ABGs Flashcards
1
Q
A
2
Q
What is pH?
A
- pH = measurement of the acidity of the blood
- This reflects the number of hydrogen ions present
- An acid produces hydrogen ions when it breaks down, making a solution acidic
- More acidic = more hydrogen ions = pH falls (more acidic)
- Normally the body’s pH is 7.35 – 7.45
- pH is maintained through buffering and excretion of acids
- Hydrogen ions are excreted via the kidney and controlled via respiration via carbon dioxide
3
Q
What is pH?
A
- If the buffers and excretion mechanisms are overwhelmed = acid is continually produced = pH falls = metabolic acidosis
- If the ability to excrete CO2 is compromised = respiratory acidosis
- Normal pH doesn’t rule out respiratory or metabolic pathology
- You must always look at all the values other than pH as there may be a compensated or mixed disorder
4
Q
Causes of low (acidic) and high pH (alkaline)
A
Partial pressure (PO2 and PC02)
- A way of assessing the number of molecules of a particular gas in a mixture of gases
- It is the amount of pressure a particular gas contributes to the total pressure
-
For example:
- We normally breathe air which at sea level has a pressure of 100kP
- Oxygen contributes 21% of 100kPa
- Which corresponds to a partial pressure of 21kPa.
- PaO2 is the partial pressure of oxygen in arterial blood
- PaCO2 is the partial pressure of carbon dioxide in arterial blood.
*
5
Q
PO2
- 11 - 15 kPa (normal reference range, check your local machine!)
- Increased levels seen in supplemented air and in polycythaemias
- Decreased levels seen in:
- air with decreased oxygen
- Anaemia
- Heart decompensations
- COPD
- Restrictive pulmonary disease or hypoventilation
A
How can you predict the P02
- The arterial oxygen concentration (PaO2) should be approximately 8-10 kPA lower than the FiO2 (fraction of inspired oxygen)
- i.e. FiO2 is 21% at room air so in a healthy person the PaO2 should be 11-13kPA
- If the FiO2 is 40% then the PaO2 should be 30-32kPa
- If a patient is wearing a nasal cannula or a simple face mask, each additional litre of O2 adds about 4% to their FiO2
- For example, a patient with a nasal cannula with 2L of oxygen attached would have an FIO2 of 21% + 8% =29%
6
Q
PCO2
- Reference range 4.6 – 6.4 kPa
- Increased CO2 is caused by pulmonary oedema or obstructive lung disease (COPD)
- Decreased CO2 is caused by hyperventilation, hypoxia, anxiety, pregnancy or PE
A
Base excess (1)
- This is the amount of strong base which would need to be added or subtracted from a substance in order to return the pH to normal (7.40).
- The base excess is defined as the amount of H+ ions that would be required to return the pH of the blood to 7.35 if the pCO2 were adjusted to normal.
- A value outside of the normal range (-2 to +2 mEq/L) suggests a metabolic cause for the acidosis or alkalosis.
- In terms of basic interpretation:
- BE more than +2 = metabolic alkalosis
- BE less than -2 = metabolic acidosis
7
Q
Base excess (2)
A
Bicarbonate
- Produced by the kidneys and acts as a buffer to maintain normal pH (alkaline)
- Normal range for bicarbonate is 22 – 30 mmol/l.
- If there are additional acids in the blood the level of bicarbonate will fall as ions are used to buffer these acids
- If there is a chronic acidosis additional bicarbonate is produced by the kidneys to keep the pH in range
- This is why a raised bicarbonate may be seen in chronic type 2 respiratory failure where the pH remains normal despite a raised CO2
8
Q
The other bits
A
Compensation = the confusing bit
- pH is closely controlled, there are various mechanisms to maintain it at a constant value
- The body will never overcompensate as the drivers for compensation cease as the pH returns to normal
- = compensation for an acidosis will not cause an alkalosis or visa versa
9
Q
Respiratory compensation
A
- If a metabolic acidosis develops the change is sensed by chemoreceptors centrally in the medulla oblongata and peripherally in the carotid bodies
- The body responds by increasing depth and rate of respiration
- = increasing the excretion of CO2 to try to keep the pH constant
- The classic example = ‘Kussmaul breathing’ seen in severe acidosis including DKA (https://www.youtube.com/watch?v=TG0vpKae3Js)
- Here you will see a low pH and a low pCO2 = metabolic acidosis with partial respiratory compensation
10
Q
Metabolic compensation
A
- In response to a respiratory acidosis the kidneys will start to retain more HCO3 in order to correct the pH
- Here you would see a low normal pH with a high CO2 and high bicarbonate.
- This process takes place over days
11
Q
Step by step: How to interpret an ABG
A
What is the pH?
What is the pCO2?
12
Q
What is the PO2?
Now look at the compensation?
A
BE = Base XS
- The base excess ( a + value means excess of base, should mirror a raised HCO3 level=metabolic alkalosis)
- The base excess ( if a – value means a lack of base, should be mirrored by a reduced HCO3 level =metabolic acidosis)
REMEMBER: metabolic compensation takes 3 days, respiratory compensation is fast!
13
Q
Respiratory acidosis
A
Respiratory acidosis is very straightforward. It is always due to retention of CO2, (Type II Respiratory failure):
- COPD
- Depressed respiratory drive
- Brain injury
- OD
- CO2 retention causing inc drowsiness
- Hypoventilation of any cause
14
Q
Respiratory alkalosis
A
- This is due to hyperventilation.
- As PaCO2 lowers, so pH rises
- Any cause of hyperventilation:
- Anxiety
- Pain
- fever
- Sepsis
- Hypoxia
15
Q
Metabolic alkalosis
A
Caused by:
- Loss of hydrogen ions (diarrhoea/vomiting/burns)
- XS bicarbonate (diuretics/ingestionof alkaline substances)
16
Q
Metabolic acidosis
(anion gap)
A
- ….moves us onto the anion gap
- This is used to help diagnose acid base disorders.
- It is usually used in suspected cases of metabolic acidosis. It can either be normal, or raised. Low anion gap does not usually occur.
17
Q
Anion gap
A
- Anions are negatively charged ions. The two most common ones are chloride and bicarbonate.
- Anions are hard to measure accurately. The anion gap is the difference between the number of measured anions, and the number of unmeasured anions.
- Negatively charged proteins make up most of the unmeasured anions in a normal individual, and the main one is albumin. And thus in a normal individual, there is an anion gap of 4-16, made up mostly of albumin.
- Some causes of metabolic acidosis, do not produce a large number of unmeasured anions, and the anion gap doesn’t increase. This is normal anion gap metabolic acidosis
- Some causes of metabolic acidosis produce a large number of both measured (e.g. HCO3- ) anions, and unmeasured anions. This is High anion gap metabolic acidosis
18
Q
A
19
Q
Normal Anion Gap Metabolic Acidosis
A
HIGH ANION GAP METABOLIC ACIDOSIS (HAGMA)
23
Q
Case 5: a bit more detailed…
A
24
Q
Case 6
A
Case 7
What is the anion gap?
Anion gap = ([Na+] + [K+]) − ([Cl−] + [HCO3−]) = 28.5
Normal range is 7 – 16.
N.B. Some analysers won’t include potassium in their calculations therefore for them >15 constitutes a raised anion gap.
Either way, this is a raised anion gap metabolic acidosis.
Differential diagnosis?
MUDPILES!
25
Q
Case 8
A
Case 9
A 17 year-old girl presents to the emergency department after an argument with her boyfriend. He says that she took lots of tablets. She denies this. You persuade her to let you do an ABG:
- pH: 7.46 (7.35-7.45)
- pO2: 12.5 (10–14)
- pCO2: 3.5 (4.5–6.0)
- HCO3: 22 (22-26)
- BE+1 (-2 to +2)
A few hours later she says she feels increasingly unwell and is complaining of ringing in her ears. A repeat gas shows:
- pH: 7.15 (7.35-7.45)
- pO2: 11.0 (10–14)
- pCO2: 3.2 (4.5–6.0)
- HCO3: 9 (22-26)
- BE: -18 (-2 to +2)
26
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A
