In previous tutorials I discussed the problem of ventilation perfusion mismatch, intrapulmonary shunt and physiologic dead space. I explained how different injuries to the lung (the 6 s approach – slimy, soggy, sticky etc.) resulted in poorly aerated airways and atelectasis. Before moving on to a discussion about CPAP/PEEP we need to explore the problem of low lung volumes. Although the lungs can hold up to 6L of air – in reality most of the time there is 2-2.5L in the alveoli. This is the resting lung volume that is found at end expiration and results when the tendency for the chest wall to spring outwards is balanced by the tendency for the lungs to collapse inwards. That resting lung volume is established by negative pleural pressure and it represents the expiratory reserve volume and residual volume – together the functional residual capacity (FRC).

FRC is the lung capacity in which most oxygenation takes place, in which lung compliance is highest, airway resistance lowest and pulmonary vascular resistance optimal. Loss of FRC (“low lung volumes”) – results in hypoxemia, increased work of breathing, autopeep and pulmonary hypertension.

During the tutorial I elaborate on lung volumes – how they are affected by position and age, how airway closure becomes a major issue as we get older – particularly in the supine position, and I introduce the volume pressure curve which is essential for understanding dynamic respiratory system compliance.