A primer on mechanical ventilation modes and uses. Targeted towards residents and nurse practitioners

1. Mechanical
Ventilation
Primer
Firas Rabi, MD

2. Indications
 Airway Compromise
 Airway patency is in doubt
 Loss of gag or cough reflex
 Respiratory Failure
 Hypoxemic – generally, PaO2 ≤ 60 mmHg
 Needs help oxygenating
 Hypercarbic – generally, PaCO2 ≥ 50 mmHg
 Needs help ventilating

3. Origins of Ventilators
 Negative pressure
ventilators
 “Iron” lung
 First used in Boston
in 1928
 Used extensively for
polio

4. Mechanical Ventilators
 Depend on place of employment
 All have similar ventilator patterns, but
may call them different names
 ALL MODES ARE HARMFUL TO THE LUNG 
Goal is always to extubate ASAP.

5. Alveoli (cartoon)

6. Alveoli (actual)

7. Servo 300

8. Servo i

9. High Frequency
Oscillating
Ventilator (HFOV)

10. LTV 1000

11. Ventilator Terminology
 A/C – Assist/Control
 IMV – Intermittent mechanical ventilation
 SIMV – Synchronized IMV
 PRVC – Pressure regulated, volume
control
 PEEP – Positive end-expiratory pressure
 CPAP
 NIPPV

12. Volume vs. Pressure
 Volume control – tidal volume is constant,
pressure will vary
 Pressure control – pressure is constant,
tidal volume will change based on lung
compliance

13. Pulmonary Compliance
 Compliance = Volume/Pressure
 This equation is worth memorizing since it
provides the basis for understanding
pulmonary and ventilator interactions

14. Modes of Ventilation

15. Control Mode
 Each breath has a pre-set volume, time,
and flow rate
 Patient cannot generate spontaneous
breaths

16. Assist/Control Mode
 Each breath has a pre-set volume, time,
and flow rate
 Each patient generated respiratory effort
over and above the set rate are delivered
at the set volume and flow rate

17. Pressure Control
 If the pressure is set at PC 16 above PEEP
of 4, then the ventilator will deliver a top
pressure (peak pressure) of 20 (PC level of
16 plus PEEP) with an end pressure of 4.
 This keeps the airways slightly open,
making it easier to inflate them, and helps
prevent collapse and consolidation

18. SIMV
 Each breath has a pre-set volume, time,
and flow rate
 Allows patient to generate own breaths
with own volumes and flow rates
 If patient initiates a breath, machine will
not initiate a ventilator breath  no
breath stacking

19. PRVC
 Each breath is volume controlled (you set
the tidal volume)
 The machine decides how much pressure
to use to deliver that tidal volume based
on the required pressures for previous 2-3
breaths

20. PRVC
 You get to limit the amount of pressure
used to deliver the volume. If this pressure
is reached, the ventilator will alarm
“regulation pressure limited”, switch to
expiration, and will not be able to deliver
the preset tidal volume

21. Automode (only in servo i)
 Automatically controls the transition
between controlled (vent triggered) and
support (patient triggered) mode in
accordance with patient’s effort
 PC  PS
 VC  VS
 PRVC  VS

22. Pressure Support
 Each patient-initiated breath is supported
by the machine

23. Combo modes
 SIMV VC with PS
 SIMV PC with PS
 SIMV/PRVC with PS

24. PEEP
 Not a specific mode but rather an
adjunct to any of the other modes
 PEEP is the amount of pressure remaining
in the lung at the END of the expiratory
phase.
 Utilized to keep otherwise collapsing lung
units open while hopefully also improving
oxygenation

25. Improving oxygenation
 FiO2
 Simplest maneuver
 Free radical damage with prolonged
exposure to > 60%
 PEEP
 Reverses pulmonary shunting
 Atelectasis, pneumonia, ARDS, CHF,
pulmonary hemorrhage

26. PEEP Side-effects
 Barotrauma
 Diminished cardiac output
 Regional hypoperfusion

27. CPAP
 This IS a mode and simply means that a
pre-set pressure is present in the circuit
and lungs throughout both the inspiratory
and expiratory phases of the breath
 CPAP serves to keep alveoli from
collapsing, resulting in better oxygenation
and less WOB

28. CPAP
 The CPAP mode is very commonly used as
a mode to evaluate the patients
readiness for extubation

29. CPAP vs. BiPAP
 CPAP is essentially PEEP
 BiPAP is PEEP plus pressure support

30. Summary of Modes
Controlled
Modes
Supported
Modes
Combined
Modes
Spontaneous
breaths
VC VS Automode:
VC + VS
CPAP
PC PS Automode:
PC + PS
BiPAP
PRVC Automode:
PRVC + PS
SIMV: VC/PS
SIMV: PC/VS
SIMV:
PRVC/PS

31. Initial Settings
 Decide on mode
 Rate based on age
 Infants  30
 Children  20
 Adolescents  10
 Most can start with tidal volume of 6mL/kg
 PEEP of 5

32. Inspiratory Trigger
 Normally set automatically
 Two modes
 Airway pressure
 Flow triggering

33. I:E Ratio
 Normally 1:2
 Asthma 1:3 or 1:4
 Severe hypoxia 1:1 or 2:1 (inverse ratio)

34. FiO2
 Start 100%
 Target lowest possible for PaO2 >
60mmHg or sat > 90%

35. High Frequency
Ventilation

36. Oxygenation in HFOV
 Oxygenation is primarily controlled by the
Mean Airway Pressure (Paw) and the FiO2
 Mean Airway Pressure is a constant
pressure used to inflate the lung and hold
the alveoli open.
 Since the Paw is constant, it reduces the
injury that results from cycling the lung
open for each breath

37. Ventilation in HFOV
 Generally controlled by frequency
 Increased frequency reduces amount of
time for exhalation, so leads to decreased
ventilation
↑ Hz  ↑ pCO2
 Increased amplitude/power will increase
ventilation
 Not usually the primary reason for using
HFOV

38. Troubleshooting
 Sudden deterioration
 Disconnection
 Obstruction
 Pneumothorax
 Equipment failure
 Anxious patient
 Appropriate mode
 Trigger set appropriately

39. Troubleshooting
 Call the respiratory therapist

40. Thank You
 Questions