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 (VE)= total gas exhaled per minute

• VE= 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