Mechanical ventilation: optimizing patient-ventilator interactions and respiratory outcomes

• Respiratory mechanics and control of breathing:
  • Clinical management of patients with acute respiratory failure is based on understanding respiratory mechanics and control of breathing.
  • Significant changes in respiratory mechanics and respiratory muscle performance contribute to acute respiratory failure.
• Matching ventilator settings to patient physiology:
  • Optimizing mechanical ventilation requires accurate measurements of lung and chest wall mechanics, respiratory muscle function, and respiratory drive.
  • Ventilator settings should be adjusted to match the patient's respiratory physiology.
• Respiratory physiology:
  • Respiratory muscles generate pressure and volume to provide adequate alveolar ventilation with tolerable work of breathing.
  • During spontaneous breathing, the equation for describing the act of breathing is Pmus=(Flow×RRS)+(Volume×ERS)+PEEPi.
• Interaction between variables:
  • The complex interaction between variables in respiratory mechanics and mechanical ventilation can be summarized by the concept of neuroventilatory coupling.
  • Positive pressure ventilation modes may compromise the coupling between effort and ventilatory output.
• Patient and ventilator variables:
  • Patient variables include respiratory drive, ventilatory requirements, and timing of the breathing pattern.
  • Ventilator variables include delivery mechanism, inspiratory trigger, and cycle-off criterion.
• Total ventilator-controlled mechanical support:
  • In this mode, the ventilator replaces the patient's breathing pattern completely.
  • Sedation and/or paralysis may be required, but there are potential risks associated with this mode.
• Partial patient-controlled mechanical support:
  • In this mode, spontaneous breathing activity is partially preserved.
  • The absence of patient-ventilator asynchrony is crucial for effective partial patient-controlled mechanical support.
• Patient-ventilator asynchrony:
  • Patient-ventilator asynchrony occurs when there is a mismatch between patient physiology and ventilator function.
  • Effective inspiratory trigger and phase synchronization are important to reduce patient effort and improve asynchrony.
• External PEEP and inspiratory effort reduction:
  • Application of external PEEP below auto-PEEP level can reduce inspiratory effort.
  • Double triggering, a result of limited expiratory phase caused by short ventilator inspiratory time, can be addressed by increasing Ti, adjusting expiratory threshold time, or optimizing pressure rise time.
  • Reverse triggering, observed in heavily sedated patients, may cause injurious inflation pattern.
  • New triggering algorithms aim to improve patient-ventilator interaction during sudden changes and air leaks.
• Gas delivery asynchrony:
  • Increasing flow rate can reduce respiratory drive and active respiratory muscle work.
  • Inspiratory time imposed during mechanical ventilation determines respiratory frequency.
  • Pressure-targeted breaths may effectively match patient's ventilatory requirements.
  • Optimal patient-ventilator synchrony requires tracking patient's inspiratory flow.
• Inspiratory timing-ventilator cycling asynchrony:
  • Ventilator-patient asynchrony occurs when patient is trying to exhale, but ventilator is still delivering gas.
  • Prolonging mechanical inflation into neural expiration increases inspiratory effort.
  • Delayed opening of exhalation valve exacerbates dynamic hyperinflation.
  • Inspiratory phase asynchrony occurs when patient's neural inspiratory time is not matched with ventilator inflation time.
  • Proper cycling-off requires tracking patient's inspiratory flow.
• Patient-ventilator asynchrony during pressure support ventilation:
  • Threshold value of inspiratory flow decay (expiratory trigger) affects patient-ventilator synchrony.
  • Pressure rise time (pressure slope) can modify inspiratory flow.
  • Pressure support level affects patient-ventilator interactions and air leaks.
  • Cycling-off criteria and inspiratory flow modulation can correct dyssynchrony in PSV breaths.
• Closed-loop control systems:
  • Closed-loop control systems in mechanical ventilators continuously measure physiologic variables and adapt the ventilator to match spontaneous variations in these variables.
  • Design features of closed-loop control systems include the input that activates the system, the output it produces, and the controlling algorithm used to link input and output.
• Modes of mechanical ventilation:
  • Proportional assisted ventilation (PAV) and proportional pressure support (PPS) enable the ventilator to amplify patient effort without imposing specific targets.
  • Proportional assisted ventilation plus (PAV+) reduces patient-ventilator asynchronies compared to conventional PAV.
  • Neurally-adjusted ventilatory assistance (NAVA) allows the patient to retain full control of the breathing pattern, improving patient-ventilator synchrony.
  • Adaptive support ventilation adjusts ventilator settings in response to changes in respiratory impedance, spontaneous efforts, and end-tidal PCO2.
• Patient-ventilator asynchrony:
  • Patient-ventilator asynchrony occurs when the patient's breathing pattern, ventilatory drive, and timing components do not match ventilator trigger, delivered flow, and cycling criteria.
  • Optimization of patient-ventilator interactions requires continuous matching of triggering, flow delivering, and cycling functions with the patient's physiologic variables.
  • Patient-ventilator asynchrony is common during mechanical ventilatory support and often goes unrecognized or untreated in clinical settings.
• Future developments in ventilator technology:
  • Future developments should focus on systems that interface between physiologic parameters and ventilator output.
  • Closed-loop algorithms and total patient-controlled mechanical support should be pursued in ventilator technology.
• Reverse Triggering in ARDS Patients:
  • Deeply sedated, mechanically ventilated patients with acute respiratory distress syndrome (ARDS) frequently experience reverse triggering.
  • This phenomenon is often unrecognized but can have important implications for the ventilation of ARDS patients.
• Improved Ventilation in COPD Patients:
  • Proportional assisted ventilation (PAV) has been shown to improve ventilation and decrease inspiratory muscle effort in difficult-to-wean patients with chronic obstructive pulmonary disease (COPD).
  • Combining PAV with continuous positive airway pressure can further unload the inspiratory muscles and improve ventilation.
• Lung Volume and Neuromechanical Coupling:
  • Variations in end-expiratory lung volume between breaths can affect the transformation of respiratory muscle activation into mechanical output (neuromechanical coupling).
  • This highlights the importance of lung volume in determining respiratory muscle function.