D9 Respiratory monitoring

Topic

Respiratory monitoring of the ventilated patient combines three main sources of information—oxygen saturation, exhaled carbon dioxide concentration, and respiratory system mechanics—with the aim of ensuring safe and effective mechanical ventilation.

Oxygen saturation is measured continuously and non-invasively via pulse oximetry, which relies on the differential absorption of red light (around 660 nm) and infrared light (around 940 nm) by oxygenated and deoxygenated hemoglobin. An algorithm based on empirical calibration curves converts pulse-related absorption variations into a saturation value. Measurements can be subject to error: carboxyhemoglobin can lead to overestimation, methemoglobin tends to drive the reading toward approximately 85%, and factors such as poor peripheral perfusion, skin pigmentation, or nail polish can also affect the result.

Capnography continuously measures end-tidal carbon dioxide (EtCO₂), which approximates arterial CO₂ pressure. The difference between the two provides information regarding dead space—air that reaches ventilated but under-perfused regions and thus does not effectively participate in gas exchange. This wasted fraction of ventilation can be estimated using the relationship described by the Bohr equation. A very high dead-space fraction indicates severe impairment of gas exchange, as seen in conditions such as pulmonary embolism, emphysema, or severe acute lung injury. During cardiac arrest, EtCO₂ levels drop sharply due to the lack of pulmonary blood flow; during resuscitation, monitoring changes in EtCO₂ allows for the assessment of chest compression efficacy and the detection of the return of spontaneous circulation.

Respiratory mechanics allow for the differentiation between resistance and compliance issues. Airway resistance is estimated based on the difference between the peak pressure reached during inspiratory flow and the pressure measured when flow ceases. An increase suggests bronchospasm or obstruction, such as in asthma, COPD, or secretion accumulation. Respiratory system compliance relates the delivered volume to the difference between the no-flow pressure and the pressure at the end of expiration. Reduced compliance indicates increased lung stiffness—seen in conditions like edema, fibrosis, or acute lung injury—or a stiff chest wall, such as in obesity or ascites. The difference between plateau pressure and end-expiratory pressure—known as driving pressure—is generally kept below approximately 15 cmH₂O.

Oxygenation is typically assessed using the PaO₂/FiO₂ ratio, which relates arterial oxygen pressure to the fraction of inspired oxygen. Values ​​above 300 indicate relatively preserved oxygenation; values ​​between 200 and 300, 100 and 200, and below 100 reflect progressively greater degrees of oxygenation impairment, although clinical interpretation also depends on the context and ventilatory settings.

Another parameter, the oxygenation index, also incorporates mean airway pressure, thereby relating the achieved oxygenation to the intensity of ventilatory support required to attain it. Persistently high values ​​despite optimized ventilation may contribute—alongside other clinical criteria—to the consideration of advanced therapies such as extracorporeal membrane oxygenation (ECMO).

Taken together, pulse oximetry, capnography, ventilatory mechanics, and oxygenation indices provide complementary information. Pulse oximetry monitors oxygenation and requires the recognition of potential erroneous readings; Capnography provides information on ventilation, perfusion, and dead space; resistance and compliance help distinguish airway obstruction from a stiff lung or chest wall; and oxygenation indices allow for monitoring the severity of respiratory failure and assessing the need for increased support. Integrating these parameters enables the adjustment of ventilation to each patient's pathophysiology and the early detection of instances where conventional support proves insufficient.

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