MECHANICAL VENTILATION SARASWATI KACHE, M.D. Clinical Assistant Professor
Spontaneous respiration vs. Mechanical ventilation Natural Breathing Negative inspiratory force Air pulled into lungs Mechanical Ventilation Positive inspiratory pressure Air pushed into lungs
Initiate Mechanical Ventilation Hypoxia Hypercarbia Airway protection (Decrease demand in cases of poor cardiac output)
Ventilators: a Schematic
IMPORTANT TERMS TIME I - Time: amount of time spent in inspiration E - Time: amount of time spent in expiration Volume Amount of tidal volume that a patient receives Pressure Measure of impedance to gas flow rate Flow Measure of rate at which gas is delivered
A Few More Terms PEEP = positive end expiratory pressure Pressure maintained in the airways at the end of exhalation Keeps Alveoli from collapsing PIP = peak inspiratory pressure Point of maximal airway pressure Delta P = the difference between PIP – PEEP MAP = mean airway pressure
ICU Ventilator: Evita 4
Types of Ventilation ….
Compliance = Volume Pressure
Volume Ventilation Preset Volume PEEP Rate I-time FiO2 Ventilator Determines Pressure required Advantages Guaranteed minute ventilation More comfortable for patient Draw-backs Large ETT leak Not optimal for poorly compliant lungs
Pressure Ventilation Preset PIP PEEP Rate I-time FiO2 Vent determines Tidal volume given Advantages Provides more support at lower PIP for poorly compliant lungs Draw back Minute ventilation not guaranteed
Volume vs. Pressure
Amount of support to give…
MODES OF VENTILATION Controlled Mechanical Ventilation (CMV) Assist Control (AC) Continuous Positive Airway Pressure (CPAP) Intermittent Mandatory Ventilation (IMV) Synchronized Intermittent Mandatory Ventilation (SIMV) Pressure Support Volume Support Pressure Regulated Volume Control (PRVC)
Assist Control Volume or Pressure control mode Parameters to set: Volume or pressure Rate I – time FiO2
Assist Control Machine breaths: Delivers the set volume or pressure Patient’s spontaneous breath: Ventilator delivers full set volume or pressure & I-time Mode of ventilation provides the most support
SIMV Synchronized intermittent mandatory ventilation Volume or Pressure mode Parameters set: Volume or pressure Respiratory rate I – time FiO2 Pressure support
SIMV Synchronized intermittent mandatory ventilation Machine breaths: d Delivers the set volume or pressure Patient’s spontaneous breath: Set pressure support delivered Mode of ventilation provides moderate amount of support Works well as weaning mode
Pressure Support Parameters set: Pressure support, FiO2 Machine breaths: none ***** Patient’s spontaneous breaths: set pressure support delivered Purposes: Final step prior to extubation Re-train muscle strength
Continuous Positive Airway Pressure (CPAP) Positive airway pressure maintained throughout respiratory cycle: during inspiratory and expiratory phases Can be administered via ETT or nasal prongs
Hypoxia Hypoventilation: decreased alveolar ventilation, i.e. CNS depression Diffusion impairment: abnormality at pulmonary capillary bed Shunt: blood flow without gas exchange Intra-pulmonary Intra-cardiac Ventilation-perfusion mismatch: Both dead space and shunt abnormalities
Treating Hypoxia Increase FiO 2 : >60% toxic to lung parenchyma Increase mean airway pressure PEEP : not too much, not too little PIP I-time
Hypercarbia Decreased minute ventilation Respiratory rate Tidal volume Treatment: Increase respiratory rate: assure I-time not too short as rate increased Increase tidal volume Allow permissive hypercarbia
Pulmonary Disease: Obstructive Airway obstruction causing increase resistance to airflow: e.g. asthma Optimize expiratory time by minimizing minute ventilation Bag slowly after intubation Don’t increase ventilator rate for increased CO 2
HIFI - Theory Resonant frequency phenomena: Lungs have a natural resonant frequency Outside force used to overcome airway resistance Use of high velocity inspiratory gas flow: reduction of effective dead space Increased bulk flow: secondary to active expiration
HIFI - Gas Transport Conventional bulk flow Coaxial flow: different flow directions in central and peripheral air columns Taylor dispersion: gas molecules disperse beyond the bulk flow front
HIFI - Gas Transport Molecular diffusion: gas mixing within alveoli Pendelluft phenomenon: inter- alveolar gas mixing due to impedance differences
HIFI - Advantages Advantages: Decreased barotrauma / volutrauma: reduced swings in pressure and volume Improve V/Q matching: secondary to different flow delivery characteristics Disadvantages: Greater potential of air trapping Hemodynamic compromise Physical airway damage: necrotizing tracheobronchitis Difficult to suction Often require paralysis
HIFI – Clinical Application Adjustable Parameters Mean Airway Pressure: usually set 2-4 higher than MAP on conventional ventilator Amplitude: monitor chest rise Hertz: number of cycles per second FiO2 I-time: usually set at 33%
Scenario #1 The following blood gas is presented to you for a 4yr patient that is now 3hours post-op from an OLT / 24 / 250 / 20 / -4 The ventilator settings are SIMV PC/PS PEEP – 4, Delta P-28, FiO2 – 50%, RR – 12.
Scenario #2 A 8yr female with ALL s/p chemo presents to the PICU with fever and neutropenia 1day prior. She is found with positive blood cultures this AM and got intubated secondary to respiratory failure. It is now 4am and the morning labs show the following ABG: 7.23 / 60 / 58 / 22 / -2 The ventilator settings are SIMV TV - 10cc/Kg, PEEP – 5, PIP – 38, PS – 14, FiO2 – 70%, RR – 20, I-time – 0.7 You go to examine the patient and she is agitated, hypertensive, and with a respiratory rate of 40.
Scenario #3 There is a 6 month old patient that presents with RSV bronchiolitis that progresses to severe disease and the patient is now on a HIFI ventilator. The patient’s ABG is as follows: 7.24 / 58 / 75 / 21 / -3 The ventilator settings are as follows: HIFI with MAP – 20, Amp – 28, Hz – 8, FiO2 – 40%. As you are looking at the chest X-ray, the nurse mentions the patient looks more edematous this evening compared to last night.
References Editors: Rogers MC & Nichols DG. Textbook of Pediatric Intensive Care. Baltimore, Willimams & Wilkins, Cairo JM & Pilbeam SP. Mosby’s Respiratory Care Equipment. St. Louis, Mosby, Evita 4 Intensive Care ventilator, Operating instructions, West JB. Pulmonary Pathophysiology. Baltimore, Willims & Wilkins, 1992.