QuestionSBA
QuestionChoice
Infective exacerbation of COPD would be a likely differential for this patient's initial clinical presentation. However, the history of new-onset drowsiness and this man's ABG findings in the context of COPD and the initiation of oxygen therapy is more suggestive of acute on chronic retention of CO2
33575
c
Infective exacerbation of COPD
QuestionChoice
Pulmonary embolism would typically present with type 1 respiratory failure. This would mean they have a low oxygen and a low to normal CO2 on an ABG. As this patient has a high CO2, they are in type 2 respiratory failure, not type 1
33576
d
Pulmonary embolism
QuestionChoice
Pneumonia would be a likely differential for this patient's initial clinical presentation (differentiated based on chest x-ray findings). However, the history of new-onset drowsiness and this man's ABG findings in the context of COPD and the initiation of oxygen therapy is more suggestive of acute on chronic retention of CO2
33574
b
Pneumonia
QuestionChoice
Myasthenia gravis can present with type 2 respiratory failure, and the typical age of onset in men is between 60 and 70 years old. However, it usually presents with diplopia, swallowing difficulties and arm weakness that worsens with repeated usage. Given this patient's history, acute on chronic retention of CO2 on a background of COPD and high flow oxygen administration is the more likely diagnosis
33577
e
Myasthenia gravis
QuestionChoice
This patient is presenting with type 2 respiratory failure secondary to oxygen therapy. Patients with COPD can develop a reduction in hypercapnic drive due to increased carbon dioxide content in the blood. This means they are reliant on hypoxic drive. Therefore, when given high flow oxygen therapy, they lose hypoxic drive, which means they have a lower respiratory effort and retain CO2. This presents clinically as drowsiness. This patient may also have an element of chronic hypercapnoea, as they have a raised bicarbonate. This is a typical ABG of someone in type 2 respiratory failure: a low pH and high CO2 indicating respiratory acidosis and high bicarbonate, suggesting this is an acute on chronic retention of carbon dioxide. Of note, if a patient is presenting with severe hypoxia and a history of COPD, they should still be commenced on high flow oxygen as their hypoxia is immediately threatening to life. However, in this case (patient with COPD who is a known retainer), it is more appropriate to keep them on a Venturi mask and aim for saturations of 88-92%
33573
a
High flow oxygen therapy
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
6715
A 66-year-old man presents to the emergency department with breathlessness, productive cough and fever. He is admitted and commenced on 15L high flow oxygen as his oxygen saturation is 89% and his respiratory rate is 22/min. He has a past medical history of COPD.

Thirty minutes later, he is drowsy and has a respiratory rate of 13/min. His ABG is shown below:

pH: 7.29 [7.35-7.45]

PaO2: 10 [9.5-14]

PaCO2: 7.1 [4.6-6]

HCO3-: 33 [22-30]

What is the most likely cause of this patient's new symptoms?
a