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Respiratory Failure - The Ins and Outs
Reading time: 10 minutes
Introduction
As a student I always found it hard to identify the type of respiratory failure, let alone its management. With time things changed and I will give you my understanding of it in the simplest form possible.
There are two types of respiratory failure (figure 1):
Type 1 or Hypoxemic failure – problem in getting oxygen into the blood (alveolar-capillary interface problems).
Type 2 or Hypoventilatory failure – problem in getting oxygen into the lungs.
I prefer to use the latter terms as they tell you where the problem is. It also helps in understanding their treatment.
Figure 1
We can identify problems in the alveolar-capillary interface by the alveolar-arterial oxygen gradient (A-a gradient). This indirectly measures how much oxygen is getting into the bloodstream by subtracting arterial oxygen tension from alveolar oxygen tension (PAo2 – PaO2). Normal is 2 kpa in the young and 4 kpa in the old (Figure 2).
Figure 2
Hypoxaemic Respiratory Failure
Obviously if there are problems in the alveolar-capillary interface this difference increases because less oxygen is getting into the blood stream (lower PaO2) even though alveolar oxygen tension remains the same. This can happen if there is a problem on the alveolar side e.g. pneumonia or on the capillary side e.g. pulmonary embolism (Figure 3).
Figure 3
In Hypoxaemic Respiratory Failure A-a Gradient is increased!
Hypoventilatory Failure
In hypoventilatory respiratory failure, there is nothing wrong with the alveolar-capillary interface. Remember, the problem is with getting air into the lungs, not into the bloodstream! Both Alveolar and arterial oxygen tensions drop, therefore the A-a gradient is normal (Figure 4).
Figure 4
The treatment of hypoventilatory failure is to ventilate. This is often achieved with non-invasive ventilation where air is driven under pressure into the lungs. This splints the airways open and ensures the alveoli are ‘ventilated’ (Figure 1).
In Hypoventilatory Respiratory Failure, A-a Gradient is Normal!
A-a Gradient
Psst.. how do you calculate the A-a gradient? Well, there is a formula but make your life easier and just use one of the innumerable apps available on all your smart phones.
For those of you who are old school (the rest can look away!):
PAo2 = 20 − 5/4(PCO2) in kpa
PCO2 is of course available on your ABG!
Treatment of Hypoxaemic Respiratory Failure
Hypoxemic respiratory failure is a little more complicated (figure 5). Initially when the person is hypoxemic the physiologic response is to increase the respiratory rate (RR) and tidal volume (TV).
Hypoxemia → ↑(RR x TV) or hyperventilation (Figure 5a)
This blows off CO2 resulting in hypocapnia.
But physiologically the respiratory rate is capped around 30/min and eventually the person starts getting tired. This results in a drop in tidal volume.
The latter in turn causes the CO2 to normalise and then when they are really tired, it results in hypercapnia (Figure 5b and c). The ABG now ‘looks’ like ‘type 2 respiratory failure’ because of the high CO2.
Figure 5
The only way you can tell that this is all hypoxemic respiratory failure is that the A-a gradient is increased through all the stages. Therefore the treatment is NOT non-invasive ventilation!
What is the treatment I hear you ask?
Well there is little oxygen getting into the bloodstream so you increase the FiO2 (fraction of inspired oxygen) which increases the amount of oxygen in the alveoli (alveolar oxygen tension). How can we do this?
Oxygen delivery devices, of course! However, remember that they all have their limitations in giving a high enough FiO2. You can only be sure that a high FiO2 is being given when you use high flow delivery devices like high flow nasal cannula or the most efficient way – endotracheal tube intubation.
You can of course, start with low flow oxygen delivery devices and if they fail, go for high flow delivery devices.
Would you like to guess why COPD patients with an exacerbation often present with hypoventilatory respiratory failure?
Put your answers in the comments section below and I will respond!
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medbookprof
I'm a Consultant Pulmonologist having experienced medicine first-hand across two continents stretching from the South Indian peninsula to the British Isles, the emerald nation of Ireland, to the Mediterranean climes of Italy and to the happiest country on earth, Denmark. Along the way, I have served as an honorary Associate Clinical Teacher and Royal College Associate tutor at University Hospital of Wales, collected a Masters in Medical Education and a passion for teaching. My enthusiasm is matched only by my thirst for simplifying clinical knowledge for medical students and making it accessible on a global scale.
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