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CPAP (Continuous Positive Airway Pressure)
Reading time: 10 minutes
CPAP = Continuous positive airway pressure = used in hypoxemic respiratory failure
BPAP = Bi-level positive airway pressure = used in hypoventilatory respiratory failure
In the UK non-invasive ventilation (NIV) is synonymous with BPAP. BiPAP is just the registered trade name for BPAP.
Strictly speaking, both CPAP and BPAP are modes of non-invasive ventilation as opposed to invasive ventilation with endotracheal intubation.
Now that we’ve got the terminology out of the way, we can start discussing the more exciting stuff!
Introduction
When someone has hypoxemic respiratory failure, the problem is in the alveolar-capillary interface as discussed in this blog. Pneumonia and ARDS are two good examples of this problem. The recent pandemic due to COVID-19 tends to cause both. When we zoom out a little, there are three different processes taking place in the lungs in ARDS – Acute Respiratory Distress Syndrome (Figure 1).
- Some alveoli are filled with gunk, inflammatory debris being the technical name 😉 – figure 1(a)
- Some alveoli are depressed and have given up – collapsed – figure 1(b)
- Some alveoli are overworking – hyper-inflated and trying to compensate – figure 1(c)
Figure 1
Now imagine blood flowing in the capillaries around these alveoli. The blood around the gunk filled and collapsed alveoli is wasting its time since there is no oxygen here. So what can we do to get more alveoli to participate in gas-exchange? Can’t remove the gunk since it takes time (e.g. with antibiotics). The overworked alveoli are doing more than they can. So it’s now up to the depressed alveoli to ‘get over it’!
How do we do this?
I’m glad you asked.
We can either do it non-invasively using CPAP or invasively (endotracheal intubation and ventilation) by applying something called PEEP – positive end-expiratory pressure.
In order to understand this, we will have to see what happens in normal breathing.
Normal Breathing
At resting state, the intra-alveolar pressure is zero. During inspiration (active process), this pressure drops to say, -1 cm of H2O. This draws in air from the outside where the atmospheric pressure is zero. When we exhale (passive recoil), the intra-alveolar pressure rises to +1 cm of H2O and the airflow stops when the pressure equalises with the atmospheric pressure of zero (Figure 2).
Figure 2
CPAP
Say, we attach a tight face mask onto this person and blow air under 5cm of H2O pressure continuously throughout the respiratory cycle. For ease of understanding, this is the new atmospheric pressure. In effect we have increased the baseline intra-alveolar pressure to 5 cm of H2O instead of zero. The patient now initiates a breath.
During inspiration the intra-alveolar pressure drops to 3cm of H2O. Air rushes in down the gradient from 5 cm of H2O to 3 cm of H2O. Although the machine is driving air at a pressure of 5cm of H2O assisting the person to get some air in, they are unlikely to feel it. Collapsed alveoli as in ARDS, now open up at this higher pressure and get ‘recruited’.
When they exhale, the intra-alveolar pressure will have to rise above 5 cm of H2O to expel the air. So it rises to 7 cm of H2O and air gets expelled. Since the alveoli are now cycling around a higher positive pressure (5 cm of H2O in this case) the ‘recruited’ alveoli remain open thereby improving oxygenation.
We know from simple physiology that smaller alveoli are more likely to collapse because of surface tension (surfactant). Now that the alveolar size is bigger they are less likely to collapse. The energy spent in each breath to open collapsed alveoli will now be saved which improves the ‘work of breathing’ (Figure 3).
Figure 3
PEEP (Positive End-Expiratory Pressure)
Functionally we can achieve the same during invasive ventilation by adding something called PEEP. Say, we attach a valve on the expiratory port of the trachea that is set at 5 cm of H2O. The intra-alveolar pressure now equates to this value at the end of expiration keeping the ‘recruited’ alveoli open. They now start participating in gas exchange (see video below).
Video of Effects of PEEP on Lungs. Credit: Gerard Carroll
Problems with CPAP/ PEEP
All of this is good news. Now for the bad news:
If we set the CPAP at 20 cm of H2O, the intra-alveolar pressure will have to rise above this in order to exhale! Now exhalation becomes an energy burning process. Too much CPAP/ PEEP can also drop cardiac output i.e. blood pressure. This is because the constant positive intra-thoracic pressure squeezes the great veins and the thin-walled right heart thereby reducing the venous return into the right heart. Less blood flows into the lungs → left heart → aorta → ↓BP. So there is something called too much CPAP/ PEEP, beware of this.
Would you like to take a guess as to how CPAP helps in cardiogenic oedema? Put your answers below in the comments 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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