The Art and Science of Intraoperative Ventilator Management Ross Blank, MD Assistant Professor Division of Critical Care Director, Thoracic Anesthesia rossblan@med.umich.edu

How Should We Ventilate Patients in the Operating Room? What we have done Pathophysiology of general anesthesia and mechanical ventilation Recent clinical data on protective ventilation strategies

Older Anesthesia Machines Ventilator had two modes: Bag Volume Control

Older Machines - Volume Control CMV (Continuous Mandatory Ventilation) No attempt to synchronize with patient effort Constant flow rate Ascending pressure Set rate and I:E ratio determine inspiratory time Flow x Inspiratory time = Tidal Volume Tidal volume changed with fresh gas flow

Pressure vs. Volume Control Pressure control Volume control (really flow control) Tobin MJ. Principles and Practice of Mechanical Ventilation, 2nd Ed. 2006.

Older PEEP

Newer Anesthesia Machines Pressure dial Flowmeters No PEEP

What Tidal Volume Should We Use Under General Anesthesia? Normal tidal volume in adult humans breathing spontaneously is approximately: 6 mL/kg predicted body weight Should there be a different normal for mechanical ventilation?

Predicted Body Weight Depends on height and gender only; as patients become more or less obese, their lungs stay the same size Males: PBW (kg) = 50.0 + 2.3 x (height in inches – 60) Females: PBW (kg) = 45.5 + 2.3 x (height in inches – 60) http://www.ardsnet.org/node/77460

Predicted Body Weight http://www.ardsnet.org/system/files/pbwtables_2005-02-02_0.pdf

Average Americans Male Female Height: 5’9” PBW: 70.7 kg TV (6 mL/kg): 424 mL Height: 5’4” PBW: 54.7 kg TV (6 mL/kg): 328 mL The old default U of M tidal volume (600 mL) worked out to 8.5 mL/kg for males and 11 mL/kg for females; these are supraphysiologic

Current/Recent Practice

Current/Recent Practice Observational study of 2937 patients undergoing GA with MV in 49 hospitals in France over a 6-month period in 2006 Female sex and obesity independent risk factors for high tidal volumes per PBW PEEP 4 cm H2O or less in 91% of patients 7.7 mL/kg 8.8 mL/kg PBW

Why do we use large tidal volumes in the OR?

“A Concept of Atelectasis” Spontaneous breathing includes periodic deep breaths or sighs Mechanical ventilation typically delivers constant tidal volumes Over time, mechanical ventilation may lead to decreases in oxygenation and compliance due to alveolar collapse or atelectasis Atelectasis may be reversible with periodic hyperinflations Bendixen et al. NEJM 1963;269:991-996

“A Concept of Atelectasis” Declines in PaO2 and compliance reversible with hyperinflation maneuvers No use of PEEP in this study Bendixen et al. NEJM 1963;269:991-996

“A Concept of Atelectasis” Large TV “Shallow” TV No definition of magnitude of TVs Bendixen et al. NEJM 1963;269:991-996

“Perhaps the best course of action, during controlled ventilation, is “Perhaps the best course of action, during controlled ventilation, is . . . in providing reasonably large tidal volumes . . . [and] periodic passive hyperinflation of the lungs.” Bendixen et al. NEJM 1963;269:991-996

Pathophysiology of General Anesthesia Hedenstierna G. Acta Anaesthesiol Scand 2012;56:675-685

Pathophysiology of General Anesthesia Atelectasis occurs with anesthesia induction Supine position Loss of muscle tone Decrease in FRC Airway closure Oxygen absorption Lung compression Surfactant deficiency Shunting -> hypoxemia Increased VD/VT -> wasted ventilation May predispose to infection Hedenstierna and Edmark. Best Pract Res Clin Anaesthesiol 2010;24:157-69

Atelectasis Tusman and Bohm. Best Pract Res Clin Anaesthesiol 2010;24:183-197

How to Reverse Atelectasis? Large tidal volumes? Recruitment maneuvers? PEEP? Alveolar Recruitment Strategy? Inhaled Gas Composition?

Compliance Curve – The Lungs as a Single Balloon Compliance low: Overinflation High VD/VT Best Compliance Compliance low: Atelectasis, Shunt Blanch et al. Curr Opin Crit Care 2007;13:332-337

Ventilator-Induced Lung Injury Slutsky and Ranieri. NEJM 2013;369:2126-2136

Ventilator-Induced Lung Injury Slutsky and Ranieri. NEJM 2013;369:2126-2136

Open the Lungs . . . and Keep Them Open Sequential CT scans, n = 13 Neumann et al. Acta Anaesthesiologica Scandinavica 1999;43:295-301

Alveolar Recruitment Strategy 20/5 25/10 30/15 35/20 40/20 Tusman et al. Br J Anaesth 1999;82:8-13 Tusman and Bohm. Best Pract Res Clin Anaesthesiol 2010;24:183-197

Compliance Curve – The Lungs as a Single Balloon Tidal Volume PEEP Blanch et al. Curr Opin Crit Care 2007;13:332-337

Possible Methods to Limit Atelectasis at Induction Pre-oxygenation with < 100% FiO2 Pre-induction CPAP Sitting position Recruitment maneuver after induction Hedenstierna G. Acta Anaesthesiol Scand 2012;56:675-685

Pre-Oxygenation with < 100% FiO2? No pre-induction CPAP No RM after intubation PEEP 3 cm H2O after intubation Edmark et al. Acta Anaesthesiol Scand 2011;55:75-81

Methods to Limit Atelectasis during Maintenance and Emergence PEEP + Periodic recruitment maneuvers Limit FiO2 to decrease absorption atelectasis At emergence, maintain PEEP/CPAP, minimize disconnections/suctioning, avoid 100% FiO2 Hedenstierna G. Acta Anaesthesiol Scand 2012;56:675-685

Emergence with < 100% FiO2? Intervention 10 minutes before end of surgery; patients transported to CT scanner after extubation; supplemental O2 only prn Least atelectasis and highest PACU PO2 observed with RM followed by 40% FiO2 Positive pressure not maintained after RM Benoit et al. Anesth Analg 2002;95:1777-1781

Role for CPAP after Extubation? Multi-center RCT 209 patients with hypoxemia after elective open abdominal surgery Mask O2 vs. O2 + CPAP 7.5 cm H2O Stopped early after CPAP group showed lower rates of reintubation and pneumonia, and less ICU days Squadrone et al. JAMA 2005;293:589-595

Postoperative Pulmonary Complications “The main outcome was the development of at least one of the following: Respiratory infection, respiratory failure, bronchospasm, atelectasis, pleural effusion, pneumothorax, or aspiration pneumonitis.”

ARISCAT Population-based surgical cohort of 2464 patients were followed prospectively for development of postoperative pulmonary complications -> incidence of at least one PPC = 5.0% Regression modeling identified seven independent risk factors Canet et al. Anesthesiology 2010;113:1338-1350

Postoperative Pulmonary Complications What works: Postoperative lung expansion modalities Selective nasogastric decompression Avoidance of long-acting neuromuscular blockers Laparoscopic approaches when feasible Lawrence et al. Ann Intern Med 2006;144:596-608

Postoperative ALI/ARDS > 50,000 low-risk surgical admissions 0.2% incidence of ALI/ARDS Blum et al. Anesthesiology 2013;118:19-29

What is ALI/ARDS? Acute Lung Injury/Acute Respiratory Distress Syndrome First described by Ashbaugh et al. in 1967 Definition formalized in 1992 American European Consensus Conference Acute onset, bilateral infiltrates on CXR PCWP ≤ 18 mmHg or no clinical evidence of left atrial hypertension PaO2/FiO2 (P/F) Ratio ≤ 300 for ALI ≤ 200 for ARDS Ashbaugh DG et al. Lancet 1967;290:319-323 Bernard GR et al. AJRCCM 1994;149:818-824

Postoperative ALI/ARDS 4,366 high-risk operations 2.6% incidence of ALI/ARDS Kor et al. Anesthesiology 2011;115:117-128

Surgical Lung Injury Prediction Kor et al. Anesthesiology 2011;115:117-128

Small Prospective Trials of Lung Protective Ventilation in the OR Author Population LPVS Control Outcome Mascia (JAMA 2010; 304:2620-7) Organ donors TV 6-8 mL/kg PEEP 8-10 CPAP for apnea testing Closed circuit for suctioning TV 10-12 mL/kg PEEP 3-5 Vent disconnect Open circuit Increased # of eligible and harvested lungs Yang (Chest 2011; 139:530-537) Lung cancer resection TV 6 mL/kg PEEP 5 FiO2 50% PCV TV 10 mL/kg PEEP 0 FiO2 100% VCV Lower rate of lung dysfunction (hypoxemia, infiltrate, atelectasis) within 72h of surgery Sundar (Anes 2011; 114:1102-10) Elective cardiac surgery mechanical ventilation at 6h and lower reintubation rate

What about more routine cases? 56 open abdominal operations randomized to protective vs. standard ventilation strategies Outcomes = CXR, oxygenation, postoperative pulmonary infection score, and PFTs Severgnini et al. Anesthesiology 2013;118:1307-1321

Severgnini RCT Protective Standard TV = 7 mL/kg PBW PEEP = 10 cm H2O Prescribed RMs after induction, after any circuit disconnection, and before emergence TV = 9 mL/kg PBW PEEP = 0 cm H2O No prescribed RMs Severgnini et al. Anesthesiology 2013;118:1307-1321

Severgnini Results Severgnini et al. Anesthesiology 2013;118:1307-1321

Severgnini Results Pulmonary infection score includes points for temperature, white blood cell count, secretions, P/F ratio, and CXR Severgnini et al. Anesthesiology 2013;118:1307-1321

The IMPROVE Trial 400 major abdominal surgeries (open and laparoscopic) Primary Outcome = composite of major pulmonary (pneumonia, respiratory failure) and extrapulmonary (sepsis, death) complications Futier et al. NEJM 2013;369:428-437

The IMPROVE Trial Lung-Protective Nonprotective TV = 6-8 mL/kg PBW PEEP = 6-8 cm H2O Prescribed RMs after induction and every 30 minutes TV = 10-12 mL/kg PBW PEEP = 0 cm H2O No prescribed RMs Futier et al. NEJM 2013;369:428-437

Las Vegas 10,000 patients in 142 centers Enrollment closed in 3/2013

Conclusions The common practice of using supraphysiologic tidal volumes without PEEP will support oxygenation and not cause overt harm in the majority of patients.

Conclusions Atelectasis develops quickly and reliably after induction of anesthesia and can be minimized with RMs after induction and circuit disconnections, application of PEEP after RMs, minimization of FiO2 when possible, and continuation of lung expansion modalities into the recovery room and postoperative ward.

Conclusions A comprehensive strategy of lung-protective ventilation aims to minimize both atelectasis and ventilator-induced lung injury and is increasingly being shown to be beneficial in varied surgical populations. There is no evidence that such strategies confer harm. rossblan@med.umich.edu