Memory Joggers: Ventilator/Respiratory
Some measurements used for determining the need for mechanical ventilation
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Measurement
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Values
Normal Mech Vent Indicated
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Tidal volume (VT), mL/kg
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5-8
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<5
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Vital capacity(VC), mL/kg
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65-75
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<10; <15
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Forced expiratory volume in 1 second(FEV1), mL/kg
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50-60
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<10
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Functional residual capacity(FRC), % of predicted value
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80-100
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<50
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Respiratory rate(f), breaths/min
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12-20
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>35
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Maximum inspiratory force(MIF), cmH20
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80-100
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<20; <25; <30
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Minute ventilation (VE), L/min
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5-6
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>10
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Maximum voluntary ventilation(MVV), L/min
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120-180
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<20; <(2 X VE)
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Dead space fraction (VD/VT), %
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0.25-0.40
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>0.60
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PaCO2, mmHg
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36-44
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>50; >55
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PaO2, mmHg
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75-100 (breathing air)
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<50(air); <70(mask O2)
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Alveolar-to-arterial PO2 gradient [P(A-a)O2], breating 100% O2, mmHg
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25-65
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>350; >450
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Arterial/alveolar PO2 ratio(PaO2/PAO2)
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0.75
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<0.15
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Arterial Po2/inspired O2 fraction ratio(PaO2/FiO2), mmHg
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350-450
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<200
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Intrapulmonary right-to-left shunt fraction (Qs/QT), %
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<=5
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>20; >25; >30
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From Pierson
Indications for Mechanical Ventilation in Acute Respiratory Failure, Classified by Physiologic Mechanism
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Mechanism
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Best Available Indicator
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Inadequate alveolar ventilation
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PaCo2 and pH
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Inadequate lung expansion
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VT; VC; f
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Inadequate respiratory muscle strength
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MIF; MVV; VC
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Excessive work of breathing
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VE required to keep PCO2 normal; VD/VT; f
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Unstable ventilatory drive
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Breathing pattern, clinical setting
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Severe hypxemia
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P(A-a)O2 PaO2/PAO2; PaO2/FIO2; QS/QT
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From Pierson
Determinants of Ability to Wean
Oxygenation
Criteria of Adequacy
- PaO2 >60 mm Hg on FIO2 <0.35 at minimal PEEP, PaO2/FIO2 >200; P(A-a)O2<350 mmHg
Selected causes of failure
- Hypoventilation due to neurologic injury or drugs
- Ventilation/perfusion mismatch (e.g., airspace disease, congestive heart failure)
- Anatomic (right-to-left) shunt (e.g., intracardiac shunt, pulmonary arteriovenous malformation), decreased venous O2 content/excessive O2 extraction due to low cardiac output or hypermetabolism
Ventilation
Criterion of Adequacy
- PaCO2 <50 mmHg or within 8 mmHg of baseline
Selected Causes of failure
- Decreased respiratory drive (e.g., sedation, obesity/hypoventilation syndrome)
- Decreased repiratory bellows function; diaphragmatic weakness; neuromuscular disase (e.g., Duchenne muscular dystrophy)
- Increased CO2 production without compensatory increase in alveolar ventilation (fever, hypermetabolism, carbohydrate overfeeding)
- Increased dead space ventilation without compensatory increase in alveolar ventilation (e.g., pulmonary embolus, bullous emphysema)
Neuropsychiatry Integrity
Criteria of adequacy
- Awake, alert, cooperative, with intact gag and swallowing
Selected causes of failure
- Cerebrovascular accident
- Sleep deprivation/ICU psychosis
- Drug therapy
- Depression
- Psychological dependency on ventilator
Two Phases of Decision Making for Weaning
Phase 1: general criteria to be satisfied
- Resolution of the process for which mechanical ventilation was initiated
- Absence of septicemia
- Hemodynamic stability
- Management secretions
Phase 2: satisfaction of specific weaning criteria
- Oxygenation/O2 transport
- PaO2 >60 mmHg with FIO2 <0.35 on low level PEEP
- P(A-a)O2 <350 mmHg
- Cardiac index >2.1 L/min/m2
- No lactic acidosis
- Mechanical function of the respiratory system
- VC >10 mL/kg ideal body weight
- VE(rest) <10 L/min
- MVV at least twice resting VE
- TTdi < 0.15
- Tracheal occlusion pressure (P0.1) <6 cmH2O
- Patient/ventilator system compliance >25 mL/cmH2O
- NIF <30 cmH2O
- F/VT <=105
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