Makindo Medical Notes

Related Subjects: |Respiratory Failure |Non invasive ventilation (NIV) |Intubation and Mechanical Ventilation |Critical illness neuromuscular weakness |Multiple Organ Dysfunction Syndrome |Haemodialysis |Dobutamine

About

• Mechanical ventilation takes over the work of breathing for the patient.
• Historically, it was developed in response to the polio epidemic.
• During the epidemic, medical students manually operated "iron lungs" to assist patients with breathing.

Benefits

• Decreases the work of breathing: Reduces the energy required for each breath.
• Maintains oxygenation: Delivers an FiO₂ of up to 100%, improving oxygenation and providing positive end-expiratory pressure (PEEP), which is helpful in patients with refractory hypoxemia.
• Helps remove carbon dioxide: By increasing respiratory rate or tidal volume.
• Provides stability: Allows time for treatment to be effective while maintaining respiratory function.

Risks

• Barotrauma: Alveolar overdistention due to high pressure levels.
• Ventilator-associated pneumonia (VAP): Infection developing 48 hours or more after intubation.
• Auto-PEEP: Positive pressure remaining in the alveoli at end-exhalation, increasing intrathoracic pressure.
• Oxygen toxicity: Cell damage from prolonged exposure to high oxygen levels.
• Ventilator-induced lung injury (VILI): Lung injury caused by mechanical ventilation.
• Reduced cardiac output: Due to increased intrathoracic pressure and reduced venous return.

Mechanical ventilation provides crucial benefits, but complications such as barotrauma and ventilator-associated pneumonia can occur.

Types of Mechanical Ventilation

• Negative pressure ventilation: Air is sucked into the lungs, similar to normal breathing (e.g., iron lung).
• Positive pressure ventilation: Air is pushed into the lungs, the most common method in modern ventilators.

Clinical Signs Indicating Need for Ventilation

• Breathlessness, cyanosis, agitation, and fatigue.
• Use of accessory muscles for breathing, hypotension, elevated jugular venous pressure (JVP).

Possible Indications for Mechanical Ventilation

• Airway protection in cases of decreased consciousness or obstruction risk.
• Respiratory arrest or respiratory rate < 8/min.
• Failure to tolerate non-invasive ventilation methods such as CPAP or BiPAP.
• PaO₂ < 8 kPa (< 60 mmHg), SpO₂ < 90%, despite high FiO₂ on CPAP.
• Removal of respiratory secretions, especially in neuromuscular diseases.
• Worsening hypercapnia or respiratory acidosis.
• Post-operative ventilation, particularly after major surgery.
• Head injury management: Lowering PaCO₂ to reduce intracranial pressure.
• Reducing cardiac work in cardiogenic shock or sepsis to minimize respiratory effort.

Objective Criteria for Mechanical Ventilation

• Respiratory rate > 35/min.
• Tidal volume < 5 ml/kg.
• Vital capacity < 15 ml/kg.
• PaO₂ < 8 kPa despite 60% oxygen.
• PaCO₂ > 8 kPa.

Procedure

• Requires anesthesia and muscle relaxation in conscious patients.
• Can cause hypotension due to the effects of positive intrathoracic pressure.

Complications

• Cardiovascular compromise due to the negative inotropic effects of sedative drugs.
• Positive intrathoracic pressures may reduce cardiac output.
• Atrophy of respiratory muscles over time due to prolonged support.
• Pneumothorax, especially when peak airway pressures exceed 40 cm H₂O.
• Ventilator-related lung injury (VILI).
• Tracheostomy may be needed after prolonged intubation (typically after 14 days) to reduce endotracheal tube complications.

Types of Ventilation Modes

• Volume-controlled ventilation
  • Synchronised Intermittent Mandatory Ventilation (SIMV): Pre-set rate and tidal volume of mandatory breaths, allowing spontaneous breaths in between.
• Pressure-controlled ventilation
  • Pressure-Controlled Ventilation (PCV): Pre-set rate and inspiratory pressure, often used in acute respiratory failure.
  • Bi-level Positive Airway Pressure (BiPAP): Provides different levels of positive pressure during inspiration and expiration.
  • Pressure Support Ventilation (PSV): Augments the patient’s spontaneous breaths with positive pressure, useful for weaning.
  • Positive End-Expiratory Pressure (PEEP): Maintains positive pressure during expiration, improves oxygenation by preventing alveolar collapse.
• Advanced modes
  • High-Frequency Oscillatory Ventilation (HFOV): Uses rapid oscillating gas flow to facilitate gas exchange in severe cases like ARDS.
  • Extracorporeal Membrane Oxygenation (ECMO): Oxygenates blood and removes CO₂ externally via a vascular bypass with an oxygenator, used in severe ARDS.

More Information

• Ventilator settings (tidal volume, respiratory rate, PEEP) depend on the cause of respiratory failure.
• Weaning from mechanical ventilation: Short-term ventilated patients often do not require a weaning process. Patients with severe lung disease (e.g., ARDS) may need gradual weaning.
• Tracheostomy: Usually performed electively after 14 days of intubation to improve patient comfort and aid in weaning.

References

• Mechanical Ventilation Made Easy: Ventilator Basics (2022)