2. PHYSIOLOGY
OF
MECHANICAL VENTILATION
Dr Deepa C MD
3. The Origin of Mechanical Ventilation
“But that life may…be restored to the animal, an opening
must be attempted in the trunk of the trachea, in which a
tube of reed or cane should be put; you will then blow into
this, so that the lung may rise again and the animal take in
air. …and as I do this, and take care that the lung is inflated
in intervals, the motion of the heart and
arteries does not stop…”
Andreas Wesele Vesalius, 1543
4. RESPIRATORY SYSTEM
• Ventilating pump
- Respiratory control centres in the brain
- Connecting tracts and nerves
- Chest wall and respiratory muscles
• Gas-exchange system - Lungs
8. SPONTANEOUS
Patm Palv ∆P Flow
BREATHING
Inspiration 0 -1 +1 Into lungs
End-inspiration 0 0 0 No flow
Expiration 0 +1 -1 Out of lungs
End-expiration 0 0 0 No flow
POSITIVE PR.
Pinsp Palv ∆P Flow
VENTILATION
Inspiration 20 0 +20 Into lungs
End-inspiration 20 20 0 No flow
Expiration 0 20 -20 Out of lungs
End-expiration 0 0 0 No flow
9. TRANSMURAL PRESSURE
“Pressure across the wall”
Difference in pressure between the inside (Pi) and the
outside (Po) of any structure
EQUILIBRIUM VOLUME of a structure
The volume it contains when the transmural pressure
(Pi - Po) is zero
10.
11. LUNG COMPLIANCE (DISTENSIBILITY)
- change in volume per unit change in pressure - ∆V/∆P
Static compliance = Tidal volume
Pplat – PEEP
(measured when there is no air flow)
Dynamic compliance = Tidal volume
Ppeak – PEEP
(measured when air flow is present)
12.
13. ELASTANCE
The retractive (recoil) force generated by the
recoil of an elastic structure
Inversely related to compliance
A less compliant lung has higher elastance
14. RESISTANCE
Resistance = ∆P/Flow
Inversely proportional to R⁴
WORK OF BREATHING - work performed by
the respiratory muscles in stretching the
elastic tissues of the chest wall and lungs
(elastic work – 65%), moving inelastic
tissues(7%) and moving air through the
respiratory passages(28%)
15. Pplat & Ppeak
• PLATEAU PRESSURE - is the pressure needed to
maintain lung inflation in the absence of air flow
• Measured by occluding the ventilator 3-5 sec at
the end of inspiration
• PEAK INSPIRATORY PRESSURE - Pressure used to
deliver the tidal volume by overcoming non-
elastic (airways) and elastic (lung parenchyma)
resistance
19. A. Increased Airway B. Decreased Compliance
Resistance of Lungs & Chest Wall
PIP PIP
Pplat
Pplat
20.
21. Tidal Volume = Inspiratory Flow x Inspiratory Time
(ml) (ml/sec) (sec)
∆ Pressure = Inspiratory Flow x Resistance
∆ Pressure = Ppeak – Pplat
22. DEAD SPACE & SHUNT
DEAD SPACE – wasted ventilation
(no gas exchange due to absent perfusion)
eg.; pulmonary embolism
SHUNT – wasted perfusion
eg.; atelectatic segment, one-lung
ventilation
23. POSITIVE END EXPIRATORY
PRESSURE (PEEP)
Increases the end expiratory or baseline airway
pressure to a value greater than atmospheric pr. on
ventilator manometer
INDICATIONS
• Intrapulmonary shunt and refractory hypoxemia
• Decreased FRC and lung compliance
• Useful in maintaining pulmonary function in non-
cardiogenic pulmonary edema, especially ARDS
24. PHYSIOLOGY OF PEEP
Opens up collapsed alveoli and prevents alveolar
collapse during exhalation
PEEP
Decreases alveolar distending pressure
Increases FRC by alveolar recruitment
Improves ventilation
Increases V/Q, improves oxygenation,
decreases work of breathing
25. • Prevents early airway closure and alveolar
collapse at the end of expiration
• Increases(and normalizes) the functional
residual capacity (FRC) of the lungs
• Facilitates better oxygenation
Note: PEEP is intended to improve
oxygenation, not to provide ventilation, which
is the movement of air into the lungs followed
by exhalation
26. Increases surface area for gas exchange by opening the
collapsed alveoli
Translocation of fluid to peribroncheal region in pulm
edema
27. DISEASES WHERE PEEP IS USED
• ARDS/ALI
• Cardiogenic pulmonary edema
• Unilateral lung ventilation & postop hypoxemia
• COPD
COMPLICATIONS ASSOCIATED WITH PEEP
• Barotrauma
• Diminish cardiac output
• Regional hypoperfusion
• Augmentation of I.C.P.
• Paradoxical hypoxemia
• Hypercapnoea and respiratory acidosis
42. MONITORING
Physical examination for all body
systems focusing on the resp system
Assess the patient for decreased cardiac
output
Administer a sedative as ordered to relax
the patient
Evaluate the settings of mechanical
ventilator
Ensure patient safety ( side rails )..
Genesis 2:7“…..& then the Lord…..breathed into hisnostrils the breath of life and man became a living being”
Spontaneous breathing – it’s a negative pressure that sucks the air into the lungs during inspiration and results in expansion.PPV – it’s the external positive pressure applied into the airway which forces the alveoli to expand during inspiration
The transmural pressure or the pressure across the wall is the difference in pressure between the inside (Pi) and the outside (Po) of any structure. The equilibrium volume of a structure is defined as the volume it contains when the transmural pressure (Pi - Po) is zero.The transpulmonarypressure or transalveolar pressure is the transmural pressure across the lungs, i.e., Palv – Ppl.The transthoracic pressure is the transmural pressure across the chest wall, i.e., Ppl – Patm.The transrespiratory pressure is the transmural pressure across the entire respiratory system, i.e., Palv – Patm. But it’s Pao-Patm in mechanically ventillated patients.The transairway pressure produces movement in the airways. i.e., Pao – Palv . Pao is the pressure at the airway opening.
Static compliance reflects the compliance of chest wall and lungs.Dynamic compliance reflects the compliance of both airways and, lungs and chest wall.
Lung inflation occurs when the transpulmonary pressure exceeds the elastic forces and a patent airway is present. The lung increases in volume until the elastance force balances the transpulmonary pressure and the volume becomes constant.
Resistance in spontaneously breathing normal adult is 0.6 – 2.4 cm H₂O/L/sec. Resistance on ventilator is much more & depends upon the size and length of ET tube, airway reactivity and circuits.
Controlled breathAssisted breathControlled breaths with PEEP