2. Clinical aspects of Acid-Base control

Question 1

1kPa=

Answer 1

7.5mmHg

Question 2

What are the main buffers that maintain body pH?

Excretion of acid by?

Answer 2

Proteins
Haemoglobin
Carbonic acid/ bicarbonate

Excretion by lungs and kidneys

Question 3

Normal values for:
• pH
• pO2
• pCO2
• Bicarbonate
(standard)

Answer 3

• pH = 7.35 – 7.45
• pO2 = 12 -13 kPa
• pCO2 = 4.5 – 5.6 kPa
• Bicarbonate (standard) = 22 – 26 mmol/l

Question 4

What is standard bicarbonate?

Answer 4

Standard bicarbonate is calculated from the actual bicarbonate but assuming 370C and a paCO2 of 5.3kPa.
It reflects the metabolic component of acid base balance

Question 5

How is ABGs assessed?

Answer 5

Step 1: Assess pO2 and oxygenation
Step 2: Assess pH
Step 3: Determine primary problem
Step 4: Is compensation occurring?

Hint: If pH and pCO2 are changing in the same direction the primary problem is metabolic
If in opposite direction, the primary problem is respiratory

Question 6

How is pO2 and oxygenation assessed?

Answer 6

PaO2/FiO2 ratio (P/F ratio)
If P/F ratio > 50 = Healthy
If P/F ratio < 40 = Acute lung injury
If P/F ratio < 26.7 = ARDS

Question 7

What is compensation?

Answer 7

Altering in function of the respiratory or renal system to change the secondary variable in an attempt to minimise an acid – base imbalance

Question 8

If pCO2 and bicarb are moving in the same direction compensation, what is happening?

Answer 8

Compensation

If moving in opposite direction, suspect a mixed disorder.

Question 9

Anion gap:
What is it?
Calculated from?
Formula?

Answer 9

The sum of routinely measured cations (+ve) in venous blood minus routinely measured anions (-ve). Indicator for metabolic acidosis
Calculated from venous blood
Anion gap= ([Na+] + [K+]) - ([Cl-] +[HCO3-])

Question 10

Increased anion gap signals?

Answer 10

Presence of metabolic acidosis

Question 11

Normal value for anion gap?

Answer 11

16

Question 12

Causes of increased anion gap (metabolic acidosis)

Answer 12

Bodies own production (lactic acidosis, ketoacidosis)
Ingestion (exogenous source)
Failure of excretion by the kidneys

Question 13

How can the bodies own production of acid result in an increased anion gap and then lactic acidosis?

Answer 13

Any condition causing hypoperfusion

• Of the whole body: Shock (carcinogenic, septic, hypovolaemic, anaphylactic)
• Of part of the body: Femoral artery embolism

Results in increased anaerobic metabolism with subsequent increased production of lactic acid = LACTIC ACIDOSIS

Question 14

What two processes result in lactic acid build up?

Answer 14

Fall in O2 delivery

Fall in the consumption of lactate by the liver

Question 15

How can the bodies own production of acid result in an increased anion gap and then ketoacidosis?

Answer 15

Situations of decreased insulin and increased glucagon.
E.g.
-Uncontrolled diabetes mellitus (severe, life threatening)
-Alcoholic ketoacidosis (common clinically)
-Starvation ketoacidosis (mild)

Question 16

How does exogenous acid load leads to an increased anion gap?

Answer 16

Accidental/deliberate ingestion of e.g.

• Methanol (industrial solvent, windscreen wash)
• Ethylene glycol (anti-freeze)

Question 17

What are the renal causes of metabolic acidosis? What are there affects on the anion gap?

Answer 17

Renal failure, both acute and chronic, increased gap

Renal tubular acidosis, leads to normal gap

Question 18

Why does the kidney function alter the anion gap?

Answer 18

As the kidneys:

• Reabsorbs filtered bicarbonate
• Regenerates bicarbonate consumed by buffering

Question 19

Two causes of a normal anion gap with metabolic acidosis?

Answer 19

Diarrhoea

Renal tubular acidosis

Question 20

GI causes of metabolic acidosis with normal anion gap?
Why?
What procedures lead to a similar pathology?

Answer 20

Diarrhoea and volume depletion (leading to RAAS and chloride retention)

Why? As gut below pylorus secretes bicarbonate into gut lumen. For every bicarbonate ion secretes, an H+ ion enters the ECF

Procedures? Ileostomy, colostomy

Question 21

Compensation for metabolic problems?
Accompanying slow metabolic correction?
Conditions for correction?

Answer 21

RESPIRATORY compensation: pCO2 must fall, so minute volume must increase

Metbalolic correction stages:
1. Leads to slow metabolic correction. Secretes more acid (therefore also makes NEW bicarbonate).
2. Plasma H+ decreases (pH rises)
3. Plasma bicarbonate rises to normal
Conditions:
-The metabolic acidaemia is of non-renal origin
-The kidneys are functioning effectively.

Question 22

How is the breathing described during compensation of metabolic acidosis?

Answer 22

Kussmaul respiration: A laboured deep, rapid pattern of breathing

Question 23

Maximal compensation for metabolic problems can take…
Limited by?
Upper range of resp volume

Answer 23

• Maximal compensation can take up to 24hrs
• Respiratory compensation is limited by the
work involved in breathing
• Can increase minute volume up to about 30l/min but difficult to maintain

Question 24

Where is carbonic anhydrase found?

Answer 24

RBC

Tubule cells

Question 25

2 processes of metabolic alkalaemia:

Answer 25

Initiating: - Loss of H+ ions (from gut above pylorus in *pyloric stenosis*, from the kidney due to diuretics) - Gain of exogenous alkali Maintenance of the alkalosis: -Process which impairs the kidneys ability to excrete bicarbonate e.g. *chloride depletion group*, potassium depletion group

Question 26

In which situation is exogenous alkali gained?

Answer 26

Massive blood transfusion

Question 27

How does chloride depletion cause metabolic alkalaemia?

Answer 27

Chloride and bicarb are the only anions present in appreciable quantities in the ECF. If chloride is low in renal tubular fluid, bicarbonate must be reabsorbed to maintain electrical neutrality

Question 28

Compensation for metabolic alkalis

Answer 28

* pH =  HCO3/pCO2 * pCO2 must increase minute volume must fall * Difficult to predict extent...

Question 29

GO LOOK AT SLIDE 39 OF LECTURE

Answer 29

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