Indications, Goals & Hazards of Mechanical Ventilation
Objectives — Identify the indications (Type I and Type II respiratory failure, impending failure, bedside parameters), contraindications, goals, and hazards of mechanical ventilation.
Indications for Mechanical Ventilation
Mechanical ventilation is indicated when the patient cannot maintain adequate ventilation or oxygenation on their own. There are three broad categories.
Type I Respiratory Failure — Hypoxemic
| Parameter | Threshold |
|---|---|
| PaO₂ | < 60 mmHg |
| PaCO₂ | Normal or decreased (< 50 mmHg) |
| P(A-a)O₂ | Increased |
Type I failure is characterized by a low PaO₂ with a normal or decreased PaCO₂. The problem is getting oxygen in, not getting CO₂ out.
Causes of Type I Failure
V/Q Mismatch
Normal V/Q = 0.8 (alveolar ventilation 4 L/min ÷ cardiac output 5 L/min).
- V/Q > 0.8 → more ventilation than perfusion (lung apices)
- V/Q < 0.8 → less ventilation than perfusion (lung bases)
| Abnormality | V/Q Ratio | Cause |
|---|---|---|
| Dead space | V/Q = ∞ (V = 4, Q = 0) | PE, shock, hypovolemia, excessive PEEP |
| Shunt | V/Q = 0 (V = 0, Q = 5) | Atelectasis, pulmonary edema, pneumonia, ARDS |
Shunt is "refractory hypoxemia" — supplemental oxygen does not correct it because blood bypasses ventilated alveoli entirely.
Normal anatomic shunt is 2–3% from bronchial and thebesian veins. Pathologic shunt includes:
- Intracardiac: ASD, VSD
- Intrapulmonary: atelectasis, pulmonary edema, pneumonia, ARDS
Diffusion Impairment — interstitial lung disease, emphysema, pulmonary vascular disease.
Decreased PiO₂ — altitude > 10,000 ft (relevant in CCATT missions).
Venous Admixture — mixing of non-oxygenated (shunted) blood with re-oxygenated blood distal to alveoli; seen in CHF with low cardiac output.
Differentiating the Cause of Hypoxemia
Use the P(A-a)O₂ gradient:
| Gradient | Implication |
|---|---|
| Increased | V/Q mismatch or shunt |
| Normal | Hypoventilation or decreased PiO₂ (high altitude) |
Common Disease Processes Causing Type I Failure
- Pneumonia
- Atelectasis
- Non-cardiogenic pulmonary edema — ALI / ARDS
- Cardiogenic pulmonary edema — CHF
- Pulmonary embolism
- Interstitial lung disease
Type II Respiratory Failure — Hypercapnic
| Parameter | Threshold |
|---|---|
| PaCO₂ | > 50 mmHg |
| Mechanism | Failure to remove CO₂ (decreased alveolar minute volume) |
Type II failure is hypercapnic respiratory failure. Hypoxemia is usually also present but responds to supplemental oxygen, distinguishing it from shunt.
Causes of Type II Failure
Decreased Respiratory Drive
- Drug overdose (opioids, sedatives)
- Brainstem lesions, CNS disease (multiple sclerosis, Parkinson's, elevated ICP)
- Carotid body resection (after carotid endarterectomy)
- Morbid obesity, hypothyroidism
Increased CO₂ Production (overwhelms removal)
- Fever, agitation, shivering, exertion
- Hypermetabolism, excess caloric intake
- Carbohydrate metabolism produces the most CO₂ per O₂ consumed
Impaired CO₂ Exhalation — Muscle Weakness Diseases
- Guillain-Barré syndrome
- Charcot-Marie-Tooth disease
- Botulism
CO₂ Exposure
- Defective CO₂ scrubber (anesthesia or PFT machines)
- Spelunkers in caves, dry ice exposure
Common diseases: COPD/asthma exacerbation, drug overdose, neurologic disease, obesity hypoventilation syndrome.
Impending Respiratory Failure
Based on physician judgment. Key considerations:
- Patient appearance and distress level
- Underlying disease process and history
- Degree of dyspnea
- Worsening hypoxemia and tachypnea
Bedside Ventilatory Parameters
These objective measurements help quantify respiratory compromise.
| Category | Parameter | Threshold Indicating MV Need |
|---|---|---|
| Alveolar ventilation (ABG) | PaCO₂ | > 55 mmHg |
| Alveolar ventilation (ABG) | pH | < 7.20 |
| Lung expansion (bedside spirometry) | Spontaneous Vt | < 5 mL/kg |
| Lung expansion (bedside spirometry) | Vital capacity | < 10 mL/kg |
| Lung expansion (bedside spirometry) | Respiratory rate | > 35 breaths/min |
| Muscle strength (NIF gauge) | MIP | > −20 cmH₂O |
A MIP of −19 cmH₂O is worse than −21 cmH₂O — remember the sign convention.
Physiologic Measurements
| Measurement | Value Indicating MV Need |
|---|---|
| PaO₂ (ABG) | 80–100 mmHg normal; intubate if severely reduced |
| P(A-a)O₂ | > 300 mmHg |
| PaO₂/FiO₂ ratio | < 200 |
| Minute volume (WOB) | > 10 L/min |
| VD/Vt ratio | > 0.6 (60%) |
The alveolar oxygen equation: PAO₂ = [(P_atm − PH₂O) × FiO₂] − (PaCO₂ / RQ)
Contraindications
Mechanical ventilation is generally contraindicated when:
- There is no indication — patient does not meet criteria
- Patient meets criteria for NIV — try less invasive approach first
- It is contrary to the patient's wishes — DNR/DNI orders
- It represents medically futile therapy
Goals of Mechanical Ventilation
- Maintain adequate alveolar ventilation
- Maintain adequate oxygen delivery
- Restore acid-base balance
- Reduce the work of breathing (WOB)
- Reduce myocardial workload secondary to hypoxemia and increased WOB
- Permit sedation and neuromuscular blockade
- Reduce intracranial pressure
- Stabilize the chest wall
Hazards of Mechanical Ventilation
Positive pressure ventilation is non-physiological — air is blown in under pressure rather than drawn in by negative pressure. This creates several hazards.
| Hazard | Mechanism |
|---|---|
| Ventilator-induced lung injury (VILI) | Overdistension and cyclic opening/closing of alveoli |
| Barotrauma | Excessive pressure ruptures alveoli → pneumothorax, pneumomediastinum |
| Nosocomial pneumonia (VAP) | Bypassed upper airway defenses, aspiration around cuff |
| Atelectasis | Inadequate PEEP, prolonged immobility |
| Oxygen toxicity | High FiO₂ (> 60%) over time damages lung parenchyma |
| Auto-PEEP | Air trapping → inadvertent PEEP → hemodynamic compromise |
| Decreased venous return | Positive intrathoracic pressure impedes venous return → ↓ cardiac output |
| Decreased urinary output | Reduced cardiac output → reduced renal perfusion |
| Lack of nutrition | Patients on MV often poorly nourished |
Newton's Equation of Motion: Pressure = Raw × Flow + Volume / Compliance. Every ventilator parameter decision flows from this relationship.