Acid Base Gas Flashcards
What is H used for
Cell membrane integrity and enzyme reactions
What are the 3 mechanisms that maintain normal pH?
- Buffer system (respond immediately)
- change strong acid and bases into weaker ones = prevent drastic changes in pH of body
- located in intracellular fluid (ICF) & extracellular fluid (ECF)
> ECF: Carbonic acid bicarbonate (disintegrate into H and HCO3/bicarb), Hb
> ICF: phosphate, proteins
- Respiration (within 1-3min)
- CO2 produced as result of cellular respi
- combine with water to form carbonic acid
- before disintegrating into H & HCO3
- resultant change in H changes pH
- cause lungs to either increase/decrease rate & depth of ventilation
> get rid of CO2 = increase RR
> retain CO2 = decrease RR
- Kidney excretion (within hrs)
- metabolic acids (except carbonic acid) can be excreted by kidney
- adjust blood bicarbonate by:
a) increase secretion/absorption
b) produce new bicarb if necessary
What does acidosis cause?
Increased H+ & nervous system depression
What does alkalosis cause?
Decreased H+ & nervous system irritability
When is respi state acidotic
When PaCO2 >45mmHg
When is respi state alkalotic
When PaCO2 is <35mmHg
When is metabolic state acidotic
When HCO3 is <22mEq/L
When is metabolic state alkalotic
When HCO3 >26mEq/L
What is respi acidosis caused by?
Hypoventilation (lung cannot exhale excess CO2)
= accumulation of CO2 in body that combines with H2O to form carbonic acid
= breakdown into bicarbonate and H+ (where H+ change pH to acidic)
May req mechanical ventilator
Causes:
1. Brainstem trauma
2. CNS depressant
3. Impaired respi muscle function
4. Lung disorders (e.g pneumonia/emphysema)
What is the treatment for respi acidosis
- Tx underlying cause
- Hold/discontinue any respi depressant drugs
- Improve ventilation/respiration
- Reverse effect of respi depressants present
- opiate = naxolone (*shd see response within 10min)
- benzodiazepines = flumazenil (*max 5 doses per series, may repeat at 20min intervals)
What are the causes of metabolic acidosis
- Conditions when large amt of metabolic acids are produced
- e.g. lactic acid -> lactic acidosis
ketoacids -> diabetic ketoacidosis
salicylic acid -> poisoning
- e.g. lactic acid -> lactic acidosis
- Impaired ability to excrete H+ by kidneys (e.g. severe kidney damage)
What is the treatment for metabolic acidosis
May req bicarbonate replacement (injection)
Na & H2O deficits must also be corrected
- Replace NaHCO3 (for pts with sodium bicarbonate loss due to diarrhoea, renal proximal tubular acidosis)
(*no definite evidence that it is beneficial for acute metabolic acidosis - incl DKA, lactic acidosis, septic shock, intranet metabolic acidosis/cardiac arrest)
- Monitor plasma e during course of therapy
- k may decrease as pH increase
- goal of HCO3 replacement: increase HCO3- to 10mEq/L & pH to 7.20 (*not to increase to normal)
What is the cause of respi alkalosis?
Hypoventilation due to:
1. Anxiety, fear, pain
2. Respi stimulants (e.g. doxapram)
3. Increased metabolic demands (e.g fever/thyrotoxicosis)
4. CNS lesions
What is the management for respi alkalosis?
- Identify & correct underlying cause
- Hold/discontinue any suspected drugs
- Initiate O2 therapy in pts with severe hypodermic (PaO2 <40mmHg)/change ventilator settings as needed
- Tx theophylline OD
What are the causes of metabolic alkalosis
Either H+ loss or excess HCO3
- Prolonged vomiting, repeated gastric suction = loss of gastric acid
- Excessive antacid usage (*not sole reason to pt having metabolic alkalosis)
What are the treatment for metabolic alkalosis
May req Cl-/K+ replacement
- Correct underlying disorder
- Hold/discounting any suspected drugs
- ECF vol contracted
a) N/S at appropriate rate for degree of vol depletion
b) replace K as needed - ECF vol overload
a) Acetazolamine (if not renal insufficiency)
b) Haemodialysis/Peritoneal dialysis (for ARF/ESRD)
- decrease bicarbonate bath if not alkalosis will worsen - Hyperaldosteronism
a) Spironolactone
b) Amiloride
What does pH test for
Tell if acidotic/alkalotic - measure of acid context/H+ in blood
What does PaO2 test for
Amount of O2 dissolved in blood (O2 in arteries)
<80 = tissue hypoxia
Have a nonlinear r/s with SpO2 (not 1:1) = SpO2 & PaO2 lvls are diff
- only 1:1 when SpO2 is at 100%
*red flag when SpO2 <90% cos PaO2 will be <60mmHg
- as SpO2 goes down, PaO2 goes down drastically
What does PaCO2 test
Measure CO2 in blood
Affected by CO2 removal in lung
*when PaCO2 is high, body compromises by vasodilating = flushed skin
What does HCO3 test
Measure of bicarbonate content in blood
Affected by renal production of bicarb
What does base excess test
Metabolic component of acid base balance reflected
Decreased or negative value = metabolic acidosis (primary/secondary to respi alkalosis)
Increased/positive value = metabolic alkalosis
*if ABG (pH, PaCO2, HCO3) is normal/body have compensated in a way but have diarrhoea etc, look at base excess
What does SaO2 test
O2 sat
What values are deranged in type 1 RF (hypoxemia without hypercapnia)
Only PaO2 (<60)
What values are deranged in type 2 RF (hypoxemia with hypercapnia)
pH becomes acidotic
PaO2 is less than 60mmHg
PaCO2 is more than 45mmHg
