Acute respiratory distress syndrome
Timing
Within 1 week of a known clinical insult or new/worsening respiratory symptoms
Chest imaginga
Bilateral opacities—not fully explained by effusions, lobar/lung collapse, or nodules
Origin of edema
Respiratory failure not fully explained by cardiac failure or fluid overload; need objective assessment (e.g., echocardiography) to exclude hydrostatic edema if no risk factor present
Mild
Moderate
Severe
Oxygenationb
200< PaO2/FiO2 ≤300 with PEEP or CPAP ≥5 cmH2Oc
100< PaO2/FiO2 ≤200 with PEEP ≥5 cmH2O
PaO2/FiO2 ≤100 with PEEP ≥5 cmH2O
In patients considered for a NPPV trial:
Ensure the appropriate environment for NPPV delivery (intensive care unit or high dependency unit) and appropriate ventilator availability (no home ventilators).
Use an oronasal mask as interface to minimize leaks.
NIV is preferred over CPAP use, especially in patients with marked distress and hypercapnia. ARDS is generally accompanied by an increased respiratory drive and places great stress on the respiratory muscles. Attenuation of inspiratory effort with PS is advisable. Another important advantage of NIV over CPAP is the ability to closely monitor Vt, which must be maintained below 6 ml/kg IBW.
Start with a low PEEP level: 5 cmH2O is probably the best choice, as it allows for subsequent reassessment of the degree of hypoxemia and appears to improve patient stratification [11].
Apply a PS of 10–15 cmH2O, closely monitoring patient-ventilator interaction, RR, and Vt.
Titrate FiO2 to a SpO2 >94 %.
After 30 min of NIV, reassess arterial blood gases.
If PaO2/FiO2 ratio is <200, the patient has moderate to severe ARDS. Invasive mechanical ventilation is indicated [11].
If PaO2/FiO2 ratio is >200, the patient has mild ARDS and may be treated with NIV in this phase.
If hypercapnia with respiratory acidosis has developed or worsened (i.e., pH <7.3 with PaCO2 >50 mmHg), consider invasive mechanical ventilation.
Once it has been established that a patient has mild ARDS on NIV with 5 cmH2O of PEEP and does not have significant or worsening respiratory acidosis:
1.
Titrate PS level to obtain a Vt ≤6 ml/kg IBW and a RR <25.
2.
Titrate FiO2 to maintain a SpO2 ≥94 %.
3.
Increase PEEP up to 10 cmH2O if necessary.
During NIV, serial monitoring of clinical and laboratory values is mandatory:
Assess patient comfort and leaks.
Assess patient-ventilator interaction and synchrony (ineffective efforts, double triggering, auto-triggering).
Measure respiratory rate and adjust ventilator settings, aiming for a RR ≤25 bpm.
Monitor Vt, aiming for ≤6 ml/kg IBW.
Repeat arterial blood gases for oxygenation and CO2 monitoring.
Maintain a low threshold for ETI and invasive mechanical ventilation in ARDS patients. Delaying a necessary intubation may harm patients and increase mortality [10].
28.3 Pneumonia
The scant literature on randomized and observational studies in the immunocompetent population does not allow for a strong recommendation for NIV for the treatment of acute respiratory failure in the course of pneumonia. The explanation for the difference of success in comparison with ACPE is twofold: (1) the effects of medical treatment on pneumonia takes much longer; and (2) the favorable effect of PEEP on oxygenation depends on the pattern of lung involvement (greater recruitment in interstitial than in consolidation), and its cardiovascular effects may be remarkably deleterious.
The results of randomized and observational trials in the immunocompromised population are more encouraging, because ETI – the alternative to NIV for severe acute respiratory failure – is frequently complicated by severe infections and a high mortality. Hence, NIV use in the immunocompromised population may decrease intubation rate and improve outcome.
According to the literature, the following suggestions can be made on the probability of NIV success in pneumonia in the immunocompetent population. The outcome is significantly better in patients with preexisting chronic cardiocirculatory or obstructive lung disease than in those with de novo acute respiratory failure [3]. Finally, early treatment of severe hypoxemic pneumonia with helmet CPAP may effectively reduce the risk of meeting ETI criteria compared with oxygen therapy [4].
Summary
1.
NIV is effective in the immunocompromised population.
2.
NIV is more effective in acute-on-chronic versus de novo respiratory failure.
3.
In immunocompetent patients without a do not intubate (DNI) order, a cautious early NIV trial may be attempted (the interface is a key factor).
4.
In immunocompetent patients with a DNI order, NIV is a possible ceiling treatment (the interface is a key factor).
28.3.1 NPPV for Pneumonia in Practice
An initial PEEP level of 5–8 cmH2O is probably the best choice.
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