Acid Base Balance Acid Base Balance CasesCases

Dr Svitlana ZheleznaClinical Teaching Fellow

UHCW NHS Trustsvitlana.zhelezna@uhcw.nhs.uk

2013/2014 academic year

Acid-base balance

Maintenance of normal ECF [H+] (pH) depends on the balance between: carbon dioxide production and excretion hydrogen ion production and excretion

Imbalances are dealt with by buffering Compensation

But reversed usually only by correction of the underlying disorder

Case 1Case 1

73 yr old man, known COPD Acute exacerbation, SOB, fever

Baseline Normal Range pH 7.28 7.35-7.45 pCO29.7 4.5-6.0 kPa

pO2 8.3 12-15 kPa Bicarb 34 22-28 mmol/L

Respiratory acidosis Cause

hypoventilation leading to CO2 retention

Correction restore normal gas exchange

Compensation increased renal acid excretion

Features acutely: low pH, n [HCO3

-], high pCO2

chronically: low/normal pH, high [HCO3-],

high pCO2

Case 2

49 y. o. woman T1DM at A&E 2/7 drowsiness O/E: dehydrated, rapid deep breathing

pH 7.12 (7.35-7.45) pCO2 2.2 kPa (4.5-6.0) pO2 15.3 kPa (12-15) bicarb 12 mmol/L (22-28)

Case 3Case 3 62 yr old woman T2DM 2/7 increasing drowsiness, breathing rapidly and deeply

Sodium 146 mmol/L 135-145 Potassium 6.1 mmol/L 3.5-5.0 Chloride 109 mmol/L 98-107 Bicarbonate 12 mmol/L 22-28 Urea 14 mmol/L 2.5-8.0 Creatinine 256 umol/L 60-110

pH 7.16 7.35-7.45 pCO2 2.9 4.5-6.0 kPa pO2 11.9 12-15 kPa Bicarb 11 22-28 mmol/L

Metabolic acidosis Causes

increased acid formation acid ingestion bicarbonate loss/AKI reduced metabolism/excretion

Correction primary cause increased renal acid excretion

Initial Compensation: hyperventilation, hence low pCO2

Features Acutely: low pH, low [HCO3

-], low pCO2

Chronically: low/normal pH, low [HCO3-], Normal pCO2

Case 4Case 4 47-year old woman – vomiting persistently Previously treated for DU, dehydrated, hypotensive, RR 9

Sodium 142 mmol/L 135 -145 Potassium 2.9 mmol/L 3.5 - 5.0 Chloride 85 mmol/L 98 -107 Bicarbonate 44 mmol/L 22 - 28 Urea 24.3 mmol/L 2.5 - 8.0 Creatinine 150 mmol/L 60 -110

pH 7.55 7.35-7.45 pCO2 8.1 4.5-6.0 kPa pO2 13.2 12-15 kPa Bicarb 42 22-28 mmol/L

Metabolic alkalosis (1) Cause

loss of gastric acid increased renal H+ excretion e.g. in

hypokalaemia Correction

primary cause increased renal bicarbonate excretion

Compensation hypoventilation with CO2 retention

Features high pH, low [H+], high [HCO3

-], N/high pCO2

Metabolic alkalosis (2) Limit to compensation

CO2 is respiratory stimulant Limits to correction

Hypochloraemia thus increased bicarbonate reabsorption with sodium

Hypovolaemia thus increased distal sodium reabsorption but potassium depletion (GI loss) so increased acid excretion (paradoxically acid urine)

Thank you!

Any questions?