Basics of Mechanical Ventilators
Transcript of Basics of Mechanical Ventilators
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Biomedical Engineer Today
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Overview of topics1. Settings
2. Modes
3. Advantages and disadvantages between modes4. Guidelines in the initiation of mechanical ventilation
5. Common trouble shooting examples withmechanical ventilation
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Settings1. Trigger mode and sensitivity
2. Respiratory rate
3. Tidal Volume4. Positive end-expiratory pressure (PEEP)
5. Flow rate
6. Inspiratory time
7. Fraction of inspired oxygen
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Tidal Volume The tidal volume is the amount of air delivered with
each breath. The appropriate initial tidal volumedepends on numerous factors, most notably thedisease for which the patient requires mechanicalventilation.
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Positive End-Expiratory Pressure
(PEEP)Applied PEEP is generally added to mitigate end-
expiratory alveolar collapse. A typical initial appliedPEEP is 5 cmH2O. However, up to 20 cmH2O may beused in patients undergoing low tidal volumeventilation for acute respiratory distress syndrome(ARDS)
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Flow Rate The peak flow rate is the maximum flow delivered by
the ventilator during inspiration. Peak flow rates of 60L per minute may be sufficient, although higher ratesare frequently necessary. An insufficient peak flow rateis characterized by dyspnea, spuriously low peakinspiratory pressures, and scalloping of the inspiratorypressure tracing
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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 exhalationtime is about twice as long as inhalation time.
If exhalation time is too short breath stacking occursresulting in an increase in end-expiratory pressure alsocalled 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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Modes of Ventilation: The BasicsAssist-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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Assist Control VentilationA set tidal volume (if set to volume control) or a set
pressure and time (if set to pressure control) isdelivered at a minimum rate
Additional ventilator breaths are given if triggered bythe patient
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Pressure Support Ventilation The patient controls the respiratory rate and exerts a
major influence on the duration of inspiration,inspiratory flow rate and tidal volume
The model provides pressure support to overcome theincreased work of breathing imposed by the diseaseprocess, the endotracheal tube, the inspiratory valvesand other mechanical aspects of ventilatory support.
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Synchronized Intermittent
Mandatory Ventilation Breaths are given are given at a set minimal rate, however if
the patient chooses to breath over the set rate no additionalsupport is given
One advantage of SIMV is that it allows patients to assumea portion of their ventilatory drive
SIMV is usually associated with greater work of breathingthan AC ventilation and therefore is less frequently used asthe initial ventilator mode
Like AC, SIMV can deliver set tidal volumes (volumecontrol) or a set pressure and time (pressure control)
Negative inspiratory pressure generated by spontaneousbreathing leads to increased venous return, whichtheoretically may help cardiac output and function
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Advantages of Each ModeMode Advantages
Assist Control Ventilation (AC) Reduced work of breathing comparedto spontaneous breathing
AC Volume Ventilation Guarantees delivery of set tidal volume
AC Pressure Control Ventilation Allows limitation of peak inspiratorypressures
Pressure Support Ventilation (PSV) Patient comfort, improved patientventilator interaction
Synchronized Intermittent MandatoryVentilation (SIMV)
Less interference with normalcardiovascular function
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Disadvantages of Each ModeMode Disadvantages
Assist Control Ventilation (AC) Potential adverse hemodynamic effects,may lead to inappropriatehyperventilation
AC Volume Ventilation May lead to excessive inspiratorypressures
AC Pressure Control Ventilation Potential hyper- or hypoventilationwith lung resistance/compliancechanges
Pressure Support Ventilation (PSV) Apnea alarm is only back-up, variablepatient tolerance
Synchronized Intermittent MandatoryVentilation (SIMV)
Increased work of breathing comparedto AC
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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 highpeak pressures
Set initial FIO2 on the high side, you can always titratedown
Initial tidal volumes should be 8-10ml/kg, dependingon patients body habitus. If patient is in ARDSconsider tidal volumes between 5-8ml/kg withincrease in PEEP
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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 expiratorytime and cause or worsen auto PEEP
When facing poor oxygenation, inadequateventilation, or high peak pressures due to intolerance
of ventilator settings consider sedation, analgesia orneuromuscular blockage
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Trouble Shooting the Vent Common problems
High peak pressures
Patient with COPD Ventilator synchrony
ARDS
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Trouble Shooting the Vent If peak pressures are increasing:
Check plateau pressures by allowing for an inspiratorypause (this gives you the pressure in the lung itself
without the addition of resistance)
If peak pressures are high and plateau pressures are lowthen you have an obstruction
If both peak pressures and plateau pressures are highthen you have a lung compliance issue
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Trouble Shooting the Vent If you have a patient with history of COPD/asthma
with worsening oxygen saturation and increasinghypercapnia differential includes: Given the nature of the disease process, patients have difficultly
with expiration (blowing off all the tidal volume)
Must be concern with breath stacking or auto- PEEP
Management options include:
Decrease respiratory rate Decrease tidal volume
Adjust flow rate for quickerinspiratory rate
Increase sedation
Adjust I:E ratio
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Trouble Shooting the Vent Increase in patient agitation and dis-synchrony on the
ventilator:
Could be secondary to overall discomfort
Increase sedation
Could be secondary to feelings of air hunger
Options include increasing tidal volume, increasing flow rate,adjusting I:E ratio, increasing sedation
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Trouble shooting the vent If you are concern for acute respiratory distress
syndrome (ARDS)
Correlate clinically with HPI and radiologic findings ofdiffuse patchy infiltrate on CXR
Obtain a PaO2/FiO2 ratio (if < 200 likely ARDS)
Begin ARDSnet protocol:
Low tidal volumes
Increase PEEP rather than FiO2
Consider increasing sedation to promote synchrony withventilator