Mechanical Ventilaton Ramon Garza III, M.D.

Indications Airway instability Most surgical patients or trauma Primary Respirator Failure Mostly medical i.e. ARDS, CHF, COPD

Basic Principles Facilitate Gas exchange for oxygen delivery to tissues Ventilation for removal of carbon dioxide Minimize detrimental effects

Ventilation vs Oxygenation Ventilation= CO2 gas exchange Oxygenation= equilibrium of oxygen tension gradient between alveoli and oxygen in blood

Ventilation Minute Ventilation (V E )= total gas exhaled per minute V E = Tidal Volume x Respiratory Rate

Ventilation Goal of mechanical ventilation is maintaining normal pCO2 Dead space ventilation can inhibit elimination of CO2

Oxygenation Oxygen tension between alveoli and capillaries favors oxygen transfer to blood Most important factor is V/Q matching

Oxygenation Evaluation of Oxygenation by A-a gradient A-a gradient= PAO2-PaO2 Normal PaO2= 90mmHg Normal A-a gradient= (Age+10)/4 P/F ratio is useful to evaluate degree of hypoxemia Normal PaO2/FiO2= 90mmHg/0.21=500

Oxygenation Improve oxygenation by increasing FiO2 or by adjusting mean airway pressure Minute ventilation does NOT change oxygenation *except in extremely low ventilation

Mechanical Ventilation 3 Variables Trigger Limit Cycle Modes and settings are varying combinations of these 3 variables

Trigger Signal that tells ventilator to give a breath Signal comes from Pt Change in flow w/in circuit Change in pressure w/in circuit Time trigger If pt does not initiate breath w/in allotted time- >machine will give breath

Limit Variable Maximal set inspiratory flow or pressure How much “breath” they are going to take Volume control vs Pressure control Volume= flow x time

Cycle Factor that terminates inspiratory cycle Time Flow Pressure Volume

Specific Types of Ventilation

Pressure Support Simplest form of pressure limited ventilation Pt breathing + ventilator support until target pressure Passive exhalation

Pressure Control Differs from pressure support b/c inspiratory time is set by ventilator Can be used in Assist Control or in SIMV Drawback is when lungs have decreased compliance-> Lower Tidal Volume

Intermittent Mechanical Ventilation Only a set number of breaths are supported Can be synchronized to pt’s inspiratory efforts Pt breaths above set number are not supported Most common ventilator mode you will see is SIMV

Mechanical Ventilation in Respiratory Failure After 30min stabilization period check ABG and adjust vent Use pulse oximetry as a guide for adjusting FiO2 and PEEP

Oxygenation Goal of mechanical ventilation is normal pCO2 and oxygen delivery to tissues pO2 of 60mmHg = 90% saturation and is adequate for O2 delivery to tissues

How to increase PaO2? Increase FiO2 Does not work if intrapulmonary shunt present Prolonged high FiO2 can be detrimental to pulmonary function

How to increase PaO2? Change patient to an upright position Increase mean airway pressure Increase PEEP to improve FRC Improves V/Q mismatching Have to balance increasing PEEP to improve oxygenation and risk of decreasing preload *Goal of ventilation is maximize oxygen delivery to tissues

Management of Ventilator Initial settings depend on patient Otherwise healthy post op pt FiO2 30% PEEP 5cm H2O Multiply injured trauma pt FiO2 100% PEEP of 15cm H2O

Mechanical Ventilation Check ABG early (w/in 30min) If sats decrease -> Suction Check ABG CXR to confirm tube placement, assess lungs, check for pneumo Pt may need w/u for PE, MI, etc