QuestionSBA
QuestionChoice
An ABG performed in a patient with simple hyperventilation would likely show low PaCO<sub>2</sub> levels (due to losses in expiration) with normal PaO<sub>2</sub> (no pathology present to cause hypoxia). This may cause a respiratory alkalosis if hyperventilation is maintained for long enough, not an acidosis. Additionally, there would be a change in the metabolic axis (i.e. bicarbonate) due to the acuity of the respiratory derangement
33325
c
Hyperventilation
QuestionChoice
This patient is acidotic, however a low bicarbonate level would be expected if this were metabolic in origin. Additionally, respiratory compensation for a metabolic acidosis would involve hyperventilation in order to reduce PaCO<sub>2</sub> levels - in this ABG example both the bicarbonate and PaCO<sub>2</sub> are elevated
33326
d
Metabolic acidosis with partial respiratory compensation
QuestionChoice
In chronic type 2 respiratory failure there is hypoxia with hypercapnia (as is the case here) - however, as this is a chronic process you would expect the pH to be largely corrected by metabolic compensatory processes (pH normal/close to normal with elevated bicarbonate)
33324
b
Chronic type 2 respiratory failure
QuestionChoice
In type 1 respiratory failure there is hypoxia without hypercapnia. In this example, the PaCO<sub>2</sub> is raised
33327
e
Type 1 respiratory failure
QuestionChoice
In acute-on-chronic type 2 respiratory failure the usual compensatory metabolic processes have been overwhelmed by the additional acute respiratory insult. The resultant pattern on ABG is a marked increase in PaCO<sub>2</sub> with significant respiratory acidosis, alongside a highly elevated HCO3 that is not adequate to normalise/nearly normalise the pH. Therefore, this patient is likely to need bilevel positive airway pressure (BiPaP) non-invasive ventilation to aid clearance of the CO<sub>2</sub> and improve the acidosis as part of their treatment plan
33323
a
Acute-on-chronic type 2 respiratory failure
Concept
Chapter
### Mean pressures in pulmonary circulation 

- Pulmonary artery 12-15mmHg
- Left atrium 5mmHg
- Pressure gradient 7-10mmHg

### Ventilation perfusion ratio

V/Q ratio - the ratio of alveolar ventilation (V) to alveolar blood perfusion (Q).

- To attain efficient gaseous exchange, ventilation and perfusion must be matching.
- The normal V/Q ratio is 0.8, which means that alveolar ventilation has to be at least 80% of pulmonary blood flow.
	- 4L/min of air enters the alvoeli
	- 5L/min of blood flows through the lungs

V/Q ratio is different across different lung zones:

- Zone 1 (upper zones, lung apices) has the lowest ventilation and perfusion;
- Zone 3 (lower zone, lung bases) has the highest ventilation and perfusion due to gravity.
- However, the regional differences in ventilation are less marked than perfusion, resulting in a higher V/Q ratio in zone 1 (3.3 apically) and lower in zone 3 (0.63 basally).

The significance of the V/Q ratio lies in its influence on the partial pressure of oxygen (PO2) and the partial pressure of carbon dioxide (PCO2).

- At a high V/Q ratio, the gaseous exchange is more efficient, so PO2 is higher and PCO2 is lower, and vice versa.
- Normal PO2 in arterial blood is 100mmHg, while normal PCO2 in arterial blood is 40mmHg when the V/Q ratio is at its optimum value, which is 0.8.

### Right-to-left shunt 

When there is a right-to-left shunt or airway obstruction, where there is no ventilation to a well-perfused alveolus, V/Q = 0; hence, gaseous exchange does not occur. PO2 and PCO2 in the arterial blood remain the same as that in the venous blood, which are 40mmHg and 46mmHg, respectively.

### Dead space 

When there are alveoli which are well-ventilated but not perfused, this is termed a physiological dead space. V/Q = ∞; hence, gaseous exchange does not occur. An example of this is a pulmonary embolism. PO2 and PCO2 in the alveoli remain the same as that in the inspired air, which are 150mmHg and 0mmHg, respectively.
2759
4742
Pulmonary circulation
Topic
257
Respiratory Physiology
6665
A 73-year-old woman with a history of chronic obstructive pulmonary disease (COPD) comes in via ambulance to the emergency department with worsening shortness of breath.


An arterial blood gas (ABG) is performed as part of her assessment:


||||
|--------------|:-------:|------------------|
|pH|7.27|7.35 - 7.45|
|PaO₂|7.1 kPa|11 - 15|
|PaCO₂|9.47 kPa|4.6 - 6.4|
|Bicarbonate|43 mmol/L|22 - 30|
|Lactate|1.1 mmol/L|0.6 - 1.4|

 - Inspired oxygen 28%


What is the most likely explanation for this blood gas result?
a