Arterial blood gas (ABG analysis)

Question 1

The respiratory system has 2 main components

Answer 1

ventilatory air pump

Gas exchange surface

Question 2

Effective gas exchange requires

Answer 2

adequate and matching flow of gas and perfusion of respiratory tissues

V/Q matching

Question 3

Basic respiratory mechanism

Answer 3

Concs of gases are refreshed on a breath-by-breath basis

Narrow diffusion distance between the alveoli and pulmonary caps

the gap is filled by pulmonary interstitial fluid

02 in Co2 out

Question 4

Physiological shunting (V/Q matching)

Answer 4

“blood passes lungs without contributing to has exchange”

Low V/Q ratio

Ventilation to one lung or area is decreased or absent

That area is still perfused as body hasnt adapted yet = no gas exchange

Oxygenated and non-oxygentaed blood mixes

e.g., obstruction or ashma attack

Question 5

Physiological deadspace (V/Q mismatch)

Answer 5

High V/Q ratio

Lung is ventilated but not perfused

No deoxygenated blood mixes

e.g., pulmonary emboli

Question 6

VQ matching

Answer 6

Compensatory mechanisms to ensure that blood flow matches ventilation

Question 7

hypoxic vasoconstriction-

Answer 7

hypoxic vasoconstriction-when oxygen levels drop in a lung, the arterioles that supply that area will vasoconstrict, reeucing blood flow to the area that is not perfused. If CO remains static, more BF goes to the areas that are ventilated.

Similar but less effective method for areas that aren’t perfused. Bronchoconstriction and reduced surfactant secretion helps divert air to well-perfused alveoli.

Question 8

Respiratory failure

Answer 8

“localised mismatching of V/Q ratio”

Question 9

type 1 respiratory failure

Answer 9

pO2 low and pCO2 normal

hypoxaemic

Question 10

type 2 respiratory failure

Answer 10

global reduction in flow of either air to alveoli or blood to pulmonary caps

pO2 low and pC02 high

hypercapnic

Question 11

Acid base balance

Answer 11

↑CO₂ + H₂O ↔︎ H2CO3 ↔︎ HCO3 + ↑H+

Therefore if you increase pCO2 or decrease HCO3- you decrease arterial pH

pH is proportional to 6.1+log x conc of (HCO3-)/ 0.03 x pCO2

Question 12

What does the body do in response to chornic acidosis?

Answer 12

Liver

• decrease urea production
• increase production of glutamine

Kidney

• increase glutamine dehydrogenase + PEPCK to catalyse the breakdown of glutamine into NH4+ (ammonium) and HCO3- in the proximal tubule
• Ammonium is then released through micturition

Question 13

What does the body do in response to chronic respiratory alkalosis?

Answer 13

Kidney

• Decreases plasma [HCO3-]
• Increase the number and activity of type-B intercalated cells in the collecting ducts (they secrete HCO3- from the blood into the tubule lumen helping to increase conc in the final urine therefore lowering the blood conc and pH

Question 14

Metabiolic factors can affect acid/base balance and therefore blood gases (4)

Answer 14

Alkaline tide

kidney failure

ketone bodies

vomiting

Question 15

Alkaline tide

Answer 15

when bicarbonate is produced in the intestine is not absorbed by the gut. bicarb bypassses the reabsorption mechanisms

e.g., bowel obstruction

Question 16

kidney failure

Answer 16

reduced excretion of uric acid leading to metabolic acidosis

Question 17

ketone bodies

Answer 17

in starvation, diabetic ketoacidosis, sepsis or severe exercise can result in greater lactic acid production

Question 18

vomiting

Answer 18

loss of H+ from vomiting

Question 19

Kussmaul’s breathing

Answer 19

Breathing patterns can affect acid/base balance

When we have metabolic acidosis due to:

K= Ketones (diabetic ketoacidosis)

U= Uremia

S= Sepsis

S= salicylates (aspirin)

M= menthanol

A= Aldehydes

U

L= lactic acid

breathing pattern changes to kussmaul’s breathing, deep sighing respiration pattern where we increase tidal volume, allowing us to breathe out more CO2

Question 20

Lungs excrete excess non volatile gases in the form of

Answer 20

CO2

Question 21

Reflex increase in ventilation rate

Answer 21

1. Increase firing of chemoreceptors
2. detected peripherally and centrally
3. transmitted to the Central Pattern Generator in the brainstem
4. Increased and more frequent phrenic and intercostal nerve activity
5. more forceful and drequent contraction of diaphragm and intercostals
6. increase rate and depth of breathing (increase V rate)

Question 22

Respiratory compensation for chronic metabolic acidosis

Answer 22

decrease pH

Increased firing of peripheral chemoreceptors

transmitted APs via the glossopharyngeal and vagus nerves to

Central Pattern Generator in Brainstem

Increased more frequent phrenic and intercostal nerve activity

Increased ventilation rate

increased exhalation of CO2

decreased PCO2 and increased pH

Question 23

Arterial blood gas

Answer 23

take arterial blood sample (usually from radial artery)

send to specific analyser

electrodes for arterial pH, O2 and CO2

gives info on pH, pCO2, Bicarbonate, pO2 and (base excess?)

Go though steps O, A, B, C

Question 24

Step O

Answer 24

Normal pO2= 13.3 kPa

• <10 suggests respiratory problem
• >10 but pH is normal suggets a metabolic problem

If the patient is breathing artificial oxygen concentrations then it should

Where pO2 is less than 10 we should look at:

• Normal/ low (type 1 RF) caused by V/Q mismatch
• High, then (type II RF) due to conditions that reduce our tidal volume

Question 25

Normal ranges for ABG

Answer 25

O2- 13.3 kPa (<10 resp problem // >10 but pH normal is metabolic problem)

pH- 7.35-7.45

CO2- <4.7 is alkalitic >6.00 is acidic

HCO3- <20mM is acidotic >28 is alkolitic

Question 26

Step A

Answer 26

Acidosis or alkalosis? (look at pH)

normally under a very tight control of 7.35-7.45

• <7.35 = too acidic = acidosis
• >7.45 = to
• o basic = alkalosis

NB- type 1 RF or mixed/ base disorder may be balancing each other out for a patient to have a normal pH. e.g., patient with ketoacidosis who is vomiting.

Question 27

Step B

Answer 27

Buffers (CO2/HCO3-)

Acidotic blood- look at CO2

• CO2 high - resp failure - resp failure
• CO2 normal/ low - bicarb low - metabolic acidosis

Alkalotic blood

• CO2 too low- resp. alkalosis - breathing too much- blow off too much CO2
• Bicarb levels too high - metabolic alkalosis

​

Question 28

Step C- Chronic/ compensated

Answer 28

Identify how acute the acid/base disturbance is by whether the body has started its compensatory mechanisms yet

Resp. acidosis- Bicarb level – body tries to produce more bicarb

Met. acidosis- PCO2 level –Falls as we start to breathe more

Resp. alkalosis- Bicarb level –Falls to balance poeple breathing too much

Met. Alkalosis- PCO2 –risen to compensate for too much bicarb

Question 29

Base excess

Answer 29

useful in conjunction with [HC03-] to confirm any component of an acid-base disorder

“the amount of strong acid that has to be added to fully oxygenate blood held at body temperature and normal PCO2 to return ut to a pH of 7.40”

negative value indicates a base deficit

-2 t0 +2 is normal range

+2 is metabolic alkalosis

-2 is metabolic acidosis

Question 30

Example 1

Patient is brought into casualty department semi-conscious. Patient was found at home with an empty bottle of sleeping pills nearby.

pH 7.16 (low 7.16< 7.35)

PCO2 10.7 (high 10>6)

Bic 28 (normal)

pO2 5.3 (low 5.3< 10)

Base excess -0.4 (normal)

Answer 30

Step O- O2 low

Step A- acidosis

Step B- high CO2 + low O2 (resp. acidosis)

Step C- bicarb is normal (acute resp.acidosis)

Presentation fits with clinical history – reduced consciousness causes a reduction in tidal volume

Alternatively:

Question 31

Example 2

Patient suffers catastrophic stroke and following the event was seen to be irregular and inadequate. Patient was intubated and ventilated with an inspired oxygen concentration of 40%.

pH 7.63 High

pCO2 low

Bic 20 mmol.L-1 normal

pO2 35.3 high

base excess normal

Answer 31

O- high

A- alkalosis

B- CO2 low Bic low? (respiratory alkalosis)

C- Bicarb starting to get low- beginings of compensation?

Patient is being overventilated. Setting on ventilator needs to be changed.

Alternatively it is someone who is hyperventilating.

Question 32

Example 3

Patient with abdominal pain due to a duodenal ulcer has been admitted to the medical ward with persistent vomiting. He was also taking large quantities of sodium bicarbonate to ease the pain.

pH- 7.54 High

pCO2- 6.7 High

Bic- 33 mmol high

pO2- 11.1 High

Base excess- 17.6 (large amounts of acid had to be added - excess bicarb)

Answer 32

O- high

A- high- alkalosis

B- CO2 high Bic high (metabolic alkalosis)

C- starting to see a degree of compensation as CO2 Is starting to rise

Patient has been vomiting too much acid out- not enough left to balance base

Question 33

Example 4-

A 52-year-old man was admitted unconscious to casualty. He was a known diabetic on daily insulin. One week ago he had developed a chest infection. He stayed at home and because he stopped eating he stopped his insulin. Over the preceding days he had become increasingly drowsy and in the morning of admission he was unrousable.

pH 7.19

PCO2: 4.0

bic: 11.1

pO2; 13.3

Base excess; 15.3

Answer 33

O; normal

A; acidic

B; low + base excess suggesting too much acid most likley metabolic

C; CO2 is starting to fall - most likley respiratory response to acidosis- KUSSMAUL breathing

Started to have ketoacidosis. Could be a septic patient with lactic acidosis.