Disorders of Respiratory Physiology

00:00 So the subject of this lecture are respiratory physiology disorders. What we're going to talk about in this lecture is type I and type II respiratory failure, the differences between them and how they should be treated. A little bit about respiratory acid-based disorders, and then we are going to move on to talk about a specific cause of type I respiratory failure, the acute respiratory distressed syndrome,ARDS and also type II, causes of type II respiratory failure which affect lung ventilation but actually the lungs are normal and that's mainly obstructive sleep apnea with some discussion of obesity hypoventilation and chest wall and muscle disease. An important point is that ventilatory diseases, which cause chronic respiratory failure, are usually worse at night with worse physiological parameters overnight and first thing in the morning. Right, so type I respiratory failure, this is respiratory failure with hypoxia alone, normal PaCO2, but a PaO2, which is, less than 8 no matter what the inspired oxygen concentration is. Now this is often an acute one-off event but it can also be a complication of chronic lung diseases.

01:15 What's the treatment? Well it depends on the cause, somebody comes in with pneumonia and hypoxia as a consequence of type I respiratory failure, you clearly treat that pneumonia, But the discussion that we are going to have today is really about oxygen, and how we use oxygen in patients with type I respiratory failure. And, essentially, the problem is how to correct the hypoxia, and we can use as much oxygen as much as it is necessary in these circumstances.

01:47 In general, we follow a progression of oxygen delivery devices based on the patients needs.

01:52 This progression starts with a nasal cannula, then progresses to a simple facemask, then a non-rebreather mask, then a high-flow nasal cannula, then non-invasive positive pressure such as CPAP or BiPAP, and finally with intubation.

02:07 And what we're trying to do is we're trying to aim for saturations above 90%, correct the hypoxia completely but not only that we would like to make the patient able to breathe at a comfortable respiratory rate. Because if they're maintaining a saturation of 95%, but they're breathing at 40 per minute, that is not a good situation, that means they're struggling very hard, they will tire, and it's quite likely that they will fail to maintain their oxygenation if they have to keep persisting with a respiratory rate of 40 per minute.

02:36 So what we do if the patient is remaining hypoxic or has a very high respiratory rate despite breathing oxygen from a mask or a non-rebreathing bag? Well there is a non-invasive form of ventilatory support the called the continuous positive airway pressure, CPAC.

02:53 Now, this is a facemask that provides a small amount of additional pressure on the inhaled air, oxygen mix, 5 to 15 cm of water, and it maintains that through the whole respiration, both inspiration and expiration, and what this does is actually recruits more alveolar units that the patient can use during their respiration. It splints open and recruits more alveolar units during inspiration and that allows the inhaled oxygen concentration to maintain a better arterial oxygen level. And this is a good treatment for isolated type I respiratory failure. Somebody presenting with say for example, community acquired pneumonia or pulmonary oedema where they have type I respiratory failure, but no other major organ damage. If this is not adequate, if this starts to work, or starts to fail to work, or the patient becomes tired despite CPAP therapy, then the next step will be intubation and artificial ventilation in the intensive care department.

04:03 So what are the common causes of type I respiratory failure? Well acutely, pneumonia, pulmonary aspiration, pulmonary oedema a large pulmonary embolus and ARDS which I'll discuss in the later slides.

04:16 Chronically, patients can have type I respiratory failure due to interstitial lung disease, if it's severe enough, if they have had chronic pulmonary emboli leading to significant pulmonary artery damage, then they'll have type I respiratory failure, as will patients with severe pulmonary hypertension.

04:33 And the phenotype of COPD that tends to get type I respiratory failure are those of the emphysematous pink puffer type phenotype.