Module C: Establishing the Need for Mechanical Ventilation

Question 1

acute ventilatory failure

Answer 1

respiratory activity is absent or is insufficient to maintain adequate oxygen uptake and carbon dioxide clearance, in spite of initial therapy

Question 2

CNS disorders that may lead to hypoventilation or respiratory failure

Answer 2

Reduced drive to breathe due to depressant drugs, brain/brainstem lesions, hypothyroidism, or idiopathic central alveolar hypoventilation

Increased drive to breathe due to increased metabolic rate (eg. Exercise), metabolic acidosis, anxiety

Question 3

neuromuscular disorders that may lead to hypoventilation or respiratory failure

Answer 3

Paralytic disorders: myasthenia gravis, tetanus, botulism, Guillain-Barre syndrome, poliomyelitis, muscular dystrophy, ALS

Paralytic drugs (curare, nerve gas, succinylcholine, insecticides, non depolarizing NMBA’s)

Drugs that affect neuromuscular transmission (aminoglycoside ABX, long term adrenocorticoids, CCB’s)

Impaired muscle function: electrolyte imbalances, malnutrition, peripheral nerve disorders, atrophy, fatigue, chronic pulmonary disease with decreasing capacity for diaphragmatic contraction as a result of air trapping)

Question 4

Disorders that increase work of breathing

Answer 4

Pleura occupying lesions (eg pleural effusions, hemothorax, empyema, pneumothorax)

Chest wall deformities (eg flail chest, rib fracture, kyphoscoliosis, obesity)

Increased airway resistance resulting from increased secretions, mucosal edema, bronchoconstriction, airway inflammation, or foreign body aspiration (eg asthma, emphysema, chronic bronchitis, croup, acute epiglottitis, acute bronchitis)

Lung tissue involvement (eg interstitial pulmonary fibrotic disease, aspiration, ARDS, cardiogenic pulmonary edema, drug-induced pulmonary edema)

Pulmonary vascular problems (eg pulmonary thromboembolism, pulmonary vascular damage)

Other problems (eg increased metabolic rates with acompanying pulmonary problems)

 Postoperative pulmonary complications 
Dynamic hyperinflation (air trapping)

Question 5

Vital capacity

Answer 5

the volume of air that can be maximally exhaled following a maximum inspiration

VC is measured in mL/kg.

Normal adult range is 65 to 75mL/kg.

Critical value that may indicate the need for getting tubed: <10-15mL/kg

Question 6

Maximum inspiratory pressure

Answer 6

lowest (ie most negative) pressure generated during a forceful inspiratory effort against an occluded airway

MIP is measured in cm H20

Normal adult range is -100 to -50 cm H20

Critical value that may indicate the need for getting tubed: -20 to 0 cm H20

Question 7

Forced expiratory volume in 1 second (FEV1)

Answer 7

a pulmonary function parameter that can be used to measure airway resistance

FEV1 is measured in mL/kg IBW

Normal adult range 50 to 60 mL/kg

Critical value that may indicate the need for getting tubed: <10 mL/kg

Question 8

Respiratory frequency (f)

Answer 8

breaths per minute

Respiratory frequency is measured in breaths/minute (f)

Normal range 12 - 20

Critical value that may indicate the need for getting tubed: >35 or <10

Question 9

Peak expiratory flow rate (PEF)

Answer 9

a good indicator of airway resistance and a patient’s ability to maintain airway patency

PEF is measured in L/min

Normal adult range 350-600 L/min

Critical value that may indicate the need for getting tubed: <75-100 L/min

Question 10

Dead Space:Tidal Volume ratio

Answer 10

the ratio of physiologicdead spaceovertidal volume(VD/VT)

Question 11

PaO2/FiO2 ratio

Answer 11

the ratio of arterial oxygen partial pressure(PaO2 in mmHg) to fractional inspired oxygen (FiO2 expressed as a fraction, not a percentage).

P/F ratio is measured in mmHg

Normal adult range: 400-500 mmHg

Critical value that may indicate the need for getting tubed: <300mmHg

Question 12

A-a gradient

Answer 12

The A-a gradient can be used to determine the cause of altered oxygenation

Normal A-a gradient

Alveolar hypoventilation (elevated PACO2)
Low PiO2 (FiO2 < 0.21 or barometric pressure < 760 mmHg)

Raised A-a gradient

Diffusion defect (rare)
V/Q mismatch
Right-to-Left shunt (intrapulmonary or cardiac)

Question 13

four standard criteria for the institution of mechanical ventilatory support.

Answer 13

Apnea or absence of breathing

Acute ventilatory failure

Impending ventilatory failure

Refractory hypoxemic respiratory failure with increased work of breathing or an ineffective breathing pattern

Question 14

physiologic goals of therapy for the mechanically ventilated patient

Answer 14

Support or manipulate pulmonary gas exchange

Increase lung volume

Reduce the work of breathing

Question 15

clinical goals of therapy for the mechanically ventilated patient

Answer 15

1. Reverse acute respiratory failure
2. Reverse respiratory distress
3. Reverse hypoxemia
4. Prevent or reverse atelectasis and maintain FRC
5. Reverse respiratory muscle fatigue
6. Permit sedation or paralysis
7. Reduce systemic or myocardial oxygen consumption
8. Minimize associated complications and reduce mortality