Understanding the Complexities of Mechanical Ventilation: An Overview of Respiratory Mechanics in Clinical Practice

• Mechanical Ventilatory Support:
  • Provides pressure and flow to the airways for oxygen and carbon dioxide transport.
  • Goal is to maintain appropriate levels of arterial blood gases and support ventilation without harming the lungs.
• Design Features of Modern Mechanical Ventilators:
  • Use high-pressure gas sources for generating tidal breaths.
  • Classified based on trigger, target, and cycle criteria into different breath categories.
• Modes of Mechanical Ventilatory Support:
  • Defined by the availability and delivery logic of different breath types.
  • Include volume assist-control, pressure assist-control, and others.
• Advanced Monitoring and Feedback Functions:
  • Incorporate continuous adjustments in breath delivery patterns.
  • Involves feedback control mechanisms such as pressure-regulated volume control and adaptive support ventilation.
• Novel Assist Modes:
  • Proportional assist ventilation and neutrally adjusted ventilatory assistance are introduced.
  • These modes are designed to unload inspiratory muscles and adjust flow and pressure in accordance with patient effort.
• Physiologic Effects of Mechanical Ventilation:
  • Alveolar ventilation is essential for oxygen and carbon dioxide transport.
  • Mechanical ventilation interacts with respiratory system compliance, resistance, and equation of motion.
• Equation of Motion in Mechanically Ventilated Patients:
  • Describes the interaction of pressure, flow, and volume with respiratory system mechanics.
  • Highlights the importance of ventilator circuit pressure and inspiratory muscle pressure.
• Measurement and Considerations in Ventilator Settings:
  • Discusses the measurement of airway pressure and transpulmonary pressure.
  • Considers the influence of chest wall compliance on pressure measurements.
• Flow-targeted vs. Pressure-targeted Breaths:
  • Flow targeting sets inspiratory flow with circuit pressure as the dependent variable, while pressure targeting sets an inspiratory pressure target with flow and volume as dependent variables.
  • Flow-targeted breaths guarantee minimal tidal volume, while pressure-targeted breaths enhance gas mixing and patient synchrony.
• Intrinsic (Auto) PEEP:
  • PEEPi depends on minute ventilation, expiratory time fraction, and the respiratory system's expiratory time constant, with potential effects in both pressure-targeted and flow-targeted ventilation.
  • In pressure-targeted ventilation, rising PEEPi decreases delivered tidal volume, accommodating to limit the buildup of PEEPi.
• Distribution of Ventilation:
  • Positive-pressure breath distribution is influenced by regional resistances, compliances, functional residual capacities, delivered flow pattern, and patient inspiratory efforts.
  • In lungs with severe mechanical heterogeneity, an active diaphragm may promote regional overinflation.
• Alveolar Recruitment and Gas Exchange:
  • Alveolar recruitment improves V˙/Q˙ matching and gas exchange, with potential benefits in maintaining patent alveoli and surfactant generation.
  • Optimizing PEEP is a balance between recruiting recruitable alveoli and avoiding regional overdistention, with additional recruitment possible through recruitment maneuvers and inspiratory time prolongations.
• PEEP and PEEPi:
  • PEEP and intrinsic PEEP (PEEPi) affect ventilation and oxygenation.
  • PEEP can redistribute ventilation and affect V/Q ratios.
• Effect on Cardiac Output:
  • Increased intrathoracic pressures reduce cardiac output during positive-pressure ventilation.
  • Inspiratory times and 1:1 I:E ratios affect intrathoracic pressures and may require sedation or paralysis.
• Mechanical Loads on Inspiratory Muscles:
  • Mechanical loads on respiratory muscles are described in terms of pressure-time product, work, and power.
  • Inspiratory muscle overload can result from excessive mechanical loads or muscle dysfunction.
• Inspiratory Muscle Dysfunction:
  • Inspiratory muscle dysfunction can result from various factors including systemic inflammatory response syndrome and metabolic disturbances.
  • Inspiratory muscle fatigue can be detected through rapid, shallow breathing patterns and paradoxical abdominal motion.
• Ventilator-Induced Lung Injury (VILI):
  • Excessive alveolar stretch due to positive-pressure ventilation can lead to VILI.
  • VILI may manifest as diffuse alveolar damage and is associated with cytokine release and bacterial translocation.
• Lung-Protective Ventilator Strategies:
  • Lung-protective strategies aim to minimize alveolar overdistention and prevent VILI.
  • Monitoring parameters such as driving pressure and stress index can help fine-tune ventilator settings.
• Cardiac Effects of Intrathoracic Pressure:
  • Changes in intrathoracic pressure can affect right and left ventricular function.
  • Ventilator-induced changes in perfusion pressures may lead to high V/Q units and increased dead space.