Mechanical ventilation Dr Samir Sahu Senior Consultant, Critical Care & Pulmonology Apollo Hospitals, Bhubaneswar
Modes • Assist control/Volume Control • Pressure control • Pressure support • Synchronized intermittent mandatory ventilation
Conventional Modes of Ventilatory Support The traditional modes of mechanical ventilation—Flow-regulated volume Assist Control (“Volume Control”, AMV, AC)) or Pressure-Targeted Assist Control (“Pressure Control”), Synchronized Intermittent Mandatory Ventilation (SIMV)—with flow or pressure targeted mandatory cycles), Continuously Positive Airway Pressure (CPAP) and Pressure Support can be used to manage virtually any patient when accompanied by adequate sedation and settings well adjusted for the patient’s needs. Their properties are discussed in the “Basic Mechanical Ventilation” unit of this series.
Pressure and Volume Targeted Ventilation VT Marini, Wheeler. Crit Care Med. The Essentials. 1997.
Positive Airway Pressure Can Be Either Pressure or Flow Controlled—But Not Both Simultaneously Dependent Variable Set Variable Set Variable Dependent Variable
Assist Control/Volume Control • Patient does ONLY the work necessary to “trigger” the vent. • Typically, minimal (i.e., 2 cm H2O) • TV is always the “set” TV. • Overall, a very good mode for resting the patient.
Assist control • Nomeclature • Volume control (Maquet) • IPPV (Drager) • S-CMV (Hamilton)
Assist control • Set tidal volume • Inspiratory-expiratory cycling
Assist Control/Volume Control “Set” TV
Assist control • Breaths: • Ventilator initiated (control breaths) • Patient initiated (assist breaths) • Set minimum frequency • Characteristics of each inspiration are the same • Not affected by whether breath is control breath or an assist breath
Assist control 50 cmH2O Pressure 70 l/min Flow -70 700 ml Volume
Assist control T 50 cmH2O Pressure 70 l/min Flow -70 700 ml Volume
Assist control • Set • Minimum respiratory rate • Patient’s spontaneous respiratory rate < set rate ventilator gives additional control breaths to make up difference • Patient’s spontaneous rate > set rate no control breaths
Volume control • Set • Minimum respiratory rate • Tidal volume
Volume control • Set • Minimum respiratory rate • Tidal volume • Inspiratory time • Inspiratory pause time
Volume control • Longer inspiratory time • Improved oxygenation • Higher mean airway pressure • Re-distribution • Lower peak airway pressure • More time available to deliver set tidal volume • Shorter inspiratory time • Less risk of gas trapping and PEEPi • Less effect on cardiovascular system
Setting I:E, inspiratory flow time, pause time • Nomenclature Volume Time
Setting I:E, inspiratory flow time, pause time • Nomenclature Inspiratory flow time Volume Time
Setting I:E, inspiratory flow time, pause time • Nomenclature Inspiratory pause time Volume Time
Setting I:E, inspiratory flow time, pause time • Nomenclature Inspiratory time Volume Time
Setting I:E, inspiratory flow time, pause time • Nomenclature Inspiratory time Expiratory time Volume Time
Setting I:E, inspiratory flow time, pause time • Nomenclature Respiratory cycle time Volume Time
Example A:I:E as a ratio & inspiratory pause time as a percentage Respiratory cycle = 6 secs 1 part 2 parts 10% Volume Set: RR, I:E, Pause time % Time
Example A:I:E as a ratio & inspiratory pause time as a percentage 6 secs 2 secs 4 secs 1 part 2 parts 10% 0.6sec Volume Set: RR, I:E, Pause time % Time
Example A:I:E as a ratio & inspiratory pause time as a percentage 3 secs 1 2 Respiratory rate 10% .3s Volume Inspiratory flow rate Set: RR, I:E, Pause time % Time
Volume control • Set • Minimum respiratory rate • Tidal volume • Inspiratory time or I:E ratio • Directly/indirectly • Inspiratory pause time • Directly/indirectly • PEEP
Assist Control/Volume Control Peak Pressure depends upon TV and lung mechanics
Triggering • The trigger sensitivity determines how easy it is for the patient to trigger the ventilator to deliver a breath. • Increased sensitivity is preferable in order to improve patient-ventilator synchrony (i.e. to stop the patient "fighting" the ventilator)
Triggering • Excessively high sensitivity may result in false or auto-triggering (i.e. ventilator detects what it "thinks" is an attempt by the patient to breath even when the patient is not).
Triggering • Triggering • Flow • Pressure • Trigger sensitivity • sensitivity preferable • Flow triggering general more sensitive than pressure triggering • flow or pressure sensitivity
ModeVolume Control Automode Admitpatient Nebulizer Status Alarmprofile Set ventilation mode 12-25 15:32 Volume control Automode Ti =1.33 s (33%) Save Trends i Basic I:E Trigger . Tidal volume 500 I:E1:2.0 Trigger sensitivityV Resp. Rate14 T. pause10 Quickstart PEEP5 T. Insp. rise5 Menu Mainscreen O2 conc.100 . Additionalvalues Cancel Accept ! Startbreath O2breaths Exp.hold Insp.hold 41 Charles Gomersall 2003
Advantages Relatively simple to set Guaranteed minimum minute ventilation Rests muscles of respiration (if properly set) Disadvantages Not synchronized Patient may “lead” ventilator Inappropriate triggering may result in excessive minute ventilation lung compliance alveolar pressure with risk of barotrauma Often requires sedation to achieve synchrony. Assist control
Pressure control • Pressure preset assist/control ventilation • Similar to volume control except • pressure is preset • inspiratory time or I:E ratio set directly • no pause time
PC above PEEP Pressure PEEP Time Flow Time Volume Time
Pressure Control Ventilation Specifically Set Inspiratory Time Reduced Expiratoy Time
Pressure Control Ventilation Peak Pressure depends solely upon the amount of pressure applied.
Pressure Control Ventilation • May used as initial setting: • Reserved for refractory hypoxemia. • MAY increase gas exchange. • Limits Peak Pressure. • Prolonged Inspiratory Time results in Reduced Expiratory Time • Potential for auto-PEEP
PCV: Key Parameter to Monitor is VT What Causes a Decreased VT During PCV? • Change in mechanics • airway resistance: • . e.g., bronchospasm • respiratory system compliance . .e.g, pulmonary edema, pneumothorax • AutoPEEP • expiratory resistance • expiratory time • e.g., rate • Inspiratory time • e.g., rate if I:E ratio constant
0.35 s 0.5 s Normal Patient ARDS: Volume/Time Curve (decreased t) Inspiration Volume (cm H2O) Time (sec) Expiration
Normal inspiratory time Short inspiratory time PC above PEEP Pressure PEEP Time Flow Time Volume Time
Advantages Relatively simple Avoids high inspiratory pressures Rests muscles of respiration Improved oxygenation Disadvantages Not synchronized Inappropriate triggering may excessive minute ventilation Change in lung compliance or resistance change in tidal volume Often requires sedation Pressure control
Pressure support • Nomenclature • Inspiratory assist • Assisted spontaneous breathing
Pressure Support Ventilation Peak Pressure depends solely upon the amount of Pressure Support
Pressure Support Ventilation • Patient does a VARIABLE amount of work of breathing: • If “adequate” pressure, work is limited to simply that required to trigger. • TV depends upon the combination of patient effort/lung mechanics AND the amount of pressure applied. • Overall, CAN achieve rest if administer enough pressure. • NO back up rate: • Not appropriate if fluctuating level of mental status. • Limits Peak Pressures
Maximum inspiratoryflow Set % of maxinspiratory flow Pressure support PS above PEEP Pressure PEEP Flow Volume
Pressure Support ‘off-switch’ is a set flow value or a set % of peak inspiratory flow. The patient with airflow obstruction may need to put on the brake with muscular effort to slow flow quickly enough to satisfy his intrinsic neural timing.