Blood Gas

Question 1

1kPa

Answer 1

7.5mmHg

Question 2

Conversion H+ to pH

Answer 2

80 - [ H+ ] = two decimal places

Question 3

buffers for pH

Answer 3

proteins
haemoglobin
carbonic acid / bicarbonate

Question 4

excretions mechanisms for acid

Answer 4

lungs kidneys

Question 5

base disturbances occur when

Answer 5

• There is a problem with ventilation
• There is a problem with renal function
• Overwhelming acid or base load the body can’t handle

Question 6

Normal Values

Answer 6

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

Question 7

Standard bicarbonate is calculated from

Answer 7

the actual bicarbonate but assuming 370C and a paCO2 of 5.3kPa

Question 8

what does standard bicarb reflect

Answer 8

the metabolic component of acid base balance

Question 9

what to look at when assessing the patient

Answer 9

• Look at the pO2 - is the patient hypoxic or is the pO2 too high

Question 10

Adverse effects of high oxygen levels

Answer 10

• Increases risk of hypercapnic respiratory
failure in acute exacerbations of COPD
• Increased mortality survivors of cardiac arrest
• Increased mortality intensive care patients
• Increased mortality in acute severe asthma
it generates free radicals

Question 11

lung toxicity and a high O2

Answer 11

 Collapse of alveoli due to atelectasis
 Irritating to mucous membranes
Ocular toxicity, myocardial damage, neuro…

Question 12

targets for O2 stats

Answer 12

• Targets 94-96% (normally)

- 88-92% (type 2 resp failure)

Question 13

Therapeutic uses of high inspired conc. of oxygen

Answer 13

• Pneumothorax

* Carbon monoxide poisoning

Question 14

Normal Alveolar –arterial (A-a) gradient is

Answer 14

less than 3kPa - ..you can expect the arterial pO2 to be approx 2/3 FiO2

Question 15

Alternatively think about the P/F ratio

Answer 15

PaO2 / FiO2 ratio or the P/F ratio (kpa divided by inspired fraction of oxygen)
P/F ratio > 50 = healthy
P/F ratio < 40 = acute lung injury P/F ratio <26.7 = ARDS

Question 16

• Assess the pH

Answer 16

• pH<7.35 acidaemia
• pH>7.45 alkalaemia
• pH between 7.35 and 7.45 2 options – normal
mixed acid base abnormality

Question 17

If the pH and pCO2 are changing in opposite directions

Answer 17

this suggests a respiratory problem

Question 18

• If the pCO2 and pH are changing in the same direction,

Answer 18

the primary problem is probably metabolic

Question 19

What is compensation?

Answer 19

• Altering of function of the respiratory or renal system in an attempt to correct an acid – base imbalance

Question 20

compensation equation

Answer 20

• pHα HCO3 / pCO2

Question 21

• If pCO and HCO - move in the same direction

Answer 21

compensation is possibly occurring (remember pH ∞ bicarbonate/pCO2)
• If both values move in opposite directions more than 1 pathology must be present

Question 22

• In chronic respiratory acidosis

Answer 22

the kidneys compensate by retaining bicarbonate. This takes a few days to reach its maximal value.

Question 23

Causes of hyperventilation

Answer 23

• Acute severe asthma • Pulmonary embolism
• Pulmonary oedema
anxiety attack

Question 24

Abnormal level of central respiratory drive

Answer 24

Hypoxia
Stimulation lung mechanoreceptors/chemoreceptors
Direct stimulation of respiratory centre
Psychogenic

Question 25

High Altitude

Answer 25

• Arterial pO2 can never exceed atmospheric pO2
• Top of Andes atmospheric pressure 50kPa
• Atmospheric pO2 10kPa
• Hypoxaemia induced hyperventilation is prominent
• Compensation occurs by renal excretion of bicarbonate