D. Sara Salarian,

Nov 2006 Kishore P. Critical Care Conference  Improve oxygenation  Increase/maintain minute ventilation and help CO 2 clearance  Decrease work of breathing  Protect airway

Mask based device Negative pressure ventilators “The Iron Lung”

Origins of mechanical ventilation Negative-pressure ventilators (“iron lungs”) Non-invasive ventilation first used in Boston Children’s Hospital in 1928 Used extensively during polio outbreaks in 1940s – 1950s Positive-pressure ventilators Invasive ventilation first used at Massachusetts General Hospital in 1955 Now the modern standard of mechanical ventilation The era of intensive care medicine began with positive-pressure ventilation The iron lung created negative pressure in abdomen as well as the chest, decreasing cardiac output. Iron lung polio ward at Rancho Los Amigos Hospital in 1953.

 Spontaneous/Controlled/Dual  Controlled Mechanical Ventilation (CMV)  Assist Control (AC)/Volume Control (VC)  Intermittent Mandatory Ventilation (SIMV)  Pressure Control (PCV)  Pressure Support Ventilation (PSV)

Nov 2006 Kishore P. Critical Care Conference Assist control Assisted-pressure supportControlled MV Spontaneous SIMV IMV +PEEP CPAP

Breath Types 1.Spontaneous Breath Inspiration is both initiated and terminated by the patient. Inspiration is both initiated and terminated by the patient. 2.Mandatory Breath Inspiration is either initiated or terminated by the ventilator. Inspiration is either initiated or terminated by the ventilator. >>>> <<<<

Breath Patterns 1.Continuous Mandatory Ventilation CMV CMV All breaths mandatory All breaths mandatory 2.Intermittent Mandatory Ventilation IMV or SIMV IMV or SIMV Mandatory and spontaneous breaths Mandatory and spontaneous breaths 3.Continuous Spontaneous Ventilation All breaths spontaneous All breaths spontaneous >>>> <<<<

Phase Variables TRIGGER starts inspiration Example: pressure drop when patient sucks in LIMIT preset inspiratory value Example: preset maximum inspiratory flow CYCLE stops inspiration Example: preset inspiratory time >>>> <<<<

 the controlled variables of tidal volume and inspiratory flow determine airway pressure and inspiratory time  Variations in airway resistance or lung compliance alter airway pressures but do not affect minute ventilation  There are three methods of initiating the inspiratory phase in volume-cycled mechanical ventilators: controlled, assist-control, and intermittent mandatory ventilation (IMV)

Nov 2006 Kishore P. Critical Care Conference  Ventilator applies a predefined target pressure to the airway during inspiration  Adv. - decreased risk of barotrauma  Disadv. - with decreasing compliance or increasing resistance, tidal volume and minute ventilation fall

minute ventilation is completely dependent upon the rate and tidal volume set on the ventilator. Any respiratory efforts made by the patient do not contribute to minute ventilation Controlled ventilation is the required ventilatory mode in patients who are making no respiratory effort (eg, spinal cord injury or drug overdose and those who have been subjected to pharmacologic paralysis).

 Advantages: rests muscles of respiration  Disadvantages: requires sedation/neuro- muscular blockade, potential adverse hemodynamic effects  Advantages: rests muscles of respiration  Disadvantages: requires sedation/neuro- muscular blockade, potential adverse hemodynamic effects

Press ure Flow Volume (L/min) (cm H 2 O) (ml) Time (sec) Time- Time-CycledSet PC level Time Triggered, Pressure Limited, Time Cycled Ventilation

Flow Pressure Volume ClClClCl ClClClCl Set PC level Time (sec) (L/min) (cm H 2 O) (ml)

Flow Pressure Volume Time (sec) (L/min) (cm H 2 O) (ml)

 In the assist-control (A/C) mode, the ventilator senses an inspiratory effort by the patient and responds by delivering a preset tidal volume. Every inspiratory effort that satisfies the ventilator's demand valve trigger threshold initiates delivery of the preset tidal volume  Patient work is therefore required to trigger the ventilator and continues during inspiration  A control mode back-up rate is set on the ventilator to prevent hypoventilation

 Volume or time-cycled breaths + minimal ventilator rate  Additional breaths delivered with inspiratory effort  Order: AC Vt 500, RR12, 100% FiO 2, 5 PEEP

 Advantages: reduced work of breathing; allows patient to modify minute ventilation  Disadvantages: potential adverse hemodynamic effects or inappropriate hyperventilation

 With intermittent mandatory ventilation (IMV), the degree of ventilatory support is determined by the selected IMV rate. At regular intervals, the ventilator delivers a breath based upon a preset tidal volume and rate. In addition, the patient is allowed to breathe spontaneously through the ventilator circuit at a tidal volume and rate determined according to need and capacity.  Most present day ventilators synchronize the intermittent ventilator breaths with inspiratory effort by the patient, a modality termed synchronized IMV or SIMV. However, this modification requires a trigger modality

 Potential advantages  More comfortable for some patients  Less hemodynamic effects  Potential disadvantages  Increased work of breathing

Spontaneous Breaths Flow (L/m) Pressure (cm H 2 O) Volume (mL)

Press ure Flow Volume (L/min) (cm H 2 O) (ml) SIMV + PS (Pressure-Targeted Ventilation) PS Breath Set PS level Set PC level Time (sec) Time-Cycled Flow-Cycled

 Pressure support ventilation (PSV) is flow- cycled in that, once triggered by a demand valve, the preset pressure is sustained until the inspiratory flow tapers, usually to 25 percent of its maximal value [22]. PSV tends to be a comfortable ventilatory modality because the patient has greater control over ventilator cycling and flow rates. Close monitoring is required whenever PSV is used alone because neither tidal volume nor minute ventilation is guaranteed. PSV can be added during full or partial support with SIMV to overcome endotracheal tube and ventilator circuitry resistance encountered during spontaneous breaths

 Pressure assist during spontaneous inspiration with flow-cycled breath  Pressure assist continues until inspiratory effort decreases  Delivered tidal volume dependent on inspiratory effort and resistance/compliance of lung/thorax Order: PS 10, PEEP 0, 50% FiO 2

 Potential advantages  Patient comfort  Decreased work of breathing  May enhance patient-ventilator synchrony  Used with SIMV to support spontaneous breaths

 Potential disadvantages  Variable tidal volume if pulmonary resistance/compliance changes rapidly  If sole mode of ventilation, apnea alarm mode may be only backup  Gas leak from circuit may interfere with cycling

Set PS level CPAP level Time (sec) Flow (L/m) Pressure (cm H 2 O) Volume (mL) Flow Cycling

 Ventilatory support requires consideration of trigger mode and sensitivity, respiratory rate, tidal volume, flow rate, flow pattern, and the fraction of inspired oxygen (FiO2).

 Mode  Rate  Volume (V T )  Pressure  FIO 2  PEEP  I:E

Nov 2006 Kishore P. Critical Care Conference  Determinants of CO 2 clearance - Ventilator factors * Rate * Tidal volume * Anatomical dead space - Patient factors * Physiological dead space * CO 2 production Alveolar minute ventilation

Nov 2006 Kishore P. Critical Care Conference  Determinants of Oxygenation - Ventilator factors: * FiO 2 ( fraction of oxygen in inspired air ) * Mean airway pressure * PEEP ( positive end expiratory pressure ) - Patient factors * V/Q ( ventilation/ perfusion ) mismatch * Shunt * Diffusion defect * Reduced mixed venous oxygen

Nov 2006 Kishore P. Critical Care Conference  Adjust FiO 2 and PEEP according to PaO 2 and SpO 2  Adjust TV and rate according to PCO 2 and pH

 PEEP : an elevation in alveolar pressure above atmospheric pressure at the end of exhalation  Extrinsic PEEP (ePEEP): applied through a mechanical ventilator ACV without PEEP ACV with PEEP

Normal Flow does not return to zero - Auto-PEEP

Inspiration ExpirationNormalPatient Time (sec) Flow (L/min) Air Trapping Auto-PEEP }

Volume (ml) PEFR FRC Inspiration Expiration Flow (L/min) PIFR VTVT

Inspiration Expiration Volume (ml) Flow (L/min) Does not return to baseline Normal Abnormal

Inspiration Expiration Volume (ml) Flow(L/min) Air Leak in mL Normal Abnormal

Inspiration Expiration Volume (ml) Flow(L/min) NormalAbnormal Origins