Optimizing Critical Care
Ventilation-
What Can We Learn from
Ventilator Waveforms
Luca Bigatello, MD
Department of Anesthesia and Critical Care,
Massachusetts General Hospital
Associate Professor of Anesthesia,
Harvard Medical School

Outline
 Understanding the basic physiology of ventilation
 Breath delivery:
 Basic choices
 Effects of patient’s changes
 Effects of ventilator changes
 Specific situations:
 ALI / ARDS
 Weaning

The Equation of Motion of
the Respiratory System
PAPPL = V × R + VT × E
PAPPL = V × R + VT / C
PAPPL = PMUS + PVENT = V × R + VT / C

From the Equation of Motion
PAPPL = PMUS + PVENT = VT / C + V × R
1. The basic mechanics of ventilation do not differ
during spontaneous and mechanical breathing
2. The results of mechanical ventilation depend not
only on what we set on the vent., but also on the
patient’s physiology- mechanics and effort

From the Equation of Motion
PAPPL = PMUS + PVENT = VT / C + V × R
3. For any independent variable set on the vent.
(‘control’, e.g., PVENT), for any mechanics (C, R) and
effort (PMUS), there is only one possible value of the
dependent variable (VT )
4. If we know two ventilating variables, we can
calculate the patient’s physiologic variable:
C = VT / Paw

Breath Delivery:
the simple view
(the pt. is not breathing…)
 Volume- controlled
 Pressure- controlled
The Emerson Critical Care Ventilator

Volume- Controlled Ventilation
 Constant flow
 Set: VT, flow
 Airway pressure (Paw)
will increases with:
 VT, flow, ⇓’d C, ⇑’d R
Paw
 Will also decrease with:
 ⇑’d pt. effort
Volume
Courtesy of Claudia Crimi, MD

V- CV Flow 30 l/min, VT 0.5 l
 The effect of
changing
flow settings
Flow 60 l/min, VT 0.5 l
Courtesy of Dean Hess, RRT, PhD

V- CV Flow 60 l/min, VT 0.5 l
 Theeffect of
changes in
respiratory
mechanics Decreased compliance

Volume- Controlled Ventilation
Paw
 The
effect of
patient Flow
effort
 Insufficient
flow
 Vigorous insp. effort

Volume- Controlled Ventilation
 Descending Ramp
 Peak flow is reached early,
then decreases linearly
 Set pressure is reached earlier,
is lower, and ≈ plateau
 For the same set flow, the
insp. time has to be longer⇒
 better PaO2 (may be)
 auto-PEEP

Volume- Controlled Ventilation
 Descending
ramp: the
effect of time

Pressure- Controlled Ventilation
 Set: insp. pressure, time
 Flowis variable, and VT will
change with:
 patient’s mechanics: C, R
 patient’s effort
 capability of the vent.

Pressure- Controlled Ventilation
 Changing
ventilator
settings: the
effect of
time
Lucangelo et al.,
Respir Care
2005;50:55

P- CV Slow time-
 Changes of resp. mechanics constant
Increasing airways resistance
Decreasing lung compliance Fast time-
constant
time

P- C, Inverse Ratio Ventilation
Chan et al., Chest 1992;102:1556
Mercat et al. AJRCCM 1997;155:1637
 Higher mean Paw⇒ better lung
recruitment for the same plateau
pressure
 Higher mean Paw⇒
hemodynamic compromise, auto-
PEEP, increased need for sedation

V- CV vs. P- CV
 VCV  PCV
 It assures the delivery  It assures a limit to
of a desired minute insp. pressure
(neonates, B-PF, etc.)
ventilation ⇒ PaCO2
 It allows further
 It limits the size of the
adjustment of breath
VT (e.g., ARDS-Net) delivery (‘rise time’)
 It’s simple  It may favor pt.-vent.
 It allows bedside synchrony
measurement of resp.  It may cause
mechanics hypoventilation

Bedside Measurement of
Respiratory Mechanics
pressure
PIP
resistance
Pplat
compliance
PEEP
time

Bi-Level, Bi-PAP…
(includes inverse-ratio, APRV)
 Two different levels of pressure (high & low) are
applied at a certain rate, and spontaneous breathing is
allowed at both levels
 In the absence of spont. breaths, this is P-CV!
 The spont. breaths may be assisted by pressure
support, generally only at the low level

Bi-Level, Bi-PAP

P- C, Airway Pressure Release
Ventilation- APRV
High Pressure
Low Pressure
time
High pressure time
Low pressure time

APRV
Putensen et al., AJCCM 1999;159:1241 APRV
Habashi, Crit Care Med 2005;33:S228
 Without spont. breathing,
⇒ PCIRV
 Enhanced lung recruitment
at lower pressures, by
allowing spont. breathing CPAP
 Comfortable?
 Important: think trans-
pulmonary pressure!

Pressure- Regulated, Volume
Control- PRVC (Auto-Flow, VC+)
Dual Mode
 PCV + a guaranteed VT Effect of decreased compliance
 In each breath, the insp.
pressure is regulated ⇑ or
⇓, to target a set VT
 To maintain a set VT
(VCV) while meeting pt.
demand (PCV)
Branson, Respir Care 2005;50:187

Breath Delivery:
When the Patient Breathes
 Assist- control
 SIMV
 Pressure support
 CPAP

 SIMV  SIMV+ Pressure Support

Assist- Control Ventilation

What’s Happened to IMV ?
 The principle of IMV-
ventilator and patient
can share the work in
a fair proportion, does
not pan out
 Physiologically, it is a
difficult principle to
accept

Continuous Positive Airway
Pressure- CPAP
 Set: a pressure at the
airway, throughout the
resp. cycle- how???
 The source provides a
flow higher than the Paw
patient’s own:
 continuous high flow with a
valveless system
 just enough flow to match
the pt.’s with a ventilator

CPAP- Uses
 To recruit the lung:
 obesity, OSA, postop. atelectasis
 To increase intrathoracic pressure:
 pulmonary edema
 To offset auto-PEEP:
 asthma/COPD
 Zero CPAP for SBTs
Antonelli et al., CCM 2002;30:602

Pressure Support
Ventilation
 Set: insp. pressure
 VT, time change with:
 patient’s effort
 patient’s rate
 pt.’s mechanics: C, R
 Important: the effect of
pt. effort on the trans-
pulmonary pressure!!

Pressure Support Ventilation
Flow
25%
 The ‘cycling’ variable
is unique to PSV:
 Thefirst part of the
breath is equal to PCV
 Thebreath ends when Paw
flow reaches a set low
value, or % of the peak

Progressive Withdrawal of PSV
Paw
PSV= 0 PSV= 5
Peso PSV=10 PSV=15
Yamada et al., J Appl Physiol ’94;77:2237

Pressure Support Ventilation
 PSV: effect of resp. mechanics
 Pts.
with a fast time-constant (low compliance-
ALI/ARDS) have a sharp decline in insp. flow,⇒
low VT, low Paw, possible dyssynchrony
 Pts.
with a slow time-constant (high compliance,
high resistance- COPD) have a slow decline in insp.
flow⇒ large VT, auto-PEEP, dyssynchrony

PSV with Low Compliance: the Sigh
 PCV+,
BiLevel:
 You can
‘trick’ the
vent. by
adding 1, 2
PC breaths
during PSV
Patroniti et al., Anesthesiology 2002;96:788

PSV with High Resistance:
Dissynchrony
 Backup cycling criteria: pressure, time
Branson, Resp Care 1998; 43:1045

Volume- Assured Pressure
Support- VAPS
Dual Mode
 Combines the
initial flow pattern
of PSV, with the
constant flow of a
VC breath
 Important to learn
the proper settings
to take advantage
of the dual mode
Branson, Respir Care 2005;50:187

Proportional Assist Ventilation-
PAV Younes M, ARRD 1992
PAPPL = PMUS + PVENT = VT x E + V × R
PAPPL = PMUS + PVENT = K1 x E + K2 × R
 PAV supports a % of ‘the patient effort’
 In reality, a % of E and R
 More ‘physiological’?

 The support PAV
increases with the
pt. effort
 Needs an intact
ventilatory drive
 Generates a
variable breathing
pattern
 May be more
interesting
physiologically
than clinically Marantz et al., J Appl Physiol ‘96

PAV
 In the US version,
the support is
expressed as % of
the work of
breathing, calculated
from periodical
measurement of R
and E

What We Have NOT Discussed
 Bedside
measurement of respiratory
mechanics
 Loops
 Triggering
 Missed triggering
 Tube
compensation
 NAVA
Emerson Iron Lung

Conclusions
 There is a gazillion modes of ventilation
 Rather,there is a gazillion names of modes
of ventilation
 Understanding the physics of ventilation
(equation of motion….) greatly simplifies
understanding mechanical ventilation

Conclusions
 Ventilatorwaveforms display helps guiding
the use of the many modes of ventilation
 Very
little evidence exists that using one
mode over another improves outcome
 Patient
outcome is affected more by how a
mode is used than by the mode itself

Dyssynchrony
 Missed triggering from dynamic hyperinflation

PSV: the ‘Rise Time’
 PSV, PCV: the rate of
rise of the insp. flow
can be adjusted:
 a high drive, a fast
time-constant may
require a high rate of
rise
 Quiet breathing,
bronchospasm may
benefit from a slower
rate of rise Chiumello et al., Eur Respir J. 2001;18:107

PSV+ Tube Compensation
 ATC:
 To overcome resistive
WOB imposed by the ETT:
PSV
 The vent. applies additional
pressure to the airway
pressure (cm H2O)
throughout the resp. cycle,
based on known resistance of
ETTs and measured flow,
resulting in more even ATC
tracheal pressure
Fabry, Intensive Care Med 1997; 23:545

PSV
 Expiratory
sensitivity
Tokioka et al.,
Anesth Analg.
2001;92:161