Acute severe asthma induces marked alterations in respiratory mechanics, characterized by a critical limitation of expiratory flow and a heterogeneous and reversible increase in airway resistance, resulting in premature airway closure, lung, and chest wall dynamic hyperinflation and high intrinsic PEEP. These abnormalities increase the work of breathing and can lead to respiratory muscle fatigue and life-threatening respiratory failure, in which case mechanical ventilation is life-saving. When instituting mechanical ventilation in this setting, a major concern is the risk of worsening lung hyperinflation (thereby provoking barotrauma) and inducing or aggravating hemodynamic instability. Guidelines for mechanical ventilation in acute severe asthma are not supported by strong clinical evidence. Controlled hypoventilation with permissive hypercapnia may reduce morbidity and mortality compared to conventional normocapnic ventilation. Profound pathological alterations in respiratory mechanics occur during acute severe asthma, which clinicians should keep in mind when caring for ventilated asthmatics. We focus on the practical management of controlled
hypoventilation. Particular attention must be paid to ventilator settings, monitoring of lung hyperinflation, the role of extrinsic PEEP, and administering inhaled bronchodilators. We also underline the importance of deep sedation with respiratory drive-suppressing opioids to maintain patient-ventilator synchrony while avoiding as much as can be muscle paralysis and the ensuing risk of myopathy. Finally, the role of noninvasive positive pressure ventilation for the treatment of respiratory failure during severe asthma is discussed.

/pdf/management-of-mechanical-ventilation-in-acute-severe-asthma-2dojgmfue1.pdf

Management of mechanical ventilation in acute severe asthma: practical aspects

Objective. To compare triggering, pressurization, and cycling of the home ventilator VPAP II with those of three ICU ventilators (Evita 4, Galileo, and Servo 300).

/pdf/comparative-bench-study-of-triggering-pressurization-and-3w3it2cxpz.pdf

Comparative bench study of triggering, pressurization, and cycling between the home ventilator VPAP II and three ICU ventilators

Objective: To test the hypothesis that replacing 70:30 nitrogen: oxygen (Air-O-2) with 70:30 helium:oxygen (He-O-2) can decrease dynamic hyperinflation ("Intrinsic" positive end-expiratory pressure) in mechanically ventilated patients with chronic obstructive pulmonary disease (COPD), and to document the consequences of such an effect on arterial blood gases and hemodynamics. Design: Prospective, interventional study. Setting: Medical intensive care unit, university tertiary care center. Patients: Twenty-three intubated, sedated, paralyzed, and mechanically ventilated patients with COPD enrolled within 36 hrs after intubation. Interventions: Measurements were taken at the following time points, all with the same ventilator settings: a) baseline; b) after 45 mins with He-O-2; c) 45 mins after return to Air-O-2. The results were then compared to those obtained in a test lung model using the same ventilator settings. Main Results (mean +/- so):Trapped lung volume and intrinsic positive end-expiratory pressure decreased during He-O-2 ventilation (215 +/- 125 mL vs. 99 +/- 15 mL and 9 +/- 2.5 cm H2O vs. 5 +/- 2.7 cm H2O, respectively; p < .05). Likewise, peak and mean airway pressures declined with He-O-2 (30 +/- 5 cm H2O vs. 25 +/- 6 cm H2O and 8 +/- 2 cm H2O vs. 7 +/- 2 cm H2O, respectively; p < .05). These parameters all rose to their baseline values on return to Air-O-2 (p < .05 vs, values during He-O-2). These results were in accordance with those obtained in the test lung model. There was no modification of arterial blood gases, heart rate, or mean systemic arterial blood pressure. In 12/23 patients, a pulmonary artery catheter was in place, allowing hemodynamic measurements and venous admixture calculations. Switching to He-O-2 and back to Air-O-2 had no effect on pulmonary artery pressures, right and left ventricular filling pressures, cardiac output, pulmonary and systemic vascular resistance, or venous admixture. Conclusion: In mechanically ventilated COPD patients with intrinsic positive end-expiratory pressure, the use of He-O-2 can markedly reduce trapped lung volume, intrinsic positive end-expiratory pressure, and peak and mean airway pressures. No effect was noted on hemodynamics or arterial blood gases. He-O-2 might prove beneficial in this setting to reduce the risk of barotrauma, as well as to improve hemodynamics and gas exchange in patients with very high levels of intrinsic positive end-expiratory pressure. (Crit Care Med 2000; 28:2721-2728).

Effects of helium-oxygen on intrinsic positive end-expiratory pressure in intubated and mechanically ventilated patients with severe chronic obstructive pulmonary disease.

Purpose of reviewThe aim of this article is to discuss the various factors that influence aerosol delivery in mechanically ventilated patients and clarify optimal techniques for aerosol administration in this patient population. Clinical use of various inhaled therapies in patients receiving invasiv

Inhalation therapy in invasive and noninvasive mechanical ventilation

The application of noninvasive ventilation (NIV) to treat acute respiratory failure has increased tremendously both inside and outside the intensive care unit. The choice of ventilator is crucial for success of NIV in the acute setting, because poor tolerance and excessive air leaks are significantly correlated with NIV failure. Patient-ventilator asynchrony and discomfort can occur if the physician or respiratory therapist fails to adequately set NIV to respond to the patient's ventilatory demand, so clinicians need to fully understood the ventilator's technical peculiarities (eg, efficiency of trigger and cycle systems, speed of pressurization, air-leak compensation, CO(2) rebreathing, reliability of fraction of inspired oxygen reading, monitoring accuracy). A wide range of ventilators of different complexity have been introduced into clinical practice to noninvasively support patients in acute respiratory failure, but the numerous commercially available ventilators (bi-level, intermediate, and intensive care unit ventilators) have substantial differences that can influence patient comfort, patient-ventilator interaction, and, thus, the chance of NIV clinical success. This report examines the most relevant aspects of the historical evolution, the equipment, and the acute-respiratory-failure clinical application of NIV ventilators.

http://rc.rcjournal.com/content/respcare/53/8/1054.full.pdf

Ventilators for Noninvasive Ventilation to Treat Acute Respiratory Failure

The study evaluated seven intensive care unit (ICU) ventilators (Veolar FT, Galileo, Evita 2, Evita 4, Servo 900C, Servo 300, Nellcor Puritan Bennett 7200 Series) with helium-oxygen (HeO2), using a lung model, to develop correction factors for the safe use of HeO2. A 70:28 helium-O-2 mixture (heliox) replaced air and combined with O-2 (HeO2). Theoretical impact of HeO2 on inspiratory valves and gas mixing was computed. True fraction of inspired oxygen (Ro,del) was compared with fraction of inspired oxygen (FIo2) set on the ventilator (FI(o2)set). True tidal volume (VTdel) was compared with VT set on the ventilator (VTset) in volume control and with control VTdel at FIo2 1.0 in pressure control. FI(o2)del minimally exceeded FI(o2)set (less than or equal to 5%) except with the 7200 Series (FI(o2)del > FI(o2)set by 125%). In volume control, with the Veolar FT, Galileo, Evita 2, and Servo 900C, VTdel > VTset, with the 7200 Series VTdel VTset (nonlinear relationship), whereas with the Servo 300 VTdel = VTset. In pressure control, VTdel was identical to control measurements, except with the 7200 Series (ventilator malfunction). Correction factors were developed that can be applied to most ventilators.

https://www.atsjournals.org/doi/pdf/10.1164/ajrccm.160.1.9807127

Calibration of seven ICU ventilators for mechanical ventilation with helium-oxygen mixtures

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Calibration of seven ICU ventilators for mechanical ventilation with helium-oxygen mixtures