This document provides an overview of mechanical ventilation, including: 1) How mechanical ventilation helps reduce the work of breathing and restore gas exchange through invasive and noninvasive positive pressure ventilation. 2) The basics of monitoring pressure, volume, flow, and pressure-time curves at the bedside. 3) Important considerations for mechanical ventilation including potential adverse effects on hemodynamics, lungs, and gas exchange, and how to address issues like auto-PEEP.

1. Basics of Mechanical Ventilation Alain Broccard, MD John Marini, MD University of Minnesota Regions Hospital St Paul, MN

3. Physiopathology of Respiratory Failure30  Resistance AW  Compliance R  VA/Q  Work of Breathing Fatigue Hypercapnia Neuromuscular disorders  VE  VO 2  VCO 2  pH Hypoxemia AW=Airrway; R=respiratroy system; VE = minute ventilation, VO2 = Oxygen consumption, VCO2=carbon dioxide production

13. Intrinsic PEEP and Work of Breathing Volume V T V T FRC Pressure PEEPi Dynamic Hyperinflation PEEPi = intrinsic or auto PEEP; green triangle = tidal elastic work; red loop = flow resistive work; blue rectangle = work expended in offsetting intrinsic PEEP (an expiratory driver) during inflation When present, intrinsic PEEP contributes to the work of breaking and can be offset by applying external PEEP.

14. + + + + The Pressure and Work of Breathing can be Entirely Provided by the Ventilator (Passive Patient) Ventilator ₊ ₊

15. Work of Breathing Under Passive Conditions When the lung is inflated by constant flow, time and volume are linearly related. Therefore, the monitored airway pressure tracing (Paw) reflects the pressure-volume work area during inspiration. A pressure-sensing esophageal balloon reflects the average pressure change in the pleural space and therefore the work of chest wall expansion.

16. The Work of Breathing can be Shared Between the Ventilator and the Patient P AW P ES patient machine time AC mode The ventilator generates positive pressure within the airway and the patient’s inspiratory muscles generate negative pressure in the pleural space. Paw = Airway pressure, Pes= esophageal pressure

17. Relationship Between the Set Pressure Support Level and the Patient’s Breathing Effort Carrey et al. Chest. 1990;97:150. The changes in Pes (esophageal pressure) and in the diaphragmatic activity (EMG) associated with the increase in the level of mask pressure (Pmask = pressure support) indicate transfer of the work of breathing from the patient to the ventilator.

21. Pressure and Volume Targeted Ventilation Marini, Wheeler. Crit Care Med. The Essentials. 1997. VT

31. Auto-PEEP (Intrinsic PEEP, PEEPi) Marini, Wheeler. Crit Care Med. The Essentials. 1997. Note that AutoPEEP is not equivalent to air trapping. Active expiratory muscle contraction is an often under appreciated contributor (left panel) to positive pressure at the end of expiration

32. Suspecting and Measuring AutoPEEP Suspect AutoPEEP if flow at the end of expiration does not return to the zero baseline. AutoPEEP is commonly measured by performing a pause at the end of expiration. In a passive patient, flow interruption is associated with pressure equilibration through the entire system. In such conditions, proximal airway pressure tracks the mean alveolar pressure caused by dynamic hyperinflation. End expiratory pause Time Pressure PEEPe PEEPi Total PEEP

37. Minute Ventilation and Gas Exchange The relationship between PaCO 2 and minute ventilation is not linear. In patients with hypoventilation, small changes in minute ventilation may have large effect on the PaCO 2 .

38. Shunting: Effects of FiO 2 on PaO 2 Note that as the % of shunt rises, increasing the FIO 2 has less and less impact on PaO 2 . Under such conditions, reducing the shunt fraction is key to the ability to improve gas exchange, and this typically requires PPV and PEEP.

40. Mean Airway and Alveolar Pressures Adapted from: Ravenscraft et al. Crit Care Med. 1992. In normal lungs, the inspiratory resistive pressure is similar to the expiratory resistive pressure (light shaded area under the airway pressure-time curve) so that mean airway pressure (Paw) can be used to track mean alveolar pressure (Palv).

41. PEEP, Regional Lung Volume, and Shunting Lung regions with shunt tend to distribute preferentially in the dependent regions. Tidal ventilation helps open collapsed regions, and PEEP helps to maintain those regions open throughout expiration and to reduce shunt. Note that level of PEEP required to achieve such varies along the gravitational axis. ZEEP CPAP

42. Effect of PEEP-induced Alveolar Recruitment on PaO 2 Malbuisson et al. AJRCCM. 2001:163:1444-1450.

43. Approach to MV Is MV indicated ? Conservative treatment and periodic reassessment NO YES Contraindication to NIPPV ? NIPPV NO Success ? Invasive MV NO NO YES