Clinical Acid and Base

Question 1

name the key components of blood gas analysis

Answer 1

• pH: compensated or decompensated (=sick!)
• pCO2: respiratory component
• HCO3- (and ‘base excess’): metabolic component

Question 2

what other useful information is on an ABG

Answer 2

• pO2/sO2: Oxygenation
• Electrolytes: To calculate anion gap
• Other values – glucose, lactate, Hb etc

Question 3

describe an ABG

Answer 3

• Use a special type of medium such as heparin in it so the blood doesn’t clot
• Go into the radial artery most common part
• Syringe will self fill if you hit the artery

Question 4

what artery do you measure an ABG from

Answer 4

radial

Question 5

Case 1 
20 year old
Very anxious
Breathless – respiratory rate 28/min
Chest examination unremarkable
Clear lung field on CXR

pH 7.58 (7.35-7.45)
pCO2 2.9 (4.5-6 kPa)
pO2 8.1 (10-13 kPa)
HCO3- 24 (22-28mmol/L)

Na		137		(135-145 mmol/L)
K		4.0		(3.5-5 mmol/L
Cl		104		(97-107 mmol/L)

Answer 5

respiratory alkalosis

Question 6

what are the causes of respiratory alkalosis

Answer 6

• hyperventilation
• any cause of impaired oxygenation

central cerebral stimulation

• panic anxiety
• fever
• pain
• drugs
• sepsis
• hypoxia

Question 7

Case 2 
80 year old
Lifelong smoker
Brought in by ambulance
Breathless – respiratory rate 22/min
Drowsy
Wheeze throughout chest
Sats 95% on 60% oxygen

pH 7.09 (7.35-7.45)
pCO2 9.7 (4.5-6 kPa)
pO2 17 (10-13 kPa)
HCO3- 24 (22-28mmol/L)

Na		137		(135-145 mmol/L)
K		4.0		(3.5-5 mmol/L
Cl		104		(97-107 mmol/L)

Answer 7

respiratory acidosis

Question 8

what are the causes of respiratory acidosis

Answer 8

Reduced ventilation:
Airways disease
Neuromuscular or chest wall disease
Reduced respiratory drive: opiates or reduced consciousness

Question 9

case 3 
80 year old
Lifelong smoker, COPD
Admitted to hospital after a fall
‘Routine’ blood gas done in emergency department

pH		7.38		(7.35-7.45)
pCO2	8.9		(4.5-6 kPa)
pO2		8.1		(10-13 kPa)
HCO3-	35		(22-28mmol/L)
sO2		88%

Na		137		(135-145 mmol/L)
K		4.0		(3.5-5 mmol/L
Cl		104		(97-107 mmol/L)

Answer 9

compensated respiratory acidosis

Question 10

describe what you would see in compensated respiratory acidosis

Answer 10

normal pH
high pCO2
compensatory high bicarbonate

Question 11

what can cause compensatory respiratory acidosis

Answer 11

• metabolic compensation may take a few days

- intercalated cells in distal nephrons actively excrete hydrogen ions and reclaim bicarbaotne

Question 12

case 4 
24 year old
Very breathless and anxious
Chest examination and CXR normal
Appears unwell

pH 7.11 (7.35-7.45)
pCO2 2.8 (4.5-6 kPa)
pO2 15 (10-13 kPa)
HCO3- 12 (22-28mmol/L)

Na		137		(135-145 mmol/L)
K		4.0		(3.5-5 mmol/L
Cl		98		(97-107 mmol/L)

Answer 12

metabolic acidosis

Question 13

what can cause high anion gap metabolic acidosis

Answer 13

Acid ‘added’ to the blood:
Ketones:
DKA
Starvation or alcoholic ketoacidosis

Lactate:
Tissue hypoxia/poor perfusion
Altered cellular respiration
Rarely: D-lactate

‘Titrable acid’
Renal failure

Ingested acid
Ethylene glycol, methanol, salicylate

Question 14

describe what lactic acidosis/ketoacidosis looks like e

Answer 14

Low pH, low HCO3-, often low CO2

Question 15

what causes lactic acidosis/ketoacidosis

Answer 15

• impaired oxygenation or glucose enters
• decrease in bicarbonate
• leads to lactate and ketones forming

Question 16

what happens in renal failure acidosis

Answer 16

• decrease in tithable or dietary acid
• decrease in bicarbonate
• failure to clear titrable acid using phosphate/ammonium

Question 17

Case 5
18 year old man
Attends A+E with renal colic

pH 7.25 (7.35-7.45)
pCO2 4.2 (4.5-6 kPa)
pO2 12 (10-13 kPa)
HCO3- 13 (22-28mmol/L)

Na		137		(135-145 mmol/L)
K		3.2		(3.5-5 mmol/L
Cl		116		(97-107 mmol/L)

Answer 17

normal anion gap metabolic acidosis = Hyperchloraemic metabolic acidosis

Question 18

what is normal anion gap metabolic acidosis due to

Answer 18

Usually due to bicarbonate loss
Kidneys: Renal tubular acidosis
Gut: diarrhoea

Compensatory rise in chloride to maintain electrical neutrality

Question 19

what are the types of renal tubular acidosis

Answer 19

Bicarbonate lost in urine:
 Type 1 ‘distal’
 Type 2 ‘proximal’
Type 3 very rare
Type 4 hyperkalaemic

1,2,3 rare and genetic
4 is more common

Question 20

what is renal tubular acidosis

Answer 20

• Not able to get acid into the urine

- If you have a patient and you think if the patient has renal tubular acidosis – test the urine pH

Question 21

what are the acute consequences of acidosis

Answer 21

Negative inotropic effects
Confusion
Kussmaul’s breathing
Hyperkalaemia

Question 22

what are the chronic consequences of acidosis

Answer 22

Bone reabsorption, calciuria, stones
Insulin resistance
Progressive renal impairment

Question 23

case 6 
74 year old man
Multiple chronic health problems
Admitted two days ago with bowel obstruction
Clinical deterioration
pH		7.2		(7.35-7.45)
pCO2	6.8		(4.5-6 kPa)
pO2		9.5		(10-13 kPa)
HCO3-	16		(22-28mmol/L)

Na		137		(135-145 mmol/L)
K		6		(3.5-5 mmol/L
Cl		98		(97-107 mmol/L)
Lactate 	5.6		(<2mmol/L)

Answer 23

mixed acidosis

Question 24

describe what mixed acidosis is caused by

Answer 24

Likely two (or more) underlying processes
Sick!

Question 25

case 7 
80 year old woman
Heart failure
Long term steroid use
Admitted with vomiting and dizziness
pH		7.6		(7.35-7.45)
pCO2	6		(4.5-6 kPa)
pO2		9.5		(10-13 kPa)
HCO3-	35		(22-28mmol/L)

Na		137		(135-145 mmol/L)
K		3.2		(3.5-5 mmol/L
Cl		95		(97-107 mmol/L)

Answer 25

metabolic alkalosis

Question 26

why does bicarbonate rise

Answer 26

As H+ is lost:
Vomiting/NG drainage
Diarrhoea
Diuretics
Mineralocorticoid excess
Rare hypokalaemic disorders

As H+ moves into cells:
- Hypokalaemia
If alkali is administered

frist list is with chloride deletion
last 2 and 2nd part is with potassium depletion

Question 27

what is the treatment of metabolic alkalosis

Answer 27

Replacement of H+:

• Normal saline (NaCl) if chloride deplete
• Potassium supplementation
• Treat underlying cause