There are two tutorials on pulse oximetry. The first looks at the SpO2 and how it is measured. The second looks at the pleth waveform and problems that we commonly encounter with pulse oximetry in general. I guarantee you’ll learn something.
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.
If you treat patient with hypoxic respiratory failure you really need to understand what is going on in their lungs. These two tutorials look at diseases of the lung parenchyma and how blood flow and gas flow interact. The first tutorial focuses on alveolar oxygen content and how it is impacted by disease. I explain the concept of airway closure (which will will revisit in detail several times during this series), stale alveolar gas, the various causes of atelectasis and the six S approach to figuring out what is going on in the airways (Slimy, Soggy, Sticky, Stiff, Squished, Shunty).
The next part of the course is all about hypoxic respiratory failure. To treat hypoxemia you must understand it. The purpose of this sequence of tutorials is to lead up to discussions on CPAP and PEEP and provide a platform for understanding Pressure Controlled Modes of Ventilation. The first tutorial looks at oxyhemoglobin saturation, why the oxyhemoglobin dissociation curve is essential knowledge for the practicing clinician, how pulse oximeters work and how to quantify hypoxemia (A-aO2 gradient and PaO2/FiO2 ratio).
ORtoICU 