The Art and Science of Intraoperative Ventilator
Management
Ross Blank, MD
Assistant Professor
Division of Critical Care
Director, Thoracic Anesthesia
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:
1. Bag
2. 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
Tobin MJ. Principles and Practice of Mechanical Ventilation, 2nd Ed. 2006.
Pressure control
Volume control(really flow control)
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
Height: 5’9”
PBW: 70.7 kg
TV (6 mL/kg): 424 mL
Female
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 to8.5 mL/kg for males and 11 mL/kg for females; these aresupraphysiologic
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
Bendixen et al. NEJM 1963;269:991-996
“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
Blanch et al. Curr Opin Crit Care 2007;13:332-337
Compliance low:Atelectasis, Shunt
Compliance low:OverinflationHigh VD/VTBest Compliance
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
Neumann et al. Acta Anaesthesiologica Scandinavica 1999;43:295-301
Alveolar Recruitment Strategy
Tusman et al. Br J Anaesth 1999;82:8-13Tusman and Bohm. Best Pract Res Clin Anaesthesiol 2010;24:183-197
20/5 35/2030/1525/10 40/20
Compliance Curve – The Lungs as a Single Balloon
Blanch et al. Curr Opin Crit Care 2007;13:332-337
Tid
al V
olum
e
PEEP
Possible Methods to Limit Atelectasis at Induction
1. Pre-oxygenation with < 100% FiO2
2. Pre-induction CPAP
3. Sitting position
4. 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
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
Lawrence et al. Ann Intern Med 2006;144:596-608
What works:
•Postoperative lung expansion modalities•Selective nasogastric decompression•Avoidance of long-acting neuromuscular blockers•Laparoscopic approaches when feasible
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
1. Acute onset, bilateral infiltrates on CXR
2. PCWP ≤ 18 mmHg or no clinical evidence of left atrial hypertension
3. PaO2/FiO2 (P/F) Ratio
≤ 300 for ALI
≤ 200 for ARDS
Ashbaugh DG et al. Lancet 1967;290:319-323Bernard 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)
Organdonors
• 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 disconnectfor apnea testing• Open circuitfor suctioning
Increased # ofeligible 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
• TV 6 mL/kg • TV 10 mL/kg Lower rate of 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
TV = 7 mL/kg PBW
PEEP = 10 cm H2O
Prescribed RMs after induction, after any circuit disconnection, and before emergence
Standard
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
TV = 6-8 mL/kg PBW
PEEP = 6-8 cm H2O
Prescribed RMs after induction and every 30 minutes
Nonprotective
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.