Acute respiratory distress syndrome mechanical ventilation therapy
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Most patients with ARDS will require endotracheal intubation and mechanical ventilation at some point during the course of their illness and recovery. A mechanical ventilation strategy using lower tidal volumes of 6 mL/kg predicted body weight and higher levels of positive end-expiratory pressure (PEEP) has been shown to be most effective at improving oxygenation and minimizing volutrauma (injury to stiff lungs resulting from overdistention).
- use tidal volumes <6.5 mL/kg,
- use a PEEP of 5–24 cm H20 within a plateau pressure <30 cm H20,
- decrease FiO2 as permitted to achieve saturation of 88%–95%,
- and elevate the head of the bed. Hypercapnia with a pH >7.25 is permissible.
- Use a higher flow rate (up to 100 L/min) for obstructive airway disease when necessary to achieve a satisfactory I:E ratio
- Lower tidal volume ventilation (6 mL/kg predicted body weight) is associated with reduced mortality and a greater number of ventilator-free days
- Lower tidal volume ventilation should be continued even if the arterial partial pressure of carbon dioxide (PaCO2) rises (this is called permissive hypercapnia)
- Permissive hypercapnia usually results in a drop in blood pH, however, treatment of acidemia (e.g., intravenous administration of sodium bicarbonate or tromethamine) is not indicated if the pH remains at or above 7.15 to 7.20
- Predicted body weight (PBW) in kilograms (kg) may be calculated from height in inches (in) as follows:
- PBW (men) = 50 + 2.3 (height in inches – 60)
- PBW (women) = 45.5 + 2.3 (height in inches – 60)
- Higher positive end-expiratory pressure (PEEP) combined with lower tidal volume ventilation is associated with decreased mortality in patients with moderate or severe ARDS (PaO2/FIO2 ≤ 200)
- Prone positioning for at least 16 consecutive hours each day is associated with improved 28-day and 90-day survival in patients with ARDS and a PaO2/FIO2 ratio < 150 on an FIO2 ≥ 60% and PEEP ≥ 5 mmHg
- Cisatracurium, when started within the first 48 hours of ARDS diagnosis and continued for 48 hours, has been associated with improved 90-day survival, a greater number of ventilator-free days, and a decreased incidence of volutrauma
ARDS Network Mechanical Ventilation Protocol
In 1994 the National Institutes of Health (NIH) and National Heart, Lung, and Blood Institute (NHLBI) founded the ARDS Clinical Trial Network (often abbreviated as ARDSnet) – a consortium of over 40 hospitals that conduct clinical research trials aimed at improving care for patients with ARDS. In order to simplify the mechanical ventilation of patients with ARDS, the NIH-NHLBI ARDS Network has compiled a Mechanical Ventilation Protocol Summary that outlines the mechanical ventilation strategies associated with better outcomes in an easy-to-use format for ICU health care providers.
Non-Invasive Positive Pressure Ventilation
Many patients who develop ARDS will receive a trial of non-invasive positive pressure ventilation (NIPPV) before intubation and mechanical ventilation become necessary to maintain adequate oxygenation, or before the degree of clinical deterioration precludes the use of NIPPV and necessitates endotracheal intubation for airway protection. Several studies have examined the utility of NIPPV in the management of ARDS:
- NIPPV observational data from cohort studies: Early application of NIPPV appears to reduce the rate of intubation and mechanical ventilation in patients with mild-to-moderate ARDS (PaO2/FIO2 ratio 150 to 200)
- NIPPV versus high-flow nasal cannula (HFNC) or supplemental oxygen via face mask: 310 patients with ARDS and a PaO2/FIO2 ratio ≤ 300 were randomized to either NIPPV, high-flow nasal cannula, or supplemental oxygen via face mask
- NIPPV via face mask versus NIPPV via helmet: 83 patients with ARDS were randomized to either NIPPV via face mark or NIPPV via helmet
- At 28 days, there was a significantly lower rate of intubation and significantly more ventilator-free days in the helmet group
- At 90 days, there were significantly fewer mortalities in the helmet group
- Study was terminated early due to the significantly higher mortality rate seen in the face mask group
Alternative Mechanical Ventilation Strategies
Several specialized modes of mechanical ventilation have been tested in ARDS, however, none has been proven to carry a morbidity or mortality benefit and should only be considered if oxygenation does not improve with a judicious trial of the first-line mechanical ventilation strategies as outlined by the ARDS Network.
- High-frequency oscillatory ventilation (HFOV) may improve oxygenation in patients with moderate to severe ARDS and severe refractory hypoxemia, however, initiation of HFOV early in the course of ARDS (i.e., prior to low tidal volume/high PEEP mechanical ventilation) has been associated with increased mortality compared to lower tidal volume/high PEEP ventilation
- Airway pressure release ventilation (APRV) appears to be safe in ARDS, and may be associated with reduced paralytic and sedative use as well as an increase in the number of ventilator-free days
A recruitment maneuver is the application of very high (up to 40 cm H2O) positive airway pressure to open collapsed alveoli, thereby reducing shunting, decreasing V/Q mismatching, and improving gas exchange. The decision to apply recruitment maneuvers must take into account various factors including the extent of lung injury (due to the risk of causing volutrauma through overdistention of stiff and inflamed lungs) and patient hemodynamics (due to the risk of further worsening hypotension by impeding venous return to the right heart). Recruitment maneuvers have not been standardized and there are insufficient data to support or discourage their use in ARDS.
Extracorporeal Membrane Oxygenation (ECMO)
There is growing evidence to support the use of extracorporeal membrane oxygenation (ECMO) for severe ARDS that fails to improve despite judicious application of the ARDS Network low tidal volume/high PEEP ventilation strategy. ECMO facilitates gas exchange in circumstances where adequate oxygenation and ventilation cannot be achieved through the lungs themselves. There are two main forms of ECMO, both of which have been used successfully in the treatment of severe ARDS:
- Veno-venous (VV)-ECMO: Venous blood is removed through an outflow cannula placed in a large vein (usually the right femoral vein or inferior vena cava) and passed through an oxygenator where gas exchange occurs (CO2 is removed and O2 is introduced) before being returned to the body through an inflow cannula placed in another large vein (usually the right internal jugular vein or superior vena cava)
- Veno-arterial (VA)-ECMO: Venous blood is removed through an outflow cannula placed in a large vein (usually the right femoral vein or inferior vena cava) and passed through an oxygenator where gas exchange occurs (CO2 is removed and O2 is introduced) before being returned to the body through an inflow cannula placed in a large artery (usually the right femoral artery or right carotid artery)
The use of ECMO in the treatment of ARDS is an ongoing area of research, and referral to a medical center with ample experience in the use of ECMO for ARDS should be considered for patients with ARDS who are failing traditional management strategies and may be candidates for ECMO. The use of ECMO requires systemic anticoagulation (usually with heparin) and is associated with the risk of major hemorrhage as well as thrombosis. Additionally, the use of VA-ECMO may result in ischemic injury to the limb distal to the site of the inflow cannula (although rates of limb ischemia have been mitigated by the addition of a reperfusion cannula that takes blood from the inflow cannula and delivers it distally to the otherwise-affected limb).
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