Physiology of Mechanical Ventilation

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The very basics of physiology of mechanical ventilation


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Presentation Transcript

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“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…” The Origin of Mechanical Ventilation Andreas Wesele Vesalius, 1543

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RESPIRATORY SYSTEM Ventilating pump - Respiratory control centres in the brain - Connecting tracts and nerves - Chest wall and respiratory muscles Gas-exchange system - Lungs

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Intrapleural -10 cm H₂O Intrapleural -2.5 cm H₂O

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POSITIVE PRESSURE VENTILATION Inflate the lungs by exerting positive pressure on the airway  forcing the alveoli to expand during inspiration

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SPONTANEOUS BREATHING Patm Palv ∆P Flow 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. VENTILATION Pinsp Palv ∆P Flow 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

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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

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LUNG COMPLIANCE (DISTENSIBILITY) - change in volume per unit change in pressure - ∆V/∆P Static compliance = Tidal volume P plat – PEEP (measured when there is no air flow) Dynamic compliance = Tidal volume P peak – PEEP (measured when air flow is present)

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ELASTANCE The retractive (recoil) force generated by the recoil of an elastic structure Inversely related to compliance A less compliant lung has higher elastance

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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%)

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P plat & P peak 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

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Increased Airway Resistance Decreased Compliance of Lungs & Chest Wall PIP Pplat PIP Pplat A . B .

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Tidal Volume = Inspiratory Flow x Inspiratory Time (ml) (ml/sec) (sec) ∆ Pressure = Inspiratory Flow x Resistance ∆ Pressure = Ppeak – Pplat

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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

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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 POSITIVE END EXPIRATORY PRESSURE (PEEP)

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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

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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

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Increases surface area for gas exchange by opening the collapsed alveoli Translocation of fluid to peribroncheal region in pulm edema

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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

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Indications for mechanical ventilation Ventilatory failure Oxygenation failure Excessive ventilatory workload Impending respiratory failure

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VENTILATORY FAILURE Drug overdose Spinal cord injury Head injury & stroke Neuromuscular dysfunction Sleep disorders Acute airflow obstruction Chest trauma Postoperative – thoracic & upper abdominal Electrolyte imbalance General anaesthesia

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OXYGENATION FAILURE & INCREASED VENTILATORY WORKLOAD Acute lung injury/ARDS Acute severe airflow obstruction Dead space ventilation Shunts Congenital heart diseases Shock High metabolic rate & Obesity General anaesthesia & Postop

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Maintain patient comfort Allow a normal, spontaneous breathing pattern whenever possible Maintain a PaCO₂ between 35 - 45 mmHg Maintain a PaO₂ sufficient to meet cellular O₂ demands but avoid oxygen toxicity Avoid acid-base and electrolyte imbalances Avoid respiratory muscle fatigue and atrophy GOALS OF MECHANICAL VENTILATION

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EFFECTS OF POSITIVE PRESSURE VENTILATION Decrease in venous return Decrease in cardiac output Decrease in pulmonary capillary blood flow Increase in pulmonary vascular resistance Increase in central venous pressure Increased intracerebral venous pressure Decreased CSF absorption Increased intraabdominal pressure

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Increased vasopressin secretion Decreased GFR & urine output Increased fluid retention Paradoxical fall in PaO₂ Barotrauma/volutrauma Ventilator-associated pneumonia Oxygen toxicity Prolonged intubation – airway problems Pressure sores Ventilator dependence Contd…

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Airflow obstruction Low I:E ratios Increased respiratory rate Low flows Leads to dynamic hyperinflation AUTO-PEEP or INTRINSIC PEEP

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DYNAMIC HYPERINFLATION Expiratory flow obstruction Increased rate Decreased expiratory time

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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 ).. MONITORING

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Thank you

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