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Published on January 15, 2008

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Man and machines: Insights into ventilation strategies May 2006:  Man and machines: Insights into ventilation strategies May 2006 Dr Geoff Shaw Dept of Intensive Care Christchurch Hospital Clin Sen Lecturer Dept of Medicine CSM&HS University of Otago, NZ Senior fellow Dept of Mechanical Engineering, University of Canterbury, NZ The ventilator is our identity:  The ventilator is our identity The ventilator is our identity:  The ventilator is our identity Mechanical ventilation has been used clinically for about 80 years….. First there was Negative pressure ventilation…:  Negative pressure ventilation… The ventilator is our identity:  The ventilator is our identity Then there was Slide6:  Dr H Lassen at Copenhagen’s Blegdams Hospital PEEP:  PEEP 1967 First report of ARDS: Ashbaugh DG, Bigelow DB, Petty TL, Levine BE. Acute respiratory distress syndrome in adults. Lancet; 1967,(II) 319-323 1972 First formal investigation of the effect of PEEP Improved PaO2 by applying 0-15cm PEEP in 10 patients with ARDS. Putative mechanism  prevention of airway closure Falke KJ, Pontoppidan H, Kumar A et al Ventilation with end- expiratory pressure in acute lung disease; J Clin Invest 1972, 51:2315–2323 PEEP:  1975 “Optimum PEEP” Defined as best O2 transport (CO X O2 content) highest compliance of respiratory system. Suter PM, Fairley B, Isenberg MD. Optimum end-expiratory airway pressure in patients with acute pulmonary failure. N Engl J Med 1975; 292:284–289 “Super PEEP” that which produces minimal shunt Kirby RR, Downs JB, Civetta JM et al. High level positive end expiratory pressure (PEEP) in acute respiratory insufficiency. Chest 1975; 67:156–163 1981 “Minimal PEEP” 2cm higher than lower inflection point of inflation limb of the pressure volume curve Lemaire F, Harf A, Simonneau G et al [Gas exchange, static pressure volume curve and positive-pressure ventilation at the end of expiration. Study of 16 cases of acute respiratory insufficiency in adults]. Ann Anesthesiol Fr 1981;22:435–441 PEEP PV Curve :  PV Curve Radford EP (1957) Recent studies of the mechanical properties of mammalian lungs. In: Remington JW (ed) Tissue elasticity American Physiological Society Washington, pp 177–190 VILI:  VILI Webb & Tierney ARRD 1974;110;556 PIP=14, PEEP=0 PIP= 45, PEEP=10 PIP= 45, PEEP = 0 Other therapies:  Other therapies 1979 “ECMO”: NIH trial 90% mortality in both groups. High volumes and pressures Zapol WM, Snider MT, Hill JD et al. Extracorporeal membrane oxygenation in severe acute respiratory failure. A randomized prospective study. JAMA 1979;242:2193–2196 1980’s “ECO2R” (Extracorporeal CO2 removal) Concept of “lung rest” Normal gas exchange targets Unacceptable complications especially bleeding Gattinoni L, Agostoni A, Pesenti A et al. Treatment of acute respiratory failure with low-frequency positive pressure ventilation and extracorporeal removal of CO2. Lancet 1980;II:292–294 Gattinoni L, Pesenti A, Mascheroni D et al. Low-frequency positive-pressure ventilation with extracorporeal CO2 removal in severe acute respiratory failure. JAMA 1986; 256:881–886 Concepts of ARDS in 1980’s:  Concepts of ARDS in 1980’s Lungs homogeneous, heavy and stiff Normalise pCO2 by use of high pressures and volumes Use of PEEP to normalise pO2 Barotrauma = “complication”  Major concerns were haemodynamic impairments caused by PEEP “Baby Lung”:  “Baby Lung” Quantitative assessment of CT images in ARDS. amount of normally aerated tissue = 5-6 yr old child Gattinoni L, Pesenti A, The concept of the “baby lung”. Intensive Care Med; 2005:31:776-784 “Baby Lung”:  “Baby Lung” Gattinoni L, Pesenti A, Baglioni S et al. Inflammatory pulmonary edema and positive end-expiratory pressure: correlations between imaging and physiologic studies. 1988; J Thorac Imaging 3:59–64 Respiratory compliance correlates with amount of normally aerated tissue “Sponge Lung”:  Bone RC The ARDS lung. New insights from computed tomography. JAMA 1993; 269:2134–2135 Gattinoni L, Pesenti A, The concept of the “baby lung”. Intensive Care Med; 2005:31:776-784 “Sponge Lung” Assumes lung oedema in ARDS is evenly distributed throughout the lung from sternum to vertebrae. (not gravitationally dependent). Gas in dependent regions is squeezed out by superimposed pressure including the weight of the heart Superimposed pressure:  Superimposed pressure (modified from Gattinoni) “Permissive hypercapnia”:  “Permissive hypercapnia” 1990 Low tidal volumes to rest lung; CO2 levels allowed to permissively rise Changed the goals of ventilation Hickling KG, Henderson SJ, Jackson R. Low mortality associated with low volume pressure limited ventilation with permissive hypercapnia in severe adult respiratory distress syndrome. Intensive Care Med 1990; 16:372–377 Permissive hypercapnia:  Permissive hypercapnia Late ’90’s: clinical trials of low tidal volume ventilation Amato MB, Barbas CS, Medeiros DM, Magaldi RB, Schettino GP, Lo-Physirenzi-Filho G, Kairalla RA, Deheinzelin D, Munoz C, Oliveira R, et al. Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome. N Engl J Med 1998;338:347–354. 63 patients Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. N Engl J Med 2000;342:1301–1308. 861 patients Stewart TE, Meade MO, Cook DJ, Granton JT, Hodder RV, Lapinsky SE,Mazer CD, McLean RF, Rogovein TS, Schouten BD, et al. Evaluation of a ventilation strategy to prevent barotrauma in patients at high risk for acute respiratory distress syndrome. N Engl J Med 1998;338:355–361. 120 patients Brochard L, Roudot-Thoraval F, Roupie E, Delclaux C, Chastre J, Fernandez-Mondejar E, Clementi E, Mancebo J, Factor P, Matamis D, et al. Tidal volume reduction for prevention of ventilator-induced lung injury in acute respiratory distress syndrome. Multicenter Trial Group on Tidal Volume Reduction in ARDS. Am J Respir Crit Care Med 1998;158:1831–1838 116 patients Brower RG, Shanholtz CB, Fessler HE, Shade DM, White P Jr, Wiener CM, Teeter JG, Doddo JM, Almog Y, Piantadosi S. Prospective, randomized, controlled clinical trial comparing traditional versus reduced tidal volume ventilation in acute respiratory distress syndrome patients. Crit Care Med 1999;27:1492–1498. 52 patients Confusion and controversy!:  Confusion and controversy! Eichacker PQ,. Gerstenberger EP, Banks SM, Cui X, Natanson C. Meta-analysis of acute lung injury and acute respiratory distress syndrome trials testing low tidal volumes. Am J Respir Crit Care Med Vol 166. pp 1510–1514, 2002 Lung recruitment in ARDS:  Lung recruitment in ARDS 68 patients with ARDS: Highly variable % potentially recruitable lung 13% ±11% % potentially recruitable correlated with % lung maintained after application of PEEP Higher % potentially recruitable lung correlated with: Lung weight PaO2/FIO2 ratio Compliance Dead space Mortality Gattinoni L, Caironi P, Cressoni M, et al. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:1775-86. Slide21:  Gattinoni L, Caironi P, Cressoni M, et al. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:1775-86. Slide22:  Gattinoni L, Caironi P, Cressoni M, et al. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:1775-86. Slide23:  Mortality relates to recruitable lung Gattinoni L, Caironi P, Cressoni M, et al. Lung recruitment in patients with the acute respiratory distress syndrome. N Engl J Med 2006;354:1775-86. Lung recruitment in ARDS:  Lung recruitment in ARDS Gattinoni L Am J Respir Crit Care Med 2001; 164:1701–1711 Over-stretch = “Volutrauma”:  Over-stretch = “Volutrauma” Ventilation induced lung injury (VILI) Epithelial and endothelial cells are anchored to the lung “skeleton” Gattinoni L, Pesenti A, The concept of the “baby lung”. Intensive Care Med; 2005:31:776-784 Bunched up collagen fibres Elastin fibres Stress and strain:  Stress and strain Stress = K (Youngs module of material) x Strain Stress = PL (transpulmonary pressure) Strain = Vt (Δ Lung Vol) / “baby lung” (volume at ZEEP) K = E spec (Specific lung elastance) Hence: E spec = PL x Baby Lung / Vt E spec = Transpulmonary Pressure at which the EELV doubles (~12-13cm normally) Gattanoni’s hypothesis...:  Gattanoni’s hypothesis... If… E spec is constant within narrow limits in ARDS Then… An estimate of stress and strain can be made by knowing either size of “baby lung”, or PL (Neither are measured routinely in ICU) “Volutrauma”: Volume–dependent elastance E2:  “Volutrauma”: Volume–dependent elastance E2 Bersten AD. Measurement of overinflation by multiple linear regression analysis in patients with acute lung injury. Eur Respir J 1998; 12: 526–532. Linear portion; constant compliance; independent of volume (E1) Non-linear portion; volume dependent compliance (E2) Pressure Volume Paw = Airway pressure E1 = volume-independent respiratory elastance E2 = Volume-dependent component of elastance VT = tidal volume Po = static recoil pressure at end-expiration PEEP (tot) “Volutrauma”: :  VEI = End expiratory volume above resting volume Bersten AD. Measurement of overinflation by multiple linear regression analysis in patients with acute lung injury. Eur Respir J 1998; 12: 526–532. “Volutrauma”: “Volutrauma”: :  = safe zone = dangerous overstretched zone %E2 was >30% in 50% of data where Pel <30 cm! Bersten AD. Measurement of overinflation by multiple linear regression analysis in patients with acute lung injury. Eur Respir J 1998; 12: 526–532. Pel,dyn =Dynamic elastic airway pressure “Volutrauma”: Airway Pressure-time curves:  Airway Pressure-time curves a =the slope of the P-t relation at t = 1 s c = the pressure at t = 0. b = dimensionless number  shape of the P-t curve During inspiration, if: b <1, P-t curve is convex; ↑ compliance b >1 P-t curve is convex; ↓ compliance b =1 P-t curve is straight; ↔compliance Ranieri VM, Zhang H, Mascia L, et al. Pressure–time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model. Anesthesiology 2000; 93:1320–8 Airway Pressure-time curves:  Airway Pressure-time curves Ranieri VM, Zhang H, Mascia L, et al. Pressure–time curve predicts minimally injurious ventilatory strategy in an isolated rat lung model. Anesthesiology 2000; 93:1320–8 Hypothetical model of Pplateau vs Vt ~based on meta analysis of 5 trials of low volume ventilation:  Hypothetical model of Pplateau vs Vt ~based on meta analysis of 5 trials of low volume ventilation Eichacker PQ,. Gerstenberger EP, Banks SM, Cui X, Natanson C. Meta-analysis of acute lung injury and acute respiratory distress syndrome trials testing low tidal volumes. Am J Respir Crit Care Med Vol 166. pp 1510–1514, 2002 Under-stretch = “Atelectrauma”:  Condom model demonstrating intrapulmonary stresses Mead J, Takishima T, Leith D. Stress distribution in lungs: a model of pulmonary elasticity. J Appl Physiol 1970; 28:596-608 “Atelectasis” as modelled by applying negative pressure to a condom surrounded by an “alveolar pressure” of 20 cmH2O Under-stretch = “Atelectrauma” “Atelectrauma”:  Shear forces in the zone of lung opening, caused by stretching of densely distributed alveolar membranes, obliquely attached to bronchiolar basal membranes Jonson B. Elastic pressure-volume curves in acute lung injury and acute respiratory distress syndrome Intensive Care Med 31:205–212, 2005, with permission from: Jonson B (1982) In: Prakash O (ed) Applied physiology in clinical respiratory care. Nijhoff, The Hague, pp 123– 139 “Atelectrauma” Atelectasis = Stress:  Atelectasis = Stress Mead J, Takishima T, Leith D. Stress distribution in lungs: a model of pulmonary elasticity. J Appl Physiol 1970; 28:596-608 Mead and colleagues have postulated that: Peff =-PL (V/V0)2/3 Where PL = Palv - Ppl PL = transpulmonary pressure, Palv = alveolar pressure, Ppl = pleural pressure V = Inflated volume, V0 = collapsed volume Consider inflating a partially collapsed lung to PL=30cmH2O. Let the volume of the degassed region be 1/10 of its final inflated volume. PL is therefore amplified by 102/3 Thus the initial pressure tending to expand the atelectatic region is: 30 x 102/3 = 140 cm H2O !! Surfactant depletion in pig model:  Surfactant depletion in pig model Courtesy of Gary Nieman, Syracuse NY Recruitment occurs throughout static inflation Alveolar wall stress in 2-D condom model Wall areas stressed next to regions of hyperiflation and collapse Deflation /re-inflation PV curves:  Hickling KG: Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome. Curr Opin Crit Care 2002, 8:32–38 Published with permission from: Rimensberger PC, Cox PN, Frndova H, et al.: The open lung during small tidal volume ventilation: Concepts of recruitment and “optimal” positive end expiratory pressure. Crit Care Med 1999, 27:1946–1952. Saline-lavaged rabbits Solid lines show re-inflation plots after deflation from 30 cm H2O airway pressure to different end-expiratory pressures. Little hysteresis with deflation to 15 cm H2O,  little derecruitment above that pressure. Deflation /re-inflation PV curves Slide40:  Hickling KG. Using the expiratory pressure volume curve- VILI at the bedside. 25th Symposium of Intensive Care and Emergency Medicine, March 21-25, 2005 Galileo datalogger: Inflation deflation method Flow Pressure Slide41:  Hickling KG. Using the expiratory pressure volume curve- VILI at the bedside. 25th Symposium of Intensive Care and Emergency Medicine, March 21-25, 2005 Galileo datalogger: Inflation deflation method Pressure Volume Models of threshold opening and closing pressures:  Models of threshold opening and closing pressures Inspiratory tidal PV plots Incremental = black symbols Decremental = open symbols TOP = 0–40, TCP = 0–4. At each PEEP level the volume at equivalent pressures and the mean tidal PV slope are greater during decremental PEEP. Hickling KG: Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome. Curr Opin Crit Care 2002, 8:32–38 Redrawn from: Hickling KG: Best compliance during a decremental, but not incremental, positive end-expiratory pressure trial is related to open-lung positive end expiratory pressure: a mathematical model of acute respiratory distress syndrome lungs. Am J Respir Crit Care Med 2001, 163:69–78 Models of threshold opening and closing pressures:  Models of threshold opening and closing pressures Hickling KG: Best compliance during a decremental, but not incremental, positive end-expiratory pressure trial is related to open-lung positive end expiratory pressure: a mathematical model of acute respiratory distress syndrome lungs. Am J Respir Crit Care Med 2001, 163:69–78 The mean tidal PV slope plotted against PEEP Incremental PEEP = black symbols Decremental PEEP = open symbols Max PV slope with: Incremental PEEP is at 20 cm H2O Decremental PEEP is at 16 cm H2O Models of threshold opening and closing pressures:  Models of threshold opening and closing pressures Hickling KG. Using the expiratory pressure volume curve- VILI at the bedside. 25th Symposium of Intensive Care and Emergency Medicine, Brussels, Belgium, March 21-25, 2005 Simulated data Max change in slope corresponds to beginning of de-recruitment But very difficult to judge slope changes by eye especially when very steep Hence a flow pressure curve can indicate the max rate of de-recruitment Flow pressure curves Slide45:  Hickling KG. Using the expiratory pressure volume curve- VILI at the bedside. 25th Symposium of Intensive Care and Emergency Medicine, March 21-25, 2005 Galileo datalogger: Flow-Pressure Flow Pressure Derecruitment Models of threshold opening and closing pressures:  Hickling KG: Reinterpreting the pressure-volume curve in patients with acute respiratory distress syndrome. Curr Opin Crit Care 2002, 8:32–38 Redrawn from: Crotti S, Mascheroni D, Caironi P, et al.: Recruitment and de-recruitment during acute respiratory failure: A clinical study. Am J Respir Crit Care Med 2001, 164:131–140. Models of threshold opening and closing pressures Airway pressure vs volume /recruited volume determined from CT (open circles and dotted line; expressed as percent of maximum volume) and recruitment (black circles and solid line; expressed as percent maximum recruitment) (B) Frequency distribution of estimated opening pressures. Note that recruited volume continues throughout inflation, up to 50 cm H2O pressure. Real-time acquisition of threshold opening and closing pressures:  Skewed normal distribution Unique to a patient and condition Recruitment is described by Threshold Opening Pressure (TOP) Derecruitment is described by Threshold Closing Pressure (TCP) Real-time acquisition of threshold opening and closing pressures Chase J, Yuta T, Shaw G, Horn B, Hann C A minimal model of mechanically ventilated lung mechanics to optimise ventilation therapy in the treatment of ARDS in critical care. Proceedings of the 12th International Conference on Bioengineering, Singapore 2005 PEEP:  Unique distributions for different levels of PEEP are found PEEP TOP TCP Chase J, Yuta T, Shaw G, Horn B, Hann C A minimal model of mechanically ventilated lung mechanics to optimise ventilation therapy in the treatment of ARDS in critical care. Proceedings of the 12th International Conference on Bioengineering, Singapore 2005 Slide49:  Chase J, Yuta T, Shaw G, Horn B, Hann C A minimal model of mechanically ventilated lung mechanics to optimise ventilation therapy in the treatment of ARDS in critical care. Proceedings of the 12th International Conference on Bioengineering, Singapore 2005 Optimisation of ventilation Parameter identification = patient specific model Simulation to determine effect of settings on PV curve Optimise ventilator settings as desired Slide51:  ‘Strengths’ in using this approach….. Real time assessment of recruitment status which is dependent on PEEP, ventilation strategy, and disease Readily identifies TCP distributions  optimization of PEEP Provides opportunity to simulate a ventilation strategy before application. TOP distribution characteristics Prediction of “overstretch”. E.g. Δ recruitment < % max rate  ? Correlated with E2% or CT scan Slide52:  Although flow resistive forces through the endotracheal tube are accounted for, the model assumes the pressure at the carina will reflect what is happening to alveolar units. Unforeseen resistive changes (eg major bronchial airway obstruction) could therefore lead to incorrect inferences about recruitment status Limitations… A model for teaching and research:  A model for teaching and research Mechanical lung model with 6 units of variable compliance (weighted) bellows and variable insp /exp resistances (taps) Mimicking nature:  Mimicking nature 5cm PEEP 15cm PEEP Sponsored by NZ$8000 grant from Hamilton Medical, Switzerland Mimicking nature:  Mimicking nature Normal PV loop PV loop of “Asthma” PV loops at different levels of PEEP (Note tidal volume is referenced to zero volume) Chase JG, Yuta T, Shaw GM, Mulligan K, Hann CE. A novel mechanical lung model of pulmonary diseases to assist with teaching and research (in review ) Slide56:  Mind what you have learned. Save you it can. Slide57:  Acknowledgements Dr Chris Hann Assoc. Prof. Geoff Chase Toshi Yuta Kerry Mulligan Beverley Horn

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