Blood Gas Analysis

Question 1

Define acid and base.

Answer 1

Acid = proton donor
Base = proton acceptor

Question 2

What is the calculation for pH?

Answer 2

pH = -log10[H+]

Question 3

What is normal body pH?

Answer 3

7.35 - 7.45

Question 4

What is the relationship between pH and [H+]?

Answer 4

As pH falls, [H+] increases in bigger and bigger intervals

Question 5

Why do we need a consistent pH in the body?

Answer 5

Affects rate of enzymatic reactions/proteins denaturing
Death usually occurs if pH is less than 6.8 (acidaemia) or greater than 7.6 (alkalaemia)

Question 6

Define acidaemia and alkalaemia.

Answer 6

Acidaemia = blood pH less than 7.35
Alkalaemia = blood pH greater than 7.45

Question 7

What is the Henderson-Hasselbach equation?

Answer 7

pH = pKa + log10[HCO3] / [0.3 pCO2]

Question 8

Describe buffers.

Answer 8

Any particle capable of accepting or donating H+
Act to soak up excess hydrogen and release it when necessary
Work very fast - seconds

Question 9

Give some examples of buffers.

Answer 9

Bicarbonate (HCO3-)
Haemoglobin
Blood proteins
Phosphate
Lactate

Question 10

Define respiratory acidosis/alkalosis.

Answer 10

A change in PCO2 leading to a change in pH

Question 11

Define metabolic acidosis/alkalosis.

Answer 11

A change in anything other than PCO2 that can affect pH
Usually see a change in HCO3-, either due to loss/gain or its role as a buffer to other acids

Question 12

What is the relationship between PaCO2 and ventilation?

Answer 12

The arterial concentration of CO2 (PaCO2) is inversely proportional to ventilation

Question 13

Define hyperventilation.

Answer 13

Fall in PaCO2

Question 14

Define hypoventilation and its effect on H+ concentration.

Answer 14

Increase in PaCO2
Increased CO2 = Increased H+

Question 15

What role does the renal system play in regulating blood gases?

Answer 15

Major way of getting rid of excess acid from the body
Plays a role in regulating amount of bicarbonate (HCO3-) in the body
Works slowly - hours to days

Question 16

Describe acute respiratory acidosis.

Answer 16

Increased PCO2 (hypoventilation)
Compensatory response is to increase HCO3-

Question 17

Describe chronic respiratory acidosis.

Answer 17

2-5 days
Kidneys have had time to react
More acid excreted and HCO3- retained

Question 18

Describe respiratory alkalosis.

Answer 18

Fall in PCO2 (hyperventilation due to pain/stress/hypoxaemia)
Metabolic compensation is to decrease HCO3-

Question 19

Describe metabolic acidosis.

Answer 19

Signified by decreased HCO3- (due to direct loss or consumption due to excess acids)
Resp. compensation - fall in PCO2 due to decreased ventilation

Question 20

Describe metabolic alkalosis.

Answer 20

Signified by increased HCO3- (compensatory increase due to loss of chloride ions/albumin)
Resp. compensation - increase in PCO2 due to decreased ventilation

Question 21

Define base excess.

Answer 21

Amount of acid required to titrate 1L of blood to a pH of 7.4 at 37 degrees C and a PaCO2 of 40mmHg
Can be positive or negative

Question 22

What is normal base excess?

Answer 22

2-5mmol/L

Question 23

What is normal PaO2 and how is hypoxaemia defined?

Answer 23

Normal = 90-100mmHg (room air)
Hypoxaemia defined as less than 80mmHg

Question 24

How do we take and handle samples for blood gas analysis?

Answer 24

Arterial better for gas exchange - taken over a few breaths
Collected anaerobically
Heparinised syringes
Prevent clot formation and discard first drop of blood

Question 25

What are common artefacts on blood gas analysis?

Answer 25

Air contamination - low CO2 and high O2
Saline contamination e.g. from catheters - high chloride
Low PCV/HCT - clotting