Principles of Mechanical Ventilation

1
Principles of Mechanical Ventilation

• RET 2284L
• Module 3.0
• Initiation of Mechanical Ventilation

2
Initiation of Mechanical Ventilation

• Indications
• Indications for Ventilatory Support
• Acute Respiratory Failure
• Impending Respiratory Failure
• Prophylactic Ventilatory Support
• Hyperventilation Therapy

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Initiation of Mechanical Ventilation

• Indications
• Acute Respiratory Failure
• Inability of a patient to maintain adequate PaO2,
  PaCO2, and potentially pH
• Type I
• Type II

4
Initiation of Mechanical Ventilation

• Impending Ventilatory Failure
• Respiratory failure is imminent in spite of
  therapies
• Patient is barely maintaining (or experiencing
  gradual deterioration) of normal blood gases at
  the expense of significant WOB
• NOTE Early intervention corrects hypoxemia and
  acidosis imposed on the major organs and reduces
  stress placed on the cardiopulmonary system

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Initiation of Mechanical Ventilation

• Assessment of Impending Ventilatory Failure
• VT lt3 5 ml/kg
• Respiratory rate gt25 35 breaths/minute
• Labored or irregular
• Minute ventilation gt10 L/min
• VC lt15 ml/kg (According to Eagan lt1 L)

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Initiation of Mechanical Ventilation

• Assessment of Impending Ventilatory Failure
• Maximum inspiratory pressure lt -20 cm H2O
• PaCO2 increasing to gt50 mm Hg
• Requires serial ABGs to monitor
• Vital Signs ? heart rate, ? blood pressure

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Initiation of Mechanical Ventilation

• Prophylactic Ventilatory Support
• Clinical conditions in which there is a high risk
  of future respiratory failure
• Ventilatory support is instituted to
• Decrease the WOB
• Minimize O2 consumption and hypoxemia
• Reduce cardiopulmonary stress
• Control airway with sedation
• Examples Brain injury, heart muscle injury,
  major surgery, prolonged shock, smoke injury

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Initiation of Mechanical Ventilation

• Hyperventilation Therapy
• Ventilatory support is instituted to control and
  manipulate PaCO2 to lower than normal levels
• Acute head injury

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Initiation of Mechanical Ventilation

• Contraindications
• Untreated pneumothorax
• Relative Contraindications
• Patients informed consent
• Medical futility
• Reduction or termination of patient pain and
  suffering

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Mode
• Full Ventilatory Support (FVS)
• Assumes essentially all the work of breathing
• Majority initially require FVS
• Assist Control (A/C)
• SIMV if rate is 12 BPM or higher (Chang)
• Partial Ventilatory Support (PVS)
• Provides less than total amount of work of
  breathing
• Common during weaning
• SIMV at lower rates (usually lt8 -10 BPM)
• PSV
• Bi-PAP

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Respiratory Rate
• A range of 8 12 breaths per minute (BPM)
• Normal respiratory rate is 12-18 BPM
• Respiratory rate is chosen in conjunction with
  tidal volume to provide an acceptable minute
  ventilation
• Normal VE is 5-10 L/min
• Estimated by using 100 mL/kg IBW
• ABG needed to assess effectiveness of initial
  settings
• PaCO2 gt45
• PaCO2 lt35

? minute ventilation via rate or VT
? minute ventilation via rate or VT
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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Tidal Volume
• A range of 6 12 ml/kg IBW is used for adults
• 10 12 ml/kg IBW (normal lung function)
• 8 10 ml/kg IBW (obstructive lung disease)
• 6 8 ml/kg IBW (ARDS)
• A range of 5 10 ml/kg IBW is used for infants
  and children (Pilbeam)
• Ideal Body Weight Calculation
• Male IBW in lb 106 6 x (height in inches
  60)
• Female IBW in lb 105 5 x (height in inches
  60)

NOTE spontaneous VT for an adult is 5 7 ml/kg
of IBW
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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Tidal Volume
• Tidal volume actually delivered to the patient is
  usually lower than the ventilator delivered tidal
  volume
• Causes
• Leakage from ventilator circuit
• Leakage from ET Tube cuff
• Circuit compressible volume loss
• NOTE Newer ventilators automatically measure and
  compensate for circuit compressible volume loss

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Determination of Circuit Compressible Volume Loss
• Set VT to 100-200 and PEEP to zero
• Set inspiratory pause at 2 seconds
• Select a minimum flow rate and maximum pressure
  limit
• Occlude Y-connection and initiate a mechanical
  breath
• Record the exhaled volume (ml) and peak
  inspiratory pressure (cm H2O)
• Divide exhaled volume by PIP (V/PIP) circuit
  compression factor
• Multiply compression factor by the patients PIP
  (PIP minus PEEP if PEEP is used)
• Example
• Circuit compression factor 150 ml / 50 cm H2O
  3 ml/cm H2O
• Circuit compression volume 3 ml/cm H2O x (60 cm
  H2O PIP 10 cm H2O PEEP)
• 3 ml/cm H2O x 50 cm H2O 150 ml (circuit
  volume Loss)

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Circuit Compressible Volume Loss
• The amount of volume lost can be added to the VT
  setting to ensure that the patient is receiving
  the desired tidal volume
• Example A patients estimated VT is 400 ml.
  Her peak pressure reading during inspiration is
  30 cm H2O and circuit compression factor is 2.9
  mL/cm H2O. What is the actual VT delivery to the
  patient? How should you set the desired VT?

Volume lost 2.9 mL/cm H2O x 30 cm h2O 87 ml
actual volume received by the patient 400 87
mL 313 mL. To compensate, increase set VT to
about 487 mL to deliver the 400 mL desired.
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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Tidal Volume
• Ideally, a tidal volume should be chosen that
  maintains a PPlat lt30 cm H2O

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• FiO2
• Initially 100
• Severe hypoxemia
• Abnormal cardiopulmonary functions
• Post-resuscitation
• Smoke inhalation
• ARDS
• After stabilization, attempt to keep FiO2 lt50
• Avoids oxygen-induced lung injuries
• Absorption atelectasis
• Oxygen toxicity

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• FiO2
• Patients with mild hypoxemia or normal
  cardiopulmonary function
• Drug overdose
• Uncomplicated postoperative recovery
• FiO2 of 40
• Same FiO2 prior to mechanical ventilation

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Positive End Expiratory Pressure (PEEP)
• Increases FRC
• Useful to treat refractory hypoxemia
• Initially set at 5 cm H2O
• Subsequent changes are based on ABG results
• Contraindications
• Hypotension
• Elevated ICP
• Uncontrolled pneumothorax

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• IE Ratio
• Relationship between inspiratory and expiratory
  time
• Usually 11.5 14 (TI of 0.8 1.2 seconds)
• Principle Determinants of TI and IE Ratio
• Peak flow ( I)
• Inspiratory time (TI)
• Inspiratory time percentage (TI)
• Respiratory rate (f )
• Minute ventilation - E (VT x f )

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Adjusting IE Ratio
• I ? I ? ?TI ? smaller IE ratio
• I ? I ? ?TI ? larger IE ratio
• VT ?VT ? ?TI ? larger IE ratio
• VT ?VT ? ?TI ? smaller IE ratio
• f ?RR ? ?TE ? larger IE ratio
• f ?RR ? ?TE ? smaller IE ratio
• TI ?TI ? larger IE ratio
• e.g., TI of 20 14, TI 0f 25 13

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Flow Pattern
• Initially, a decelerating flow pattern is used
  others result in higher airway pressure

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Flow Pattern
• Some clinician choose to use a constant (square)
  flow pattern initially because it enables the
  clinician to obtain baseline measurements of lung
  compliance and airway resistance (Oakes
  Ventilation Management ch.5)

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Flow Pattern
• Changing flow patterns may affect peak I, TI
  and/or TE depending on the ventilation type
• Optimal flow pattern is highly variable and
  dependent on the individual patients lung and
  airway conditions. Pattern variation usually has
  insignificant effects on normal lungs.

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Initiation of Mechanical Ventilation

• Initial Ventilator Settings
• Flow Rate
• Set at 40 100 L/min (volume ventilation)
• Set I patients peak inspiratory demand
  and to deliver an IE ratio of 11.5 14

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Adequate Flow During Volume Ventilation
30
Adequate flow
P
aw
Time (s)
cmH2O
1
2
3
-10
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Inadequate Flow During Volume Ventilation
30
Adequate flow
P
aw
Time (s)
cmH2O
1
2
3
-10
28
The Patient Outbreathing the Set Flow
Air Starvation
Paw
Sec
cmH2O
1
2
3
4
5
6
-20
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Expiratory Flow Pattern
Inspiration
Expiratory time TE
Time (sec)
Flow (L/min)
Expiration
30
Inadequate Inspiratory Flow
Active Inspiration or Asynchrony
Normal Abnormal
Time (sec)
Flow (L/min)
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Initiation of Mechanical Ventilation

• Ventilator Alarm Settings
• High Minute Ventilation
• Set at 2 L/min or 10-15 above baseline minute
  ventilation
• Patient is becoming tachypneic (respiratory
  distress)
• Ventilator is self-triggering
• High Respiratory Rate Alarm
• Set 10 15 BPM over observed respiratory rate
• Patient is becoming tachypneic (respiratory
  distress)
• Ventilator self-triggering

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Initiation of Mechanical Ventilation

• Ventilator Alarm Settings
• Low Exhaled Tidal Volume Alarm
• Set 100 ml or 10-15 lower than expired
  mechanical tidal volume
• Causes
• System leak
• Circuit disconnection
• ET Tube cuff leak
• Low Exhaled Minute Ventilation Alarm
• Set at 2 L/min or 10-15 below minimum SIMV or
  A/C backup minute ventilation
• Causes
• System leak
• Circuit disconnection
• ET Tube cuff leak

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Initiation of Mechanical Ventilation

• Ventilator Alarm Settings
• High Inspiratory Pressure Alarm
• Set 10 15 cm H2O above PIP
• Inspiration is terminated when triggered
• Common causes
• Water in circuit
• Kinking or biting of ET Tube
• Secretions in the airway
• Bronchospasm
• Tension pneumothorax
• Decrease in lung compliance
• Increase in airway resistance
• Coughing

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Initiation of Mechanical Ventilation

• Ventilator Alarm Settings
• Low Inspiratory Pressure Alarm
• Set 10 15 cm H2O below observed PIP
• Causes
• System leak
• Circuit disconnection
• ET Tube cuff leak
• High/Low PEEP/CPAP Alarm (baseline alarm)
• High Set 3-5 cm H2O above PEEP
• Circuit or exhalation manifold obstruction
• Auto PEEP
• Low Set 3-5 cm H2O below PEEP
• Circuit disconnect

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Initiation of Mechanical Ventilation

• Ventilator Alarm Settings
• Apnea Alarm
• Set with a 15 20 second time delay
• In some ventilators, this triggers an apnea
  ventilation mode
• High/Low FiO2 Alarm
• High 5 - 10 over the analyzed FiO2
• Low 5 - 10 below the analyzed FiO2
• High/Low Temperature Alarm
• Heated humidification
• High No higher than 37? C
• Low No lower than 30? C