critical care medicine
Acute Hypoxemic Respiratory Failure (AHRF, ARDS)
Treatment of AHRF
Noninvasive oxygenation support Mechanical ventilation if oxygen saturation is<90% on high-flow oxygen AHRF is usually initially treated with 70 to 100% oxygen delivered noninvasively (eg, with a non-rebreather face mask.) However, the use of noninvasive oxygen support, such as high-flow nasal cannula (HFNC) and noninvasive positive pressure ventilation (NIPPV), for the initial management of acute hypoxemic respiratory failure has increased during the COVID-19 pandemic due to the potential ventilator sparing effects. Noninvasive oxygen support may avoid Tracheal Intubation and its complications; however, spontaneous breathing with excessive effort may induce lung damage known as patient self-inflicted lung injury. One clinical trial comparing the efficacy of HFNC, face mask NIPPV, and standard oxygen for the prevention of endotracheal intubation suggested that HFNC may prevent endotracheal intubation in patients with a PaO2/FiO2 ratio Another small clinical trial comparing oxygen delivery by helmet NIPPV with face mask found lower rates of endotracheal intubation and mortality when the helmet was used ( Treatment references ). There are limited data comparing the use of helmet NIPPV to HFNC in patients with COVID-19–related acute hypoxemic respiratory failure, suggesting that helmet NIPPV may reduce endotracheal intubation rates but does not improve days free of respiratory support ( If noninvasive oxygenation support does not result in oxygen saturation>90%, mechanical ventilation probably should be considered. Specific management varies by underlying condition. Mechanical ventilation (see also Overview of Mechanical Ventilation ) benefits the failing left ventricle in several ways. Positive inspiratory pressure reduces left and right ventricular preload and left ventricular afterload and reduces the work of breathing. Reducing the work of breathing may allow redistribution of a limited cardiac output away from overworked respiratory muscles. Expiratory pressure ( Noninvasive positive pressure ventilation (NIPPV) , whether continuous positive pressure ventilation or bilevel ventilation, is useful in averting endotracheal intubation in many patients because drug therapy often leads to rapid improvement. Typical settings are inspiratory positive airway pressure (IPAP) of 10 to 15 cm H2O and EPAP of 5 to 8 cm H2O. Conventional mechanical ventilation can use several Means and Modes of Mechanical Ventilation . Most often, assist-control (A/C) is used in the acute setting, when full ventilatory support is desired. Initial settings are tidal volume of 6 to 8 mL/kg ideal body weight, respiratory rate of 25/minute, FIO2 of 1.0, and PEEP of 5 to 8 cm H2O. PEEP may then be titrated upward in 2.5-cm H2O increments while the FIO2 is decreased to nontoxic levels. Pressure support ventilation can also be used (with similar levels of PEEP). The initial inspiratory airway pressure delivered should be sufficient to fully rest the respiratory muscles as judged by subjective patient assessment, respiratory rate, and accessory muscle use. Typically, a pressure support level of 10 to 20 cm H2O over PEEP is required. Nearly all patients with ARDS require mechanical ventilation, which, in addition to improving oxygenation, reduces oxygen demand by resting respiratory muscles. Targets include Plateau alveolar pressures30 cm H2O (factors that potentially decrease chest wall and abdominal compliance considered) Tidal volume 6 mL/kg ideal body weight to minimize further lung injury FIO2 as low as is allowed to maintain adequate oxygen saturation to minimize possible oxygen toxicity Ventilator settings should be high enough to maintain open alveoli and minimize FIO2 until a plateau pressure of 28 to 30 cm H2O is reached. Patients with moderate to severe ARDS are the most likely to have mortality reduced by use of higher PEEP. Noninvasive positive pressure ventilation (NIPPV) is occasionally useful with ARDS. However, compared with treatment of cardiogenic pulmonary edema, higher levels of support for a longer duration are often required, and EPAP of 8 to 12 cm H2O is often necessary to maintain adequate oxygenation. Achieving this expiratory pressure requires inspiratory pressures Conventional mechanical ventilation in ARDS previously focused on normalizing arterial blood gas values. It is clear that ventilating with lower tidal volumes reduces mortality. Accordingly, in most patients, tidal volume should be set at 6 mL/kg ideal body weight (see sidebar Initial Ventilator Management in ARDS ). This setting necessitates an increase in respiratory rate, even up to 35/minute, to produce sufficient alveolar ventilation to allow for adequate carbon dioxide removal. On occasion, however, Because hypercapnia or low tidal volume alone may cause dyspnea and cause the patient to breathe in a fashion that is not coordinated with the ventilator, analgesics (fentanylormorphine) and sedatives (eg,propofolinitiated at 5 mcg/kg/minute and increasing to effect up to 50 mcg/kg/minute; because of the risk of hypertriglyceridemia, triglyceride levels should be checked every 48 hours) may be needed. Sedation is preferred to neuromuscular blockade because blockade still requires sedation and may cause residual weakness. Ventilator settings improves oxygenation in ARDS by increasing the volume of aerated lung through alveolar recruitment, permitting the use of a lower FIO2. The optimal level of PEEP and the way to identify it have been debated. Routine use of recruitment maneuvers (eg, titration of PEEP to maximal pressure of 35 to 40 cm H2O and held for 1 minute) followed by decremental PEEP titration was found to be associated with an increased 28-day mortality ( The best indicator of alveolar overdistention is measurement of a plateau pressure through an Respiratory Mechanics ; it should be checked every 4 hours and after each change in PEEP or tidal volume. The target plateau pressure is Some investigators believe pressure control ventilation protects the lungs better, but supportive data are lacking, and it is the peak pressure rather than the plateau pressure that is being controlled. With pressure control ventilation, because the tidal volume will vary as the patient's lung compliance evolves, it is necessary to continually monitor the tidal volume and adjust the inspiratory pressure to ensure that the patient is not receiving too high or too low a tidal volume. Generally, the following approach is recommended for ventilator management in ARDS: Assist-control mode is used initially with a tidal volume 6 mL/kg ideal body weight, respiratory rate 25/minute, flow rate 60 L/minute, FIO2 1.0, and PEEP 15 cm H2O. Once oxygen saturation is>90%, FIO2 is decreased. Then, PEEP is decreased in 2.5-cm H2O increments as tolerated to find the least PEEP associated with an arterial oxygen saturation of 90% on an FIO2 of≤0.6. The respiratory rate is increased up to 35/minute to achieve a pH of>7.15, or until the expiratory flow tracing shows end-expiratory flow. Ideal body weight (IBW) rather than actual body weight is used to determine the appropriate tidal volume for patients with lung disease receiving mechanical ventilation: Patient positioning improves oxygenation in some patients by allowing recruitment of nonventilating lung regions. One study suggests this positioning substantially improves survival ( in patients with ARDS balances the requirement for an adequate circulating volume to preserve end-organ perfusion with the goal of lowering preload and thereby limiting transudation of fluid in the lungs. A large multicenter trial has shown that a conservative approach to fluid management, in which less fluid is given, shortens the duration of mechanical ventilation and length of stay in the intensive care unit when compared with a more liberal strategy. However, there was no difference in survival between the 2 approaches, and use of a pulmonary artery catheter also did not improve outcome ( Treatment references ). Patients not in administered early in moderate to severe ARDS suggested improvements in ventilator free days and mortality, but the trial was stopped early due to slow enrollment, which may magnify the treatment effects ( Treatment references ). Thus the role of corticosteroids in ARDS remains uncertain and more data are needed. 1.Frat JP, Thille AW, Mercat A, et al: High-flow oxygen through nasal cannula in acute hypoxemic respiratory failure.N Engl J Med372:2185–2196, 2015. doi: 10.1056/NEJMoa1503326 2.Patel BK, Wolfe KS, Pohlman AS, et al: Effect of noninvasive ventilation delivered by helmet vs face mask on the rate of endotracheal intubation in patients with acute respiratory distress syndrome: A randomized clinical trial.J AMA315(22):2435–2441, 2016. doi: 10.1001/jama.2016.6338 3.Grieco DL, Menga LS, Cesarano M, et al: Effect of helmet noninvasive ventilation vs high-flow nasal oxygen on days free of respiratory support in patients With COVID-19 and moderate to severe hypoxemic respiratory failure: The HENIVOT randomized clinical trial.JAMA325(17):1731–1743, 2021. doi: 10.1001/jama.2021.4682 4.Bellani G, Laffey JG, Pham T, et al: Noninvasive ventilation of patients with acute respiratory distress syndrome. Insights from the LUNG SAFE study.Am J Respir Crit Care Med195(1):67–77, 2017. doi: 10.1164/rccm.201606-1306OC 5.Writing Group for the Alveolar Recruitment for Acute Respiratory Distress Syndrome Trial (ART) Investigators, Cavalcanti AB, Suzumura ÉA, et al: Effect of lung recruitment and titrated positive end-expiratory pressure (PEEP) vs low PEEP on mortality in patients with acute respiratory distress syndrome: A randomized clinical trial.JAMA318(14):1335–1345, 2017. doi: 10.1001/jama.2017.14171 6.Guérin C, Reignier J, Richard JC, et al: Prone positioning in severe acute respiratory distress syndrome.N Engl J Med368(23):2159–2168, 2013. doi: 10.1056/NEJMoa1214103 7.Scholten EL, Beitler JR, Prisk GK, et al: Treatment of ARDS with prone positioning.Chest151:215–224, 2017. doi: 10.1016/j.chest.2016.06.032. Epub 2016 Jul 8 8.National Heart, Lung, and Blood Institute Acute Respiratory Distress Syndrome (ARDS) Clinical Trials Network, Wiedemann HP, Wheeler AP, et al: Comparison of two fluid-management strategies in acute lung injury.N Engl J Med354(24):2564–2575, 2006. doi: 10.1056/NEJMoa062200 9.Villar J, Ferrando C, Martinez D, et al:Dexamethasonetreatment for the acute respiratory distress syndrome: a multicentre, randomised controlled trial.Lancet Respir Med8: 267–276, 2020. doi: 10.1016/S2213-2600(19)30417-5 