workup and treatment of acid base disorders Flashcards
order lab studies
ABG: To assess pH, PCO2, HCO3-, and oxygenation; alternatively, venous blood gas
metabolic panel: To assess changes in sodium, chloride, bicarbonate, other electrolytes, and renal function, and to calculate the anion gap
a mixed acid base disorder is when
The expected compensatory response differs from the laboratory findings.
anion gap
the difference between the concentration of measured cations and measured anions
high anion gap means
increased concentration of organic acids such as lactate, ketones (e.g., beta-hydroxybutyrate, acetoacetate), oxalic acid, formic acid, or glycolic acid, with no compensatory increase in Cl-.
general principles of metabolic acidosis
Calculation of the anion gap is the first step in the evaluation of metabolic acidosis.
concommitant acid-base disturbances
Calculation of the delta gap can help determine if another acid-base disturbance is present in addition to a high anion gap metabolic acidosis. Cut-off values may vary depending on the source.
delta gap: a ratio of the change in anion gap to the change in bicarbonate
deltaa gap values
Delta gap < 1 : Hyperchloremic or normal anion gap metabolic acidosis is present in addition to high anion gap metabolic acidosis. [10]
Delta gap 1–2 : Only high anion gap metabolic acidosis is present.
Delta gap > 2 : A metabolic alkalosis is present in addition to high anion gap metabolic acidosis. [11]
urine anion gap
The difference between the concentration of measured anions and the concentration of measured cations in the urine.
workup of normal anion gap metabolic acidosis
Calculate the urine anion gap
Negative urine anion gap: Acidosis is likely due to loss of bicarbonate.
Positive urine anion gap: Acidosis is likely due to decreased renal acid excretion.
loss of bicarbonate
negative urine anion gap
causes:
- diarhhoea
- GI fistulas
- toluene ingestion
- meds
- type 2 renal tubular acidosis
decreased renal acid excretion
positive urine anion gap
causes:
- hyperchloraemia
- renal failure
- addison disease
- renal tubular acidosis
compensation in metabolic disorders
rapid within minutes through respiratory compensation
compensation in respiratory disorders
slow over several hours through changes in urine pH
calculating the expected compensation in metabolic acidosis
winter formula
expected pCO2 = 1.5 x HCO3 +8 +/- 2
what is the first step in metabolic acidosis
calculation of the anion gap
determines whether the loss of HCO3 is being compensated by an increase in Cl
high anion gap metabolic acidosis approach
exclude accumulation of endogenous acids eg. ketoacidosis (measure ketone levels in urine or serum, lactic acidosis, uraemia (measure BUN and creatinine)
consider accumulation of exogenous organic acids (ingestion) as the cause: serum or urine toxicology screen.
calculate the delta gap to exclude concommittant acid base disturbnaces
causes of lactic acidosis
related to hypoxia eg. by septic shock, hypovolaemic shock, hopoxaemia
or not relted to hypoxia eg. caused by liver failure, metformin
causes of accumulation of exogenous organic acids
ingestion of methanol
ingestion of etylene glycol (antifreeze)
ingestion of propylene glycol
toluene
salicylate toxicity
iron overdose
isoniazid overdose
workup of normal anion gap metabolic acidosis
calculate the urine anion gap
negative urine anion gap: acidosis is likely due to the los of bicarbonate
positive urine anion gap: acidosis is. likely due to the decreased renal acid excretion
when. to. calculate the urine osmolol gapp
ppreferred over. the urine anion gap if the urine pH is >6.5
decreased urine osmolol gap suggests impairment in the excretion of urinary ammonium
appraoch of metabolic alkalosis
assess blood pressure and volume status
evaluate for exogenous ingestion (eg. laxatives, calcium, alkali load, diuretics
obtin BMP and serum calcium, urinary chloride, urinary potassium levels
if there is low urine potassium
consider laxative abuse as a potential cause
testing urinary chloride in metabolic alkalosis
chloride resposive alkalosis: hypovolaemia (Gi lossess due to vomiting, NG. tube, diarrhoea), renal losses, cystic fibrosis, dietary chloride deficiency
chloride resistant metabolic alkalosis: severe magnesium deficiency, extreme hypercalcaemia or hypokalaemia, loop. or thiazide diuretics,
respiratory acidosis workup
establish chronicity based on severity of presentation: is it acute or chornic
acute respiratory acidosis
acute lung disease
acute exacerbation of COPD
CNS depression due to head trauma, postictal state, drug toxicity, central sleep apnoea
chronic respiratory acidosis
COPD, asthma
respiratory muscle weakness ego myasthenia gravis, ALS, GBS, poliomyelitis, MS, severe hypokalaemia
HCO3- is
alkaline
CO2 is
acidic
kidney compensation of respiratory aklalosis
proximal convoluted tubule secreetes more HCO3
respiratory alkalosis is seen in
hyperventilation
causes of acute respiratory alkalosis
pulmonary disease eg. pneumonia, pulmonary embolism, pulmonary oedema, ppulmonary oedema, aspiration pneumonitis, interstitial firbosis
pain, anxiety, panic attacks
fever
drug toxicity
CNS infections
stroke
severe anaemia
congestive heart fialure
sepsis
hypoxaemia
hyperventilation while on mechanical ventilation
chronic respiratory alkalosis
pulmonary. embolism
liver failure
hyperthyroidism
brainstem tumour
