Basics of Mechanical Ventilation. Dr Shrikanth Srinivasan MD,DNB,FNB,EDIC Consultant, Critical Care Medicine Medanta, The Medicity

Transcription

1 Basics of Mechanical Ventilation Dr Shrikanth Srinivasan MD,DNB,FNB,EDIC Consultant, Critical Care Medicine Medanta, The Medicity

2 Overview of topics 1. Goals 2. Settings 3. Modes 4. Advantages and disadvantages between modes 5. Guidelines in the initiation of mechanical ventilation 6. Common trouble shooting examples with mechanical ventilation

3 Goals of Mechanical Ventilation Improve pulmonary gas exchange Reverse hypoxemia Relieve acute respiratory acidosis Relieve respiratory Distress Decrease oxygen cost of breathing and reverse respiratory muscle fatigue Alter pressure-volume relations Prevent and reverse atelectasis Improve Compliance Prevent further injury Permit lung and airway healing

4 Typical Ventilator Settings FIO2 Rate Volume PIP and PEEP Flow rate, I-time, I:E Ratio Sighs, inspiratory pause Mode??? Alarms Different brands of ventilators have different control layouts, but they all accomplish essentially the same basic functions.

5 FIO2 The lowest possible fraction of inspired oxygen (FiO2) necessary to meet oxygenation goals should be used This will decrease the likelihood that adverse consequences of supplemental oxygen will develop, such as absorption atelectasis, accentuation of hypercapnia, airway injury, and parenchymal injury

6 Tidal Volume Vt Tidal volume is the amount of air delivered with each breath Initial tidal volume depends on numerous factors, most notably the disease for which the patient requires mechanical ventilation What Vt should be used? Depends on situation 6-8 ml/kg of Ideal body weight (IBW) IBW: Males: (height in inches-60) Females: (height in inches-60)

7 Respiratory Rate An optimal method for setting the respiratory rate has not been established. For most patients, an initial respiratory rate between 12 and 16 breaths per minute is reasonable

8 Flow Rate The peak flow rate is the maximum flow delivered by the ventilator during inspiration. Peak flow rates of 60 L per minute may be sufficient, although higher rates are frequently necessary. An insufficient peak flow rate is characterized by dyspnea, spuriously low peak inspiratory pressures, and scalloping of the inspiratory pressure tracing

9 PEEP/CPAP Where do you start? PEEP = 5 7 cmh 2 O Increase in increments of 2-3 cmh 2 O Where do you stop? Goals: Adequate oxygenation FIO 2 reduced to acceptable level No upper limit exists

10 Inspiratory Time: Expiratory Time Relationship (I:E Ratio) During spontaneous breathing, the normal I:E ratio is 1:2, indicating that for normal patients the exhalation time is about twice as long as inhalation time. If exhalation time is too short breath stacking occurs resulting in an increase in end-expiratory pressure also called auto-peep. Depending on the disease process, such as in ARDS, the I:E ratio can be changed to improve ventilation

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12 Phase Variables A/ Trigger mechanism What causes the breath to begin? B/ Limit variable Which parameter is sustained at a preset level during the breath? C/ Cycle mechanism What causes the breath to end? A B C

13 Flow (L/min) Who Limits? Phases of Inspiration Who determines when to cycle? Peak inspiratory flow rate Inspiration Beginning of expiration exhalation valve opens Who initiates? Time (sec) Beginning of inspiration Exhalation valve closes Expiration

14 THREE QUESTIONS STRATEGY TO UNDERSTAND MODES OF VENTILATION 1. Who initiates (triggers ) the inspiration and How? (TRIGGER VARIABLE) 2. How far is the Inspiration allowed or what is the limit for inspiration? (LIMIT VARIABLE) 3. When and how is inspiration ended allowing for expiration to start? (CYCLE VARIABLE)

15 1.Who & What initiates (triggers ) the inspiration and How? (TRIGGER VARIABLE) Machine Elapse of Time (Time triggered) Pressure Triggered Patient Flow triggered

16 Patient Trigger There are two ways to initiate a ventilatordelivered breath: pressure triggering or flow-by triggering When pressure triggering is used, a ventilatordelivered breath is initiated if the demand valve senses a negative airway pressure deflection (generated by the patient trying to initiate a breath) greater than the trigger sensitivity. When flow-by triggering is used, a continuous flow of gas through the ventilator circuit is monitored. A ventilator-delivered breath is initiated when the return flow is less than the delivered flow, a consequence of the patient's effort to initiate a breath

17 Pressure Triggering Pressure (cmh 2 0) Assisted Controlled 0.5Cms/H2O 1Cms/H2O Time (sec)

18 0.5lpm Flow Triggering E valve 0.4lpm E valve 0.5lpm 0.5lpm Expiration Early Inspiration

19 2. How far is the Inspiration allowed OR what is the limit for inspiration? (LIMIT VARIABLE) PRESSURE VOLUME Remember Volume Change Compliance of the Lungs = Pressure Change

20 Cycling vs. Limiting Once the limited parameter is reached Flow is adjusted to maintain it Pressure Limited Pressure Change from inspiration to expiration Cycled Flow continues Flow stops Time Time

21 3. When is inspiration ended allowing for expiration to start? (CYCLE VARIABLE) Time Cycled MACHINE Pressure Cycled Volume Cycled PATIENT Flow Cycled

22 Flow cycling CYCLING AT 25% 0F PEAK FLOW CYCLING AT 40% OF FLOW PRESSURE SUPPORT 23

23 VOLUME CONTROLLED Flow limited V0lume cycled F l o w PRESSURE CONTROLLED Pressure limited Time cycled PRESSURE SUPORT Pressure limited Flow cycled Expiratory valve opens SPONTANEOUS Pressure CONTROLLED ASSISTED

24 DIFFERENT TYPES OF BREATHS depending upon the above three questions Spontaneous triggered, limited and cycled by the patient Supported triggered by the patient,limited by the ventilator and cycled by the patient Assisted triggered by the patient, but limited and cycled by the ventilator Controlled triggered, limited and cycled by the ventilator

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26 Adjustments To affect oxygenation, adjust: FiO2 PEEP I time PIP MAP To affect ventilation, adjust: Respiratory Rate Tidal Volume MV

27 CONCLUSIONS Remember three questions strategy to learn modes of Ventilation Remember there are only four type of breaths Controlled (Mandatory ) Assisted Supported Spontaneous

28 What is a Mode? 3 components Control variable Pressure or volume Breath sequence Continuous mandatory Intermittent mandatory Continuous spontaneous Targeting scheme (settings) Vt, inspiratory time, frequency, FiO2, PEEP, flow trigger

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30 Basic Modes of Ventilation Assist-Control Ventilation Volume Control Assist-Control Ventilation Pressure Control Pressure Support Ventilation Synchronized Intermittent Mandatory Ventilation Volume Control Synchronized Intermittent Mandatory Ventilation Pressure Control

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32 Assist Control Ventilation A set tidal volume (if set to volume control) or a set pressure and time (if set to pressure control) is delivered at a minimum rate Additional ventilator breaths are given if triggered by the patient

33 Modes -Volume vs Pressure Control Volume Ventilation P Volume delivery constant Inspiratory pressure varies Inspiratory flow constant Inspiratory time determined set flow and VT by Pressure Ventilation F Volume delivery varies Inspiratory pressure constant Inspiratory flow varies Inspiratory time set by clinician

34 Volume Control Ventilation Set tidal volume Minimum respiratory rate Assist mode both ventilator and patient can initiate breaths Advantage Simple, guaranteed ventilation, rests respiratory muscle Disadvantages Not synchronised ventilator breath on top of patient breath Inadequate flow settings patient sucks gas out of ventilator Inappropriate triggering Decreased compliance high airway pressure Requires sedation for synchrony

35 VCV

36 Pressure Control Ventilation Set inspiratory pressure Constant pressure during inspiration High initial flow Inspiratory pause built in Advantages Simple, avoids high inspiratory pressures, improved oxygenation Disadvantages Not synchronised Inappropriate triggers Decreased compliance reduced tidal volume

37 PCV

38 Pressure Support Set inspiratory pressure Patient initiates breath Back-up mode apnoea Cycle from inspiration to expiration Inspiratory flow falls below set proportion of peak inspiratory flow (usually 25%) Advantages Simple, avoids high inspiratory pressure, synchrony, less sedation, better haemodynamics, weaning mode Disadvantages Dependent on patient breaths Affected by changes in lung compliance

39 Modes - PSV PSV is a spontaneous mode The patient must demonstrate they can trigger the ventilator and that volumes are appropriate High and low rate, apnea, and high and low tidal volume alarms need to be assessed P F

40 Synchronised Intermittent Mandatory Ventilation Mandatory breaths VCV, PCV Patient breaths depends on SIMV cycle Synchronised mandatory breath Pressure support breath Advantages Synchrony, guaranteed minute ventilation Disadvantages Sometimes complicated to set

41 SIMV

42 Advantages of Each Mode Mode Assist Control Ventilation (AC) Advantages Reduced work of breathing compared to spontaneous breathing AC Volume Ventilation AC Pressure Control Ventilation Guarantees delivery of set tidal volume Allows limitation of peak inspiratory pressures Pressure Support Ventilation (PSV) Synchronized Intermittent Mandatory Ventilation (SIMV) Patient comfort, improved patient ventilator interaction Less interference with normal cardiovascular function

43 Disadvantages of Each Mode Mode Assist Control Ventilation (AC) AC Volume Ventilation AC Pressure Control Ventilation Pressure Support Ventilation (PSV) Synchronized Intermittent Mandatory Ventilation (SIMV) Disadvantages Potential adverse hemodynamic effects, may lead to inappropriate hyperventilation May lead to excessive inspiratory pressures Potential hyper- or hypoventilation with lung resistance/compliance changes Apnea alarm is only back-up, variable patient tolerance Increased work of breathing compared to AC

44 Difference between PCV and VCV Any Evidence? A single RCT and 3 observational trials There were no statistically significant differences in mortality, oxygenation, or work of breathing PCV lower peak airway pressures a more homogeneous gas distribution (less regional alveolar over distension) improved patient-ventilator synchrony earlier liberation from mechanical ventilation than volumelimited ventilation VCV it can guarantee a constant tidal volume, ensuring a minimum minute ventilation

45 Setting the ventilator FiO2 Mode Triggering Tidal volume Inspiratory pressure PEEP Respiratory rate Inspiratory time I:E ratio Inspiratory flow Alarm settings Peak pressure PEEP Minute ventilation

46 How to set a ventilator FiO2 Begin at 100% and wean as quickly as able to < 60% Mode Volume controlled ventilation Set tidal volume 6 8 ml/kg Pressure controlled ventilation Set inspiratory pressure cmh2o Triggering Flow triggering : 1 5 L/min Pressure triggering: -0.5 to -2.0 cmh2o Inspiratory pressure (Plateau) Aim < 30 cmh2o. VCV or SIMV = plateau pressure PCV = Sum of PEEP and Inspiratory pressure PEEP Start with 5-10 cmh20 Respiratory rate Start at breaths per minute Inspiratory time Normal seconds I:E ratio Normally 1:2 Increase in obstructive airways disease (COPD/Asthma) Inspiratory flow L/min

47 Guidelines in the Initiation of Mechanical Ventilation Primary goals of mechanical ventilation are adequate oxygenation/ventilation, reduced work of breathing, synchrony of vent and patient, and avoidance of high peak pressures Set initial FIO2 on the high side, you can always titrate down Initial tidal volumes should be 6-8ml/kg IBW. If patient is in ARDS consider lower tidal volumes between 4-6ml/kg with increase in PEEP

48 Guidelines in the Initiation of Mechanical Ventilation Use PEEP in diffuse lung injury and ARDS to support oxygenation and reduce FIO2 Avoid choosing ventilator settings that limit expiratory time and cause or worsen auto PEEP When facing poor oxygenation, inadequate ventilation, or high peak pressures due to intolerance of ventilator settings consider sedation, analgesia or neuromuscular blockage

49 Troubleshooting Airway pressure Ventilator settings, malfunction Circuit kinking, water pooling, wet filter ETT kinked, obstructed, endobronchial intubation Patient bronchospasm, compliance (lungm, pleura, chest wall), dysynchrony, coughing Inspiratory pause pressure - Estimate of alveolar pressure Tidal volume Reduced respiratory acidosis Monitor in PCV/PS Changes in compliance anywhere in system Expired Vt more accurate Minute ventilation determined by RR + Vt Apnoea important in PS Intrinsic PEEP (gas trapping) Expiratory pause hold Hypotension after initiating IPPV Hypovolaemia/Reduced VR Drugs Gas trapping disconnect Tension pneumothorax Dysynchrony Patient factors Ventilator settings, eg I:E PS > SIMV > PCV/VCV

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