Blood gases Flashcards
what is normal blood pH?
average 7.4 (7.35-7.45) it is closely controlled by buffering and excretion of acids (H+ ions via the kidney) The bicarbonate buffering system is the method by which the body controls pH and is crucial to understand arterial and venous blood gas results.
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3–
The equation demonstrates an equilibrium, between carbon dioxide, and hydrogen ions + bicarbonate. In normal physiology at a normal metabolic rate, this equilibrium exists to keep the pH between 7.35 and 7.45
what is normal pO2?
10-14 kPA
PA=pressure in alveoli
Pa=pressure in artery
normal pCO2
4.5-6 kPA
normal base excess (BE)
(-2) to 2 mol/l (mEq/l)
HCO3 bicarbonate
22-26 mol/l
What is the primary way in which the concentration of H+ ions is regulated?
Changes in ventilation - so say you can’t breath properly CO2 concentration increases and a respiratory acidosis can occur because H+ ions can’t be excreted:
more CO2 + H2O↔ H2CO3↔moreH+ + HCO3–
what is the base excess BE?
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).
what is the BE in a metabolic acidosis?
< -2mEq/l
what is the BE in a metabolic alkalosis?
> +2mEq/l
what state is it if normal CO2 and pH?
normal acid base staus i.e. healthly person
normal Co2 but low pH
metabolic acidosis
normal CO2 but high pH
metabolic alkalosis
Is caused by:
Loss of hydrogen ions
Diarrhoea (sometimes vomiting too)
Burns
Excess Bicarbonate
Diuretics
Ingestion of alkaline substances
low pH and high pCO2
respiratory acidosis
Respiratory acidosis is very straightforward. It is always due to a retention of CO2, (Type II Respiratory failure) of which there are only a handful of causes:
COPD
Depressed respiratory drive (e.g. low GCS)
Brain Injury
Drug overdose (often opiates)
CO2retention in COPD patients causing worsening drowsiness
Hypoventilation of any other cause
Patient breathing room air
PaO2 6.6 – very low
PaCO2 6.5 – high
pH 7.14
HCO3 23
what acid base situation is this?
Respiratory acidosis
This is a primary respiratory acidosis without compensation – because pH is low (acidosis) and CO2 is high (respiratory) and HCO3 is normal – so there is not metabolic compensation.
This is type 2 respiratory failure.
The acidosis is acute because it is not compensated – the bicarbonate is normal.
PaO2 7.8 (low)
PaCO2 8.0 (high)
pH 7.35 (normal)
HCO3 31 (high)
respiratory acidosis
High CO2 indicates a respiratory acidosis – but the increased bicarbonate and the normal pH indicated t is fully compensated.
This is likely to be chronic respiratory failure
FlO2 .21 (21% oxygen – room air)
PaO2 8.0 low
PaCO2 5.0 (normal)
pH 7.51 High
HCO3 30
metabolic alkalosis
pH is high. This is an alkalosis
CO2 is normal – therefore not likely to be hyperventilation
This is a metabolic alkalosis – with a possible other cause of the hypoxia.
In this particularly example the alkalosis was due to diuretics. The patient’s actual presenting complaint was carbon monoxide inhalation, which explains his hypoxia.
Patient is on 3L oxygen
PaO2 9.5 (low)
PaCO2 2.8 (low)
pH 7.40 (normal)
HCO3 12 -very low
O2 sats 95%
metabolic acidosis (fully compensated)
The pH is normal, but the PaCO2 is very low. This indicates a fully compensated metabolic acidosis, as indicated by the low bicarbonate.
You are asked to review a 63-year-old female who was admitted with shortness of breath. On your arrival, the patient appears drowsy and is on 10L of oxygen via a mask. You perform an ABG and receive the following results…
PaO2: 7.0 (11-13 kPa)
pH: 7.29 (7.35 – 7.45)
PaCO2: 9.1 (4.7-6.0 kPa)
HCO3–: 26 (22-26 mEg/L)
Base excess: +1 (-2 to +2)
what does the ABG show?
Oxygenation (PaO2)
The PaO2 is low, so we know the patient is in respiratory failure, however, we don’t yet know what type.
pH
You should then note that the pH reveals an acidosis and look at the CO2 to see if it is contributing to the acidosis (↑CO2).
PaCO2
In this case, the PaCO2 is raised significantly and this is likely the cause of the acidosis. In the context of low PaO2, a raised PaCO2 suggests the patient type 2 respiratory failure.
HCO3-
The HCO3– is normal, so the metabolic system is not contributing to the acidosis and also isn’t compensating for the respiratory acidosis, suggesting that this is an acute derangement.
Base excess (BE)
The base excess is within normal limits as there has been no significant change in the amount of HCO3–. If this respiratory acidosis was chronic we would expect that the kidneys would have generated more HCO3– to compensate, which would have resulted in an increased BE.
Summary
Respiratory acidosis
A 17-year-old patient presents to A&E complaining of a tight feeling in their chest, shortness of breath, some tingling in their fingers and around their mouth. They have no significant past medical history and are not on any regular medication. An ABG is performed on the patient whilst they’re breathing room air and the results are shown below…
PaO2: 14 (11-13 kPa)
pH: 7.49 (7.35 – 7.45)
PaCO2: 3.2 (4.7-6.0 kPa)
HCO3–: 22 (22-26 mEg/L)
BE: +2 (-2 to +2)
What does the ABG show?
Oxygenation (PaO2)
A PaO2 of 14 on air is at the upper limit of normal, so the patient is not hypoxic.
pH
A pH of 7.49 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).
PaCO2
The CO2 is low, which would be in keeping with an alkalosis, so we now know the respiratory system is contributing to the alkalosis and is likely the entire cause of it. The next step is to look at the HCO3- and see if it is also contributing to the alkalosis.
HCO3-
HCO3- is normal, ruling out a mixed respiratory and metabolic alkalosis, leaving us with an isolated respiratory alkalosis.
Base Excess
Base excess is normal, suggesting there has been no addition of bicarbonate to cause the alkalosis, ruling out the metabolic system as the cause.
Compensation
The bicarbonate is on the low end of normal, but this does not represent compensation. Compensation would involve a much more significant reduction in HCO3–.
Summary
Respiratory alkalosis
A 48-year-old male has been admitted with a 24hr history of abdominal distention and profuse vomiting. A CT scan reveals a large mass causing bowel obstruction. As part of the patient’s assessment, the surgical registrar requested that you check his blood gas (on air), with the results shown below…
PaO2: 12.7 (11-13 kPa)
pH: 7.50 (7.35 – 7.45)
PaCO2: 5.5 (4.7-6.0 kPa)
HCO3-: 29 (22-26 mEg/L)
BE: +3 (-2 to +2)
What does the ABG show?
Oxygenation (PaO2)
A PaO2 of 12.7 on air is normal, so the patient is not hypoxic.
pH
A pH of 7.50 is higher than normal and therefore the patient is alkalotic. The next step is to figure out whether the respiratory system is contributing to the alkalosis (e.g. ↓ CO2).
PaCO2
The CO2 is normal, which is not in keeping with an alkalosis, so we now know the respiratory system is not the cause of this derangement. The next step is to look at the HCO3- and see if it explains the alkalosis.
HCO3–
HCO3– is high, which is in keeping with a metabolic alkalosis.
Base Excess
Base excess is increased, in keeping with an excess of HCO3–.
Compensation
The respiratory system can attempt to compensate for a metabolic alkalosis by increasing PaCO2 (decreasing ventilation), but in the short term, the respiratory system will likely maintain PaCO2 within the normal range. If the metabolic alkalosis persists, however, you would expect the PaCO2 to rise and compensate for the metabolic alkalosis, as the respiratory centre becomes progressively desensitized to the increasing levels of PaCO2.
Summary
Metabolic alkalosis
