Mechanical Ventilation
Acute respiratory failure (ARF) is defined as the inability of the respiratory system to meet the oxygenation, ventilatory, or metabolic requirements of the patient [1].
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Mechanical Ventilation
1.1 Acute Respiratory Failure Acute respiratory failure (ARF) is defined as the inability of the respiratory system to meet the oxygenation, ventilatory, or metabolic requirements of the patient [1]. Most authors divide respiratory failure based on the two gas exchange functions, oxygenation and elimination of carbon dioxide. Either, “only” oxygen replenishment may be compromised or a joint disruption occurs [2, 3]: I. Hypoxaemic respiratory failure II. Hypercapnic respiratory failure Hypoxaemic respiratory failure refers to the failure of the lungs to oxygenate mixed venous blood sufficiently, PaO2 50 mmHg in the presence of hypoxaemia [5–7]. Hypercapnic respiratory failure is called ventilatory failure as well [6], highlighting that the ventilatory part of the respiratory system – the “pump function” of the respiratory apparatus – has failed, mainly due to ventilatory muscle fatigue, rather than to the gas exchange element [5]. As such, hypercapnia is a hallmark of ventilatory failure [5, 8], and an acutely decompensated ventilatory failure is characterized by a respiratory acidosis (pH 7.65 or 35/min 10 %
No
Apply PEEP 5 cm H2O or increase by 1 cm H2O not to exceed 8 cm H2O
Yes
Set VT 6–8 ml/kg
Pressuretargeted mode
No
Increase inspiratory flow
Yes
Increase pressurization rate
Time cycled
No
Long time constant (COPD)
Yes Yes
Decrease inspiratory time
Increase flow cycleoff threshold (% of peak flow)
No
Decrease flow cycleoff threshold (% of peak flow)
1 Mechanical Ventilation
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[110, 114]. Accordingly, the airway pressure needed to inflate the lungs either during spontaneous breathing or by ventilator is affected by the compliance of the respiratory system (CRS), the airway resistance (R), the volume (V) inhaled (tidal volume, VT) and the airflow (Q) [110], depicted by the following relationship:
PAW = V / CRES + R / Q
[110]
In contrast, PTRANS measured at end inspiration is free from such influences, allowing estimation of the actual true distending pressure in the passive lungs [113, 114]. The transpulmonary pressure is negative during spontaneous inspiration, zero at the functional residual capacity (FRC) of the lungs where the opposing forces of lungs and chest wall are equal and opposite to each other [110], and at which the PPL is −5 cm H2O as most authors mention [108], and positive during expiration [110]. Of those features mentioned above affecting the airway pressure, the compliance is of special interest and relevance. The compliance of the respiratory system is made up by the compliance of the lungs (CL) and the compliance of the chest wall (CW), related by 1/CRS = 1/CL + 1/CCW (or alternatively CRS = (CCW × CL)/(CCW + CL)) as both components are arranged in series [108–110]. The elastic properties of the respiratory system, CRs, correlate well with the amount of aerated lung tissue in patients with acute lung injury and ARDS [115]. Compliance is the inverse of elastance (ETOT = EL + ECW [112]-formula (B)), with ETOT indicating the elastance of t
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