Arterial blood gases

Question 1

Where is the rhythm generator in the body? What does it do?

What area of the hindbrain is involved in modification?

Answer 1

In the medulla
It contains inspiratory and expiratory groups of neurones

It determines the pace and depth of breaths

Pneumotoxic centre in Pons

Question 2

State 7 things that cause respiratory depression.

Answer 2

Opiates/narcotics
Alcohol (acute use)
Strokes in medulla
Anaesthesia
Degenerative disease
Sedatives
Cerebral disease

Question 3

Chemosensing occurs via peripheral and central afferent nerve inputs. Describe the peripheral chemoreceptors

Answer 3

The carotid bodies- bundles of cells just outside the bifurcation of carotid arteries

The aortic bodies- bundle of cells within arch

They respond to hypoxia more than increased CO2 and H+
Carotid and aortic bodies provide back up for each other

Question 4

Chemosensing occurs via peripheral and central afferent nerve inputs. Describe the central chemoreceptor

Answer 4

Clusters of cells scattered throughout hindbrain
Sense PaO2 and concentration of H+(indirectly PaCO2)
Blood brain barrier makes it difficult for H+ to enter CSF from capillary

Question 5

Describe how chemosensing occurs in the CSF

Answer 5

CO2 diffuses in CSF and reacts with aqueous environment forming unstable hydrogen carbonate which breaks down into H+ ions which are detected by medulla oblongata.

Once H+ detected there is an immediate increase in response rate. This occurs when PaCO2> 40

Question 6

What is CO2-insensitivity?

Answer 6

It arises from chronic CO2 retention. There is a risk of type 2 respiratory failure. In case of injury, CO2 rises to critical levels which causes CO2 narcosis which can lead to death.

Type 2 respiratory failure is the inability to ventilate and it can be caused by COPD

Question 7

Efferent nerves innervate the muscles involved in respiration. Describe which and where.

Answer 7

INSPIRATION
Diaphragm: phrenic nerves C3-C5
External intercostal muscles: thoracic nerves T1-T11
Accessory muscles in neck: sternocleidomastoid (XI cranial nerve) & scalene muscles (C3-C8)

EXPIRATION
Abdominal wall
Internal intercostal muscles

Question 8

There are other inputs involved in chemosensing other than peripheral and central afferent nerve inputs, describe them.

Answer 8

Vagal
Lung stretch receptors sense lung stretch during breathing to prevent over-stretching
Sense abnormal changes in airway mechanical properties

Irritant & particulate receptors
C-fibre neurones activated by oedema and endogenous sensitisers(e.g. food) such as bradykinin leading to cough

Question 9

What is normal pH of blood?

What is the normal range for PaCO2? What is the term given to an increase or decrease out of these limits?

What is the normal range for PaHCO3? What is the term given to an increase or decrease out of these limits?

Answer 9

7.4 (sd +/- 0.02)

36-44mmHg
PaCO2 >44mmHg = Respiratory Acidosis
PaCO2< 36mmHg = Respiratory Alkalosis

22-26mmol/L
PaHCO3> 26mmol/L = Metabolic Alkalosis
PaHCO3< 22mmol/L = Metabolic Acidosis

Question 10

Outline the 6 steps involved in ABG testing

Answer 10

1. Examine pH, PaCO2, PaHCO3
2. Determine primary process
3. Calculate the anion gap and base excess (normal gap = 12mEq/L)
4. Identify the compensatory process
5. Determine if a mixed acid/base disorder is present
6. Generate a diffferential diagnosis

BE AWARE THAT OPPOSITE PROCESSES MAY LEAD TO A NORMAL pH!

Question 11

Why does the retention of CO2 cause respiratory acidosis?

Answer 11

It is a volatile acid so dissolves readily in plasma

Question 12

What is anion gap metabolic acidosis ?

What are its causes?

Answer 12

A form of metabolic acidosis caused by a lager anion gap (>12mEq/L)

G - glycols (ethylene and propylene)
O - oxoproline
L  - L-lactate
D - D-lactate
M - methanol
A  - aspirin
R  - renal failure
K  - ketoacidosis

Question 13

What is a non-anion gap acidosis?

Answer 13

Renal tubular acidosis

Three types:
1. there is an inability to secrete H+ into lumen (distal duct)

2. there is an inability to secrete H+ into lumen (proximal duct)
3. RARE

Other causes: GI losses, acetazolamide, excessive chloride administration (NaCl fluids)

Question 14

Define base excess

Answer 14

The relationship between a metabolic acidoses/alkalosis and bicarbonate levels is NOT linear. It refers to the dose of acid needed to return the blood to 7.4 under standard conditions

Question 15

For each primary disturbance outline the compensatory mechanism

Answer 15

Respiratory acidosis- retain bicarbonate
Respiratory alkalosis- reduce bicarbonate

BOTH TAKE DAYS

Metabolic acidosis- reduce CO2
Metabolic alkalosis - retain CO2

Question 16

Describe what mixed disorder means

Answer 16

Two or more primary acid-base disturbances

Clue:

1. Anion gap not equal to reduction in bicarbonate- “gap of the gap”
2. Anion gap present but pH is alkalemic
3. Incomplete compensation

Question 17

Describe what happens at high altitude

Answer 17

Hypoxaemic hypoxia leads to :
Right shift of Hb-O2
Increased Hb
Increased minute ventilation