This presentation will help to understand basic principals of pulmonary gas exchange initial selection of conventional positive pressure ventilator settings and settings adjustments

1. PRINCIPALS OF CONVENTIONAL PPV
Sergey Shushunov, MD
I have no commercial interests related to this presentation

2. RESPIRATTORY SYSTEM FUNCTION
Removal of carbon dioxide from blood
Oxygenation of blood

3. RESPIRATTORY SYSTEM FAILURE
Inability to remove carbon dioxide
Inability to oxygenate blood
Both

4. ORGAN / SYSTEM FUNCTION ETIOLOGY EXAMPLES
CNS Control
head injury, CNS infection, seizures, drugs, toxins,
liver and kidney failure, hypoglycemia, metabolic
PNS trauma, neuropathy, toxins, metabolic, drugs
Skeletal Mechanical trauma, deformities
Muscular
atrophy, myopathy, myolysis, NMB, electrolyte
imbalance, tetanus
Respiratory
upper airways
Gas exchange
croup syndrome, foreign body, tumor, mass,
laryngotracheomalacia, trauma, peripheral neuropathy
Respiratory
lower airways
RAD, asthma, COPD, bronchiolitis,
Respiratory
alveolar space
pneumonia, ARDS, NRDS, pulmonary edema,
aspiration, pulmonary contusion/hemorrhage, fibrosis
Gas delivery Gas delivery CHF, PHT, PE, pneumothorax, cardiac tamponade

5. Takes place in alveolar space
Depends on:
• Partial pressure gradient
• Alveolar surface area (mean of I and E surface area)
• Alveolar wall thickness
• Exposure time
• Gas diffusion coefficient (physical property of a gas independent
from any intervention)

6. CO2 REMOVAL
Depends on
• Partial pressure gradient across alveolar-
capillary membrane
• CO2 has 200 times higher than O2 diffusion
coefficient and 20 times higher diffusion
ability therefore alveolar surface area, wall
thickness and exposure time are less relevant

7. CO2 REMOVAL
• diffusion across alveolar-capillary membrane
takes place during inspiration.
• Alveoli don’t have to remain opened during
expiration for optimal CO2 removal
• CO2 removal is decreased in:
Hypoventilation (control, mechanical or extreme small airway
disease)
Increased dead space
• Alveolar disease does not usually lead to CO2
retention

8. CO2 REMOVAL
dead space
• Mechanical - different extensions of ventilator
tubing
• Anatomical - airways
• Alveolar - nonperfused alveolar space

9. CO2 REMOVAL
• Direct relationship with MV
• MV is required to maintain
desired PP gradient
• MV (PaCO2)is controlled by
TV (limited by over-distension)
RR (limited by auto PEEP)

10. CO2 REMOVAL
auto PEEP
• May lead to dynamic
hyperinflation
• Complications
include barotrauma
and hypovolemic
shock

11. Depends on
• Partial pressure gradient
• Alveolar surface area
• Alveolar-capillary membrane thickness
• Diffusion exposure time
• Oxygenation is decreased in
alveolar disease
large airway disease
small airway disease

12. OXYGENATION
• Normally O2 diffusion across alveolar-capillary
membrane takes place during inspiration and
expiration.
• Alveolar collapse during expiration shortens
diffusion exposure time
• Alveoli must remain opened during expiration
for best oxygenation

13. OXYGENATION
• Direct relationship with FiO2
(limited by 100%)
• Direct relationship with
Mean Airway Pressure

14. CONTROL OF MEAN AIRWAY
PRESSURE
Pressure
Time
PEEP
PIP
Increase I. Time
Increase
PEEP
Increase PIP
(TV)
I. Time E. Time
Increase
% Rise
(flow)
%Rise
I. Time E. Time
% Rise (flow)
PIP – limited by over distention
I. Time - limited by auto PEEP
PEEP – limited by over distention

15. ALVEOLAR DISEASE
• Accumulation of intra and extra alveolar fluid
• Thickening of alveolar-capillary membrane
• Alveolar collapse during expiration

16. EFFECT OF POSITIVE AIRWAY PRESSURE
redistribution of intra alveolar fluid

17. EFFECT OF POSITIVE AIRWAY PRESSURE
increase of alveolar surface area
reduction of thickness of alveolar-capillary membrane

18. RESPIRATORY FAILURE
Categories
• Neither alveolar nor small airway disease
• Small airway disease
• Alveolar disease
• Mixed small airway and alveolar disease

19. • Reasonable guess
• Patient’s age and size
• Cause of respiratory failure
• Degree of respiratory failure

20. PPV SETTINGS
• Mode of PPV
• PEEP
• FiO2
• TV / PIP
• Respiratory Rate
• I. Time

21. • Category (or etiology) of respiratory failure
• Degree of illness
• Respiratory effort

22. MODES OF CONVENTIONAL PPV
(BREATH DELIVERY)
• Controlled (Assist Controlled) PC or VC
Patients with no respiratory drive (NMB, anesthesia, coma)
Weaning is not imminent
• IMV (SIMV) PC or VC
All modern ventilation offer only SIMV
Patients with or without respiratory drive
• Support (PS or VS)
Patient with good respiratory drive
Moderate degree of respiratory failure
• Mixed (usually SIMV + PS or VS)
Patients with good respiratory drive
Preferred weaning mode

23. CMV (AC) vs. IMV (SIMV)
CMV IMV
Mandatory and spontaneous breaths
have identical TV/PIP and I. Time
Breaths vary in volume and I. Time
between mandatory and spontaneous
Hard to wean by decreasing rate Easy to wean: patient takes “own” breaths
with or without PS/VS
Minimal work of breathing Potential for increased work of breathing
with insufficient PS/VS
Feels unnatural. Patient/ventilator
asynchrony is possible in awake patients
and may need sedation +/- NMB
Feels more natural. Easier to synchronize,
requires less sedation and less likely NMB
Significant hyperventilation in agitated
patients is possible
Significant hyperventilation in agitated
patients is less likely
In heavily sedated, comatose and paralyzed patients CMV and IMV work identically

24. CMV (AC) vs. IMV (SIMV)
Assist control IMV

25. INITIAL PEEP SELECTION
Etiology of respiratory failure
Zero in small airway disease
Low (physiological) in no small airway disease / no alveolar disease
High in alveolar disease
Tricky in mixed small airway disease / alveolar disease (bronchiolitis)
Degree of hypoxemia (in alveolar disease)
Additional pathology
Increased ICP - lower
Pulmonary hypertension - lower
Increased intra-abdominal pressure - higher
FiO2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
PEEP 5-6 6-8 8-10 10-12 12-14 14-16 16-18 20-24

26. LIMITATIONS
• Barotrauma
• Pulmonary hypertension
• Increase of anatomical
dead space (CO2)
• Decrease of CO

27. • Pressure vs. Volume
• TV is age independent
• Physiological - patients wit neither small airway
nor alveolar disease or patients with isolated
small airway disease
• Small – patients with alveolar disease
• Selecting TV based on ml/kg may be misleading
• Chest rise

28. PRESSURE vs. VOLUME
PRESSUE VOLUME
TV changes as patient’s compliance and
airway resistance changes
PIP changes as patient’s compliance and
airway resistance changes
Increase of airway resistance, or decrease
of compliance will lead to decrease of TV
and hypoventilation
Increase of airway resistance or decrease
of compliance will lead to increase of PIP
and potentially to volutrauma and
barotrauma, but not to hypoventilation

29. PEAK INSPIRATORY PRESSURE
• Changes of PIP in VC mode is an important parameter
which can be used as the “sixth” vital sign
• Elevation of PIP while patient receives constant TV
always indicates a problem, including mucus
accumulation, pulmonary edema, infiltrate, atelectasis,
pneumothorax, bronchospasm, artificial airway
obstruction
• Decrease of the PIP while patient receives constant TV
always indicate clinical improvement, including
resolution of atelectasis, bronchospasm, infiltrate or
pulmonary edema

30. RESPIRATION RATE SELECTION
main principals
• Respiratory drive and effort
• Desired pCO2
• BMR (BMR and CO2 production is elevated in
sepsis, trauma, burns, etc.)
• Age and weight

31. RESPIRATION RATE SELECTION
age and weight considerations
• Metabolic rate is age
dependent.
• TV per unit of weight is the
same for all ages.
• Newborn has 4 times higher
metabolic rate per unit of
weight than average adult (120
kcal/kg/day vs. 30
Kcal/kg/day).
• Newborn has 4 times higher
CO2 production per unit of
weight than average adult.
• Newborn breathes 4 times
faster than average adult and
requires 4 times faster RR.
• Use ideal body weight.

32. INSPIRATORY TIME SELECTION
• Respiratory Rate
• Desired Expiratory Time to Inspiratory Time ratio
• Presence of small airway disease
• Targeted Mpaw

33. INSPIRATORY TIME SELECTION
• I. Time, E. Time and RR are interlinked
• Example:
RR of 10/min makes respiratory cycle 6 seconds
I. Time set at 1 second makes I:E ratio of 1:5
Changing I. Time to 1.5 seconds makes I:E ratio of 1:3 (1.5:4.5 sec)
Changing RR to 15/min (resp. cycle of 4 sec.) makes I:E ration of 1:3

34. I.TIME, RR and I:E RATIO SELECTION
Wrong selection of I. Time/RR/I:E ration can lead
to auto-peep and air trapping

35. COMPLICTIONS OF AUTO PEEP
• CO2 retention
• Barotrauma
• Decreased CO and shock

36. SUGGETSTED INITIAL VENTILATOR
SETINGS
NO ALVEAOLAR, NO SMALL AIRWAY DISEASE
• Low FiO2
• Usual TV
• Age appropriate rate
• Match ventilator I. Time to patient’s I. Time
• I:E ratio 1:3
• Physiologic PEEP

37. SUGGETSTED INITIAL VENTILATOR
SETINGS
NO ALVEAOLAR, NO SMALL AIRWAY DISEASE
AGE 0-12 M 1-5 Y 6-12 Y ADULT
TV 10-12 8-12 8-12 6-10
RATE 30-40 25-40 15-25 8-15
I. TIME 0.3-0.5 0.5-0.7 0.7-0.9 0.9-1.2

38. SUGGETSTED INITIAL VENTILATOR
SETINGS
SMALL AIRWAY DISEASE
• High FiO2
• Usual TV
• Slow rate
• Age appropriate I. Time
• I:E ratio 4:1 and longer depending on auto PEEP
• 0 PEEP

39. SUGGETSTED INITIAL VENTILATOR
SETINGS
SMALL AIRWAY DISEASE
AGE 0-12 M 1-5 Y 6-12 Y ADULT
TV 10-12 8-12 8-12 6-10
RATE 20-30 15-25 10-20 6-8
I. TIME 0.5 0.5-0.7 0.7-0.9 0.9-1.2

40. VENTILATOR SETINGS ADJUSTMENTS
SMALL AIRWAY DISESE
• To improve oxygenation
Increase flow or decrease rise time
Increase PEEP cautiously to low levels
Increase TV/PIP cautiously (PIP reflects airway resistance
rather than alveolar compliance)
• To improve CO2 removal
Increase PEEP to 5-10 cm (slowly)
Increase E. Time
Increase TV/PIP

41. SUGGETSTED INITIAL VENTILATOR
SETINGS
ALVEOLAR DISEASE
• High FiO2
• Small TV
• Age appropriate rate
• Age appropriate I. Time
• I:E ratio 1:<3
• High PEEP

42. SUGGETSTED INITIAL VENTILATOR
SETINGS
ALVEOLAR DISEASE
AGE 0-12 M 1-5 Y 6-12 Y ADULT
TV 6-12 6-10 4-10 4-8
RATE 30-40 15-25 15-25 8-15
I. TIME 0.3-0.5 0.5-0.7 0.7-0.9 0.9-1.2

43. VENTILATOR SETINGS ADJUSTMENTS
ALVEOLAR DISEASE
• To improve oxygenation
Increase I. Time (Decrease I:E ratio)
Increase PEEP to 30 cm, try recruitment maneuver with PEEP
of 40 cm, aim to keep FiO2 < 0.6
Increase TV/PIP (the least desirable option) for small chest rise
• To improve CO2 removal
Increase RR
Increase TV/PIP

44. PERMISSIVE HYPERCARBIA
(ignored CO2 retention)
• Results from predictably low MV
Decreased RR in small airway disease
Decreased TV in alveolar disease
• Sedation to overcome hypercarbic drive
(35-45 < pCO2 < 80-100)
• Well tolerated by most patients
• Main limitation is arterial pH
• Recommended to limit: pCO2 - 70, pH - 7.2
• Reports of pCO2 in excess of 200
• Increased ICP
• Pulmonary hypertension

45. WEANING PROCESS
• Consider using SIMV + PS
• D/C NMB and wean sedation
• Wean FiO2 to < 0.4
• Wean PEEP to 3-5 cm (higher in obese patients)
• Wean RR to < ½ of “normal”
• Wean PS to 5-10 cm
• Monitor
Work of breathing (RR, use of accessory muscles)
HbO2Sat or PaO2
ET CO2 or PCO2