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Optimizing Critical Care Ventilation: What can we learn from Ventilator Waveforms?
1. 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
2. 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
3. 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
4. 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
5. 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
6. Breath Delivery:
the simple view
(the pt. is not breathing…)
Volume- controlled
Pressure- controlled
The Emerson Critical Care Ventilator
7. 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
8. 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
9. V- CV Flow 60 l/min, VT 0.5 l
Theeffect of
changes in
respiratory
mechanics Decreased compliance
11. 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
13. 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.
15. P- CV Slow time-
Changes of resp. mechanics constant
Increasing airways resistance
Decreasing lung compliance Fast time-
constant
time
16. 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
17. 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
18. Bedside Measurement of
Respiratory Mechanics
pressure
PIP
resistance
Pplat
compliance
PEEP
time
19. 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
21. P- C, Airway Pressure Release
Ventilation- APRV
High Pressure
Low Pressure
time
High pressure time
Low pressure time
22. 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!
23. 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
24. Breath Delivery:
When the Patient Breathes
Assist- control
SIMV
Pressure support
CPAP
27. 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
28. 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
29. 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
30. 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!!
31. 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
32. Progressive Withdrawal of PSV
Paw
PSV= 0 PSV= 5
Peso PSV=10 PSV=15
Yamada et al., J Appl Physiol ’94;77:2237
33. 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
34. 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
35. PSV with High Resistance:
Dissynchrony
Backup cycling criteria: pressure, time
Branson, Resp Care 1998; 43:1045
36. 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
37. 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’?
38. 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
39. PAV
In the US version,
the support is
expressed as % of
the work of
breathing, calculated
from periodical
measurement of R
and E
40. What We Have NOT Discussed
Bedside
measurement of respiratory
mechanics
Loops
Triggering
Missed triggering
Tube
compensation
NAVA
Emerson Iron Lung
41. 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
42. 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
46. 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
47. 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