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MECHANICAL VENTILATION
R. Duncan Hite, M.D.Associate Professor of Internal Medicine
Section on Pulmonary and Critical Care Medicine
OBJECTIVES
Mechanics of Ventilators
Weaning (“Liberation”) from Mechanical Ventilation
Non Invasive Mechanical Ventilation (NIPPV)
Respiratory Failure
Hypoxic Respiratory Failure
Hypercarbic Respiratory Failure
VENTILATION
Physical Parameters of Ventilation
PressureVolumeFlow
VENTILATION
Negative Pressure• Spontaneous• Mechanical
Positive Pressure • Mechanical
Invasive Non Invasive (CPAP, BiPAP)
MECHANICAL VENTILATIONVentilation Modes
Volume-Cycled (Vt fixed, P variable)Assist Control (AC, CMV)Intermittent Mandatory Ventilation (IMV, SIMV)
Pressure-Cycled (P fixed, Vt variable)Pressure Support (PSV)Pressure Control (PCV)Airway Pressure Release (APRV)
MECHANICAL VENTILATIONVentilation Modes
MacIntyre, Chest, 1993, 104, 560.
COMPLIANCE
Compliance = V/P
Key physiologic variables:Lung ComplianceChest Wall ComplianceAirway Resistance
Static = Vt/Pplat-PEEP
Dynamic = Vt/PIP-PEEP
MECHANICAL VENTILATIONNomenclature
MECHANICAL VENTILATIONTriggering
MECHANICAL VENTILATIONRespiratory Rate
Assist Control Set rate = sets time duration for ventilator to give breath if not
triggered by patient. Possible for no breath to be given without patient triggering
All breaths are preset tidal volume
SIMVSet rate = sets time interval during which assisted breath must
be given with or without patient triggeringPreset tidal volume only provided during assisted breaths. Vt
for other breaths are spontaneous or PSV-assisted.
PSVNO set rate. Preset pressure provided during all breaths.
PCVAble to set rate, same as AC. Preset pressure provided for all.
MECHANICAL VENTILATIONSIMV
Terminology of PEEP
Extrinsic (i.e. Machine generated)
Terminology of PEEP
Extrinsic (i.e. Machine generated)
PEEP = CPAP
Intrinsic = autoPEEPObstructive lung disease - “air trapping”Mechanical (Inverse Ratio Ventilation)
Prolonged Inspiration/Intrinsic PEEP
Terminology of PEEP
Extrinsic (i.e. Machine generated)
PEEP = CPAP
Intrinsic = autoPEEPObstructive lung disease - “air trapping”Mechanical (Inverse Ratio Ventilation)
“Physiologic” (?)
Amato, etal. Am J Respir CritCare Med, 1995, 152, 1835.
Low Volume Ventilation StrategyARDS
ARDS Network, NEJM, 2000,342,1301.
Low Volume Ventilation StrategyARDS
ARDS Network, NEJM,2000,342,1301.
100
110
120
130
140
150
160
170
180
190
Day 0 Day 1 Day 3 Day 7
Pa
O2
/FiO
2 R
ati
o
Lower
Traditional
20
25
30
35
40
45
50
Day 1 Day 3 Day 7P
aC
O2
Lower
Traditional
Permissive Hypercapnea
Hickling, etal. Crit Care Med, 1994,22,1568.
• Management Issues– Cardiovascular
• Reduced Myocardial Contractility
• Pulmonary Vasoconstriction
• Systemic Vasoconstriction (increased afterload)
– Central Nervous System• Anesthetic
• Increased Intracranial Pressure
• Cerebral Vasodilation
– Musculoskeletal• Reduced Contractility
– Endocrine
Weaning Guidelines
“Weaning” (Extubation) Parameters
Parameters:• Strength
• MIP (< -20 cmH2O)• VC (> 10 cc/kg)• Vt (> 5 cc/kg)
• Demand• Ve (< 10 Lpm)• RR (> 30 bpm)
• Combined• f/Vt (< 105 bpm/L)
“Weaning” (Extubation) Parameters
Hilberman, etal. J Thor Cardiovasc Surg, 1976, 71, 711.
Yang, Tobin. NEJM, 1991, 324, 1445.
“Weaning” (Extubation) Parameters
Modes of Ventilation for Weaning
Volume-Cycled (Vt fixed, P variable)Assist Control (AC, CMV)Synchronized Intermittent Mandatory
Ventilation (SIMV)
Pressure-Cycled (P fixed, Vt variable)Pressure Support (PSV)Pressure Control (PCV)Airway Pressure Release (APRV)
Comparison of Weaning Modes
SIMV v. PSV??PSV - more patient
comfort, requires more observation
SIMV - less apnea alarms!
Esteban,etal. NEJM,
1995, 332, 345.
Impact of Daily Screen at NCBH
Ely, etal. N Engl J Med, 1996, 335, 1864.
MECHANICAL VENTILATIONSpontaneous Breathing Trials
Esteban,etal. Am J Resp Crit Care Med, 1999, 159, 512.
Modes: AutoFlow or Flowby PSV = 5 T-Piece Trial
Duration: 30 - 120 min
Non Invasive Ventilation
• PEEP = CPAP
• BiPAP = CPAP + PSV modes
• ePAP = CPAP
• iPAP = CPAP + PSV
Non Invasive Ventilation
• Clinical Indications:
• Hypercarbic Respiratory Failure• COPD Exacerbation
• BiPAP• OSA / Obesity Hypoventilation• Neuromuscular Disease
• Hypoxic Respiratory Failure• Cardiogenic Pulmonary Edema
• CPAP (?BiPAP)• Other ??
Mehta, Hill. Am J Respir Crit Care Med, 2001, 163, 540.
Non Invasive VentilationPatient Selection
Predictors of Success• Younger Age• Lower acuity of illness• Better Neurologic score• Cooperative and able to coordinate breathing with ventilator• Less air leaking; intact dentition• Hypercarbia; not too severe (PaCO2 > 45 and < 92 mm
Hg)• Acidemia, but not too severe (pH < 7.35, > 7.10)• Improvements in gas exchange, HR and RR within first 2 h
Mehta, Hill. AJRCCM, 2001, 163, 540.
Non Invasive VentilationDelivery Issues
Mehta, Hill. AJRCCM, 2001, 163, 540.
Post Extubation Resp Failure• Likely Improves Reintubation rate in COPD patients• No change in other etiologies• May increase mortality (?)
Staff Requirements• No increase in nursing time• Significant increase in RT time in first 8 – 48 hours• Requires MD re-evaluation after first 2 hours• Results not as favorable when performed outside of ICU
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