonic anhydrase
what does K2 of bicarbonate buffering system describe
H2CO3 H+ + HCO3-
how can we rewrite Henderson-Hasselbalch in terms of [HCO3-]
assume H2CO3 rapidly converted to H+ + HCO3-
therefore,
pH = pKa + log ( [HCO3-]/[CO2] )
how can we rewrite
pH = pKa + log ( [HCO3-]/[CO2] )
for bicarb buffering system
[CO2] = 0.03 x PaCO2 pKa = 6.1
pH = 6.1 + log [HCO3-] / (0.03 x PaCO2)
what is the pH of an arterial blood sample normally
substitute
[HCO3-] = 24 meq/L
PaCO2 = 40 Torr
pH = 7.40 `
what is normal arterial pH range
how does it change in Denver
normal pH = 7.38-7.43
higher in denver
what is normal venous pH range
7.34 - 7.37
lower than arterial because carrying more CO2
why is venous pH slightly lower and venous pCO2 slightly higher (~45 Torr) than arterial blood despite amount of CO2 being carried
deoxyhemoglobin is a good buffer
venous pH is slightly ____ than arterial blood
lower
venous pCO2 is slightly ___ than arterial blood
higher
what is acidemia
how does that affect pH
more acid in blood than normal
lower pH
what is alkalemia
how does that affect pH
more base (or less acid) in blood than normal
higher pH > 7.40
how does the body compensate to normalize pH (2 ways)
1) lungs regulate CO2 levels (minutes)
2) kidneys regulate bicarbonate (hours to days)
compensation will ____ completely correct to normal pH (nor will it over-compensate)
NEVER !!!
compensation will ____ completely correct to ____
normal pH or over-compensate
What is respiratory acidosis
how does that affect pH
due to?
too much CO2, incr PaCO2 in denominator
lower pH
due to ineffective ventilation
respiratory acidosis is acute, chronic, or both
both
acute before kidneys compensate
what are compensation rules for
1) acute resp acidosis
2) chronic resp acidosis
acute = for every 10 Torr incr in CO2, pH decr by 0.08
chronic = for every 1 Torr incr in CO2, HCO3- incr about 0.4 meq/L
too much CO2, incr PaCO2 in denominator
lower pH
due to ineffective ventilation
respiratory acidosis
a
a
what are acute causes of resp acidosis
1) CNS depressants (opiates, benzodiazepines, alcohol most common)
2) Respiratory muscle fatigue (increased work of breathing)
what are chronic causes of resp acidosis
1) Central hypoventilation (e.g. obesity hypo- ventilation syndrome)
2) Neuromuscular disease (e.g. ALS)
3) Chronic lung diseases (emphysema, bronchiectasis, etc)
4) Hypothyroidism
a
a
define resp alkalosis
how does that affect pH
too little CO2, decr PaCO2, decr denominator
higher pH
what is resp alkalosis most commonly due to?
incr ventilation
resp alkalosis can be acute, chronic, or both
both
what are compensation rules
1) acute resp alkalosis
2) chronic resp alkalosis
SAME AS RESPIRATORY ACIDOSIS
1) acute = for every 10 Torr decrease in CO2, pH increases by 0.08
2) chronic = for every 1Torr decrease in CO2, HCO3- decreases about 0.4 meq/L
acute causes of resp alkalosis
1) Pain
2) Anxiety/Panic attack
3) Fever (inflamm cascade –> resp sensing)
4) Mechanical Ventilation
what are chronic causes of resp alkalosis
1) Living at altitude
2) Brain injury
3) Chronic aspirin toxicity (activ CNS)
4) Pregnancy
too little CO2, decr PaCO2, decr denominator
higher pH
incr ventilation
respiratory alkalosis
a
a
define metabolic acidosis
how does that affect pH
too much acid, decr [HCO3-]
decr pH
in metabolic acidosis ___ compensation is quite rapid with incr ventilation –> decr pCO2
respiratory compensation
–> leads to respiratory alkalosis
in metabolic acidosis respiratory compensation is quite rapid with ___
incr ventilation –> decr pCO2
compensation rules for metabolic acidosis
winter’s formula ***MEMORIZE
expected pCO2 = 1.5 [HCO3-] + 8 +/- 2
Winter’s formula
2 categories of metabolic acidosis
1) anion gap
2) non-anion gap
describe anion gap in metabolic acidosis
what is normal value
what does it it indicate
Anion Gap = [Na+] – ([Cl-] + [HCO3-])
normally 12-14
indicates additional acid that is buffered by bicarbonate, incr amount of unmeasured anions
(incr HA, decr HCO3-, incr A-)
describe non-anion gap in metabolic acidosis
what is it caused by
caused by loss of bicarb
what are causes of anion gap medically
1) methanol
2) uremia
3) DKA (EtOH and starvation)
4) propylene glycol
5) Isoniazid
6) lactate
7) ethylene glycol
`8) salicylates
MUDPILES
what are causes of non-anion gap medically
1) GI losses = diarrhea
2) renal losses = Renal tubular acidosis
3) too much IV saline (incr Cl- with loss of bicarb)
too much acid, decr [HCO3-]
decr pH
metabolic acidosis
a
a
define metabolic alkalosis
effect on pH
too much [HCO3-],
higher pH
____ compensation is rapid with decr ventilation –> incr pCO2
our body will not allow us to ____ to point of hypoxemia
respiratory
will not hypoventilate to point of hypoxemia
respiratory compensation is rapid with ___
decr ventilation –> incr pCO2
will not hypoventilate to point of hypoxemia
compensation rules for metabolic alkalosis
incr [HCO3-] of 1mEq/L, incr PaCO2 by 0.7 Torr
causes of metabolic alkalosis
1) vomiting or NG tube suction (loss of gastric acid)
2) Ingestion NaHCO3
3) Ingestion of other alkali (milk-alkali syndrome)
4) Hypovolemia, so-called contraction alkalosis
5) Diuretics
CASE
what does this patient have?
why?
emphysema pt with exacerbation with wheezing, tight, shortness of breath
incr work of breathing
RESPIRATORY ACIDOSIS
muscles fatigue –> decr Tidal volume –> less effective ventilation, not blowing as much CO2, CO2 incr –> respiratory acidosis
too much [HCO3-],
higher pH
metabolic alkalosis
dangers of metabolic alkalosis (2)
1) ventricular arrhythmia
2) seizures
dangers of metabolic alkalosis (2)
1) ventricular arrhythmia
2) seizures
CASE 2
what does this patient have?
Patient presents to the ED with some nausea/vomiting past few days with the following blood gas pH = 7.07 paco2 = 18 Torr = low PaO2 = 78 torr = low [HCO3-] = 5 mEq/L
Metabolic acidosis due to DKA
NORMAL OXYGENATION
anion gap or non-anion gap acidosis
Patient presents to the ED with some nausea/vomiting past few days with the following blood gas pH = 7.07= low paco2 = 18 Torr = low PaO2 = 78 torr = low [HCO3-] = 5 mEq/L
[Na+] = 132 mEq/L [Cl-] = 94 mEq/L glucose = 560 mg/dL AG = 132 - (94 + 5) = 33
anion gap metabolic acidosis
compensated?
Patient presents to the ED with some nausea/vomiting past few days with the following blood gas pH = 7.07 paco2 = 18 Torr = low PaO2 = 78 torr = low [HCO3-] = 5 mEq/L
[Na+] = 132 mEq/L [Cl-] = 94 mEq/L glucose = 560 mg/dL AG = 132 - (94 + 5) = 33
use WINTER’S FORMULA
Expected PaCO2 = 1.5 (5) + 8 +/ 2 = 7.5 (round up to 8) + 8 +/- 2 = 16 +/- 2
SHOULD BE AROUND ~16 +/- 2
patient’s expected paCO2 = 18 so YES, COMPENSATED
if 24, incomplete compensation
CASE 3
what does patient have?
Patient presents with a broken arm
pH = 7.52 = high PaCO2 = 25 Torr = low PaO2 = 85 Torr [HCO3-] = 21 mEq/L
acute respiratory alkalosis
high pH
low PaCO2
acute or chronic?
Patient presents with a broken arm
pH = 7.52 = high PaCO2 = 25 Torr = low PaO2 = 85 Torr [HCO3-] = 21 mEq/L
acute because bicarb is not significantly reduced and paCO2 is nearly 15 points below normal and pH is about 0.12 points above normal which fits clinical rule so acute
cause?
Patient presents with a broken arm
pH = 7.52 = high PaCO2 = 25 Torr = low PaO2 = 85 Torr [HCO3-] = 21 mEq/L
hyperventilating due to pain when ABG was drawn
why is PaO2 higher than expected?
Patient presents with a broken arm
pH = 7.52 = high PaCO2 = 25 Torr = low PaO2 = 85 Torr [HCO3-] = 21 mEq/L
PaO2 is higher than expected because PaO2 PaO2 is what predicted by alveolar gas equation for reduced PaCO2
CASE 3
what does he have?
N/V for past few days
pH = 7.53 = high PaCO2 = 42 Torr PaO2 = 110 Torr [HCO3-] = 36 mEq/L
metabolic alkalosis
is compensation appropriate?
why is he not hypoventilating?
N/V for past few days
pH = 7.53 = high PaCO2 = 42 Torr PaO2 = 110 Torr [HCO3-] = 36 mEq/L
Metabolic alkalosis with incomplete compensation
expected PaCO2 = 14 x 0.7 = 10
[HCO3-] = 14 mEq/L and PaCO2 should incr by 0.7
INCOMPLETE but won’t hypoventilate to hypoxemia
in order to incr PaCO2 by 10 points you hypoventilate to point of hypoxemia which the resp centers in the brain generally will not allow
cause?
N/V for past few days
pH = 7.53 = high PaCO2 = 42 Torr PaO2 = 110 Torr [HCO3-] = 36 mEq/L
loss of gastric acid
contraction alkalosis
elevated PaO2 because he was on supplemental oxygen
CASE 4
what does he have?
patient was stuporous
Pulse ox shows Sp02 = 85%
pH = 7.31 = high PaCO2 = 48 = high PaO2 = 55 Torr [HCO3-] = 23 mEq/L
respiratory acidosis
acute or chronic?
patient was stuporous
Pulse ox shows Sp02 = 85%
pH = 7.31 = high PaCO2 = 48 = high PaO2 = 55 Torr [HCO3-] = 23 mEq/L
acute, within hours
bicarb is normal and approx 10 Torr change in PaCO2, pH has decr about 0.08
cause?
patient was stuporous
Pulse ox shows Sp02 = 85%
pH = 7.31 = high PaCO2 = 48 = high PaO2 = 55 Torr [HCO3-] = 23 mEq/L
urine positive for opiates = heroin
Why is he hypoxemic?
patient was stuporous
Pulse ox shows Sp02 = 85%
pH = 7.31 = high PaCO2 = 48 = high PaO2 = 55 Torr [HCO3-] = 23 mEq/L
Check A-a gradient
A-a gradient = (630-47) x 0.21 - 48/0.8 - 55 = 7
R = 0.8
Observed PaO2 = 55
NORMAL = 10 can incr with age
so why PaO2?
because CO2 high, alveoli only have so much room so CO2 high
no problem transfer O2 but CO2 problem
so hypoventilation causing hypoxemia
no p
