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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
Physiology of mechanical ventilation upload
PHYSIOLOGY OFMECHANICAL VENTILATION Dr Deepa C MD
The Origin of Mechanical Ventilation“But that life may…be restored to the animal, an openingmust be attempted in the trunk of the trachea, in which atube of reed or cane should be put; you will then blow intothis, so that the lung may rise again and the animal take inair. …and as I do this, and take care that the lung is inflatedin intervals, the motion of the heart andarteries does not stop…” Andreas Wesele Vesalius, 1543
RESPIRATORY SYSTEM• Ventilating pump - Respiratory control centres in the brain - Connecting tracts and nerves - Chest wall and respiratory muscles• Gas-exchange system - Lungs
Intrapleural-10 cm H₂O Intrapleural -2.5 cm H₂O
POSITIVE PRESSURE VENTILATIONInflate the lungs by exerting positive pressureon the airway forcing the alveoli to expandduring inspiration
SPONTANEOUS Patm Palv ∆P Flow BREATHING Inspiration 0 -1 +1 Into lungsEnd-inspiration 0 0 0 No flow Expiration 0 +1 -1 Out of lungsEnd-expiration 0 0 0 No flowPOSITIVE PR. Pinsp Palv ∆P FlowVENTILATION Inspiration 20 0 +20 Into lungsEnd-inspiration 20 20 0 No flow Expiration 0 20 -20 Out of lungsEnd-expiration 0 0 0 No flow
TRANSMURAL PRESSURE “Pressure across the wall”Difference in pressure between the inside (Pi) and theoutside (Po) of any structure EQUILIBRIUM VOLUME of a structureThe volume it contains when the transmural pressure(Pi - Po) is zero
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)
ELASTANCEThe retractive (recoil) force generated by therecoil of an elastic structureInversely related to complianceA less compliant lung has higher elastance
RESISTANCE Resistance = ∆P/Flow Inversely proportional to R⁴WORK OF BREATHING - work performed bythe respiratory muscles in stretching theelastic tissues of the chest wall and lungs(elastic work – 65%), moving inelastictissues(7%) and moving air through therespiratory passages(28%)
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
A. Increased Airway B. Decreased Compliance Resistance of Lungs & Chest Wall PIP PIP Pplat Pplat
Tidal Volume = Inspiratory Flow x Inspiratory Time (ml) (ml/sec) (sec) ∆ Pressure = Inspiratory Flow x Resistance ∆ Pressure = Ppeak – Pplat
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
POSITIVE END EXPIRATORY PRESSURE (PEEP)Increases the end expiratory or baseline airwaypressure to a value greater than atmospheric pr. onventilator manometerINDICATIONS• Intrapulmonary shunt and refractory hypoxemia• Decreased FRC and lung compliance• Useful in maintaining pulmonary function in non- cardiogenic pulmonary edema, especially ARDS
PHYSIOLOGY OF PEEPOpens up collapsed alveoli and prevents alveolarcollapse during exhalation PEEP Decreases alveolar distending pressure Increases FRC by alveolar recruitment Improves ventilation Increases V/Q, improves oxygenation, decreases work of breathing
• 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 improveoxygenation, not to provide ventilation, whichis the movement of air into the lungs followedby exhalation
Increases surface area for gas exchange by opening the collapsed alveoliTranslocation of fluid to peribroncheal region in pulm edema
DISEASES WHERE PEEP IS USED • ARDS/ALI • Cardiogenic pulmonary edema • Unilateral lung ventilation & postop hypoxemia • COPDCOMPLICATIONS ASSOCIATED WITH PEEP • Barotrauma • Diminish cardiac output • Regional hypoperfusion • Augmentation of I.C.P. • Paradoxical hypoxemia • Hypercapnoea and respiratory acidosis
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 )..