Acutely ill patient - acid/base balance Flashcards
Which organs are involved in acid base balance?
Kidneys, lungs and liver
What do the kidneys lungs and liver do in health?
Kidneys - remove acid, regenerate bicarb
Lungs - regulate the removal of acid via CO2 by varying the RR
Liver - removes and recycles lactate
What determines blood pH?
Ratio of HCO3- to CO2
What does the kidney buffering system control?
Protons and bicarb excretion or reabsorption
The conversion of ammonia to ammonium
What does the lung buffering system control?
Carbon dioxide in the blood
What diseases increase the risk of acid-base balance?
CKD (eGFR<30), resp disease and liver disease.
What is a dangerously high/low pH?
High >7.6
Low <7.1
What is the Schwartz-Bartter approach?
•Accepts the Bronsted-Lowry definition of
–acids as proton donors
–bases as proton acceptors
–approach utilises the anion gap calculation to classify acid-base disturbances
•The hydrogen ion concentration is a function of the ratio between the PCO2 and the serum bicarbonate
What is the Stewart approach?
- Termed strong ion difference, based on principle that the serum bicarb does not alter pH.
Favoured by intensivists and anaesthetists
What is the clinical presentation of an acid-base disturbance?
•acutely ill patients consider whether there may be an underlying acid-base disturbance
•Serum bicarbonate and chloride are not standard components of all U&E reports
•Severe acidaemia (pH <7.1)
–cardiac function and vascular tone
•Severe alkalaemia (pH >7.6)
–irritability of cardiac and skeletal muscle
What conditions are associated with acid-base disorders?
•vomiting/diarrhoea
• shock
– cardiogenic
– septic
• hypovolaemia
• acute kidney injury
• respiratory failure
• altered neurological status
– coma
– seizures
• decompensated diabetes
• prolonged and excessive infusions of 0.9% sodium chloride
What investigations should you perform if you suspect an acid-base imbalance?
Urea, creatinine and electrolytes. Bicarb, cholride and ABG (inc lactate).
What is the step-by-step pathway to identify the cause?
•pH to determine whether
–acidaemia or
–alkalaemia
•Change in bicarbonate and base excess = metabolic process
•Change in PCO2 = respiratory process
•Determine whether
–simple disorder i.e. either metabolic or respiratory process alone
•Evidence of compensatory changes in either bicarbonate or PCO2
– mixed disorder i.e. a combination of a metabolic and respiratory process occurring together
What is the effect of metabolic acidosis on bicarb and what is the compensation?
Effect: low bicarb
Compensation: lower PCO2
What is the effect of metabolic alkalosis on bicarb and what is the compensation?
Effect: High bicarb
Compensation: Raised PCO2
What is the effect of respiratory acidosis on bicarb and what is the compensation?
Effect: high PCO2
Compensation: High bicarb
What is the effect of respiratory alkalosis on bicarb and what is the compensation?
Effect: Low PCO2
Compensation: Low bicarb
What is the anion gap?
•determined by negative charge on serum proteins
–particularly albumin
• anion gap = unmeasured anions – unmeasured cations
[Na+] – ([HCO3–] + [Cl–])
• normal anion gap = 5-11 mmol/l
What does an increased anion gap indicate?
Acidosis
What does a decreased anion gap indicate?
Alkalosis.
What are the causes of high anion gap acidosis?
Methanol
Uraemia
Diabetes
Paraldehyde
Alcohol
Lactate
Ethelyne glycol
Salicylate
Ketoacidosis, lactic, drug toxicity, kidney disease
What is ethylene glycol toxicity?
Metabolism generates glycolate - toxic to tissues. Elevated osmolal gap >25. Calcium oxalate crystals in urine indicative.
Treatment: alcohol infusion - inhibit alcohol dehydrogenase, fomepizole - inhibit alcohol dehydrogenase, dialysis
What is normal anion gap acidosis (hyperchloraemic)?
GI bicarb loss, renal bicarb loss and infusion of 0.9% saline
What is the managment of acid-base balance problems?
•Correcting
–fluid and electrolyte balance (e.g. hypovolaemia, salt and water deficit
–underlying cause for the acid-base disorder (ketoacidosis, acute kidney injury, sepsis)
•specific therapy to correct the HCO3– or PCO2
–if the acid-base disorder is affecting organ function or
–pH is <7.1 or >7.6
