Cardio-Respiratory monitoring in ARDS

Careful cardiorespiratory monitoring in patients affected by ARDS is crucial to improve prognosis and to tailor treatment via mechanical ventilatory support.Acute respiratory distress syndrome (ARDS) is a severe condition. Resulting in life threatening impairment of lung function and oxygen levels, ARDS can come on suddenly and progress rapidly. ARDS is a result of fluid build-up in the lung.

1.ARDS

Patients with acute respiratory distress syndrome (ARDS) exhibit inflammatory pulmonary edema resulting from changes in endothelial and epithelial permeability, leading to organ damage. The severity of ARDS determines the application of different types of mechanical support. Superimposed hemodynamic impairment may complicate patient management, worsening outcomes. Therefore, a comprehensive evaluation of ARDS patients involves careful respiratory and hemodynamic monitoring, encompassing both invasive and noninvasive technologies, along with clinical and laboratory data. This approach is crucial for tailoring therapeutic strategies to individual patients and minimizing lung injury.

This manuscript reviews strategies for respiratory and hemodynamic monitoring in ARDS patients, highlighting the most recent data and clinical utility in daily management.

2.Pulse oximetry

Pulse-oximetry exploits the principle of spectrophotometry to quantify the amount of oxygenated hemoglobin in blood, allowing continuous noninvasive monitoring of arterial saturation [1]. The difference between arterial oxygen saturation (SaO₂) measured via blood gas analysis and oxygen saturation measured via pulse-oximetry (SpO₂) is normally less than 3%. However, the accuracy of SpO₂ may be lower among patients with darker skin pigmentation, thus overestimating arterial oxygen saturation. This phenomenon, as recently demonstrated by Henry et al., possibly increases the incidence of occult hypoxemia, i.e., patients in which SaO₂ is lower than 88% with an SpO₂ higher than 92%. The clinical consequences of occult hypoxemia have also been investigated during the recent pandemic. In COVID-19 patients, occult hypoxemia is more frequent in Asian, Black and non-Black Hispanic patients than in White patients, with lower treatment eligibility for these three ethnicities .

The ratio of pulse-oximetric oxygen saturation to the fraction of inspired oxygen (SpO₂/FiO₂) is an acceptable surrogate of the ratio of the partial pressure of arterial oxygen to FiO₂ (PaO₂/FiO₂). Its use has been described both in invasively and noninvasively ventilated patients. The SpO₂/FiO₂ ratio is a good outcome predictor both in patients with coronavirus disease (COVID-19) and non-COVID-19 ARDS patients. In patients with COVID-19-associated pneumonia requiring oxygen therapy, the SpO₂/FiO₂ ratio at admission showed an area under the curve (AUC) of 85% for the prediction of ARDS occurrence. Kim et al. showed that the SpO₂/FiO₂ ratio can predict high-flow nasal cannula (HFNC) failure [13]. Moreover, SpO₂/FiO₂ shows a good correlation with PaO₂/FiO₂ in invasively ventilated COVID-19 ARDS patients, and when computed on day 2 and day 3, it is associated with outcome. These data confirm the reliability of pulse oximetry for evaluating gas exchange in ARDS patients and for following this trend, as pulse oximetry is continuously measurable. It is easy to measure and is thus especially valid in contexts in which a blood gas analyzer is not promptly available.

The optimal SpO₂ concentration for ARDS treatment is still a matter of debate, ranging from 88% to 96–100% to balance the risk of hyperoxia and hypoxia. In a recent large randomized controlled trial (RCT), Semler et al. showed that, in mechanically ventilated patients, the use of a lower (90%, range from 88 to 92%), intermediate (94%, 92–96%) or higher (98%, 96–100%) SpO₂ target does not affect either ventilator-free days or hospital outcomes.

3.ETCO₂ to arterial PCO₂

A further parameter to estimate gas exchange efficiency is the computation of the end-tidal-to-arterial PCO₂ ratio (P_ETCO₂/PaCO₂), which measures the influence of venous admixture and alveolar dead space on lung performance. Ideally, this ratio should be equal to one. Bonifazi et al. showed that the P_ETCO₂/PaCO₂ ratio significantly decreases from mild to severe ARDS. Additionally, P_ETCO₂/PaCO₂ is strongly correlated with the amount of nonaerated tissue measured via computed tomography (CT) and respiratory compliance. A subsequent study revealed a relationship between the P_ETCO₂/PaCO₂ ratio, alveolar ventilation and hospital mortality. For every 0.01 increase in the P_ETCO₂/PaCO₂ ratio, the risk for mortality decreases by 1%.

Currently, weaning from venous extracorporeal membrane oxygenation (VV-ECMO) lacks well-defined criteria and is often based on acceptable blood gas analysis and the absence of excessive inspiratory effort. In a recent multicenter study, Lazzari et al. showed that the P_ETCO₂/PaCO₂ ratio, with a cutoff of 0.83, is able to predict weaning.

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