What roles do negative inspiratory flow and negative inspiratory pressure play in assessing lung mechanics and readiness for liberation from mechanical ventilation (MV) support in patients?

From the Guidelines

Negative inspiratory flow and negative inspiratory pressure are crucial parameters in assessing lung mechanics and readiness for ventilator liberation in mechanically ventilated patients, with a recommended negative inspiratory pressure of at least -20 to -30 cmH2O and an inspiratory flow of at least 60 L/min during a spontaneous breathing trial. These measurements help clinicians evaluate if patients can overcome airway resistance and generate adequate tidal volumes independently, as stated in the guidelines by the American Thoracic Society and the American College of Chest Physicians 1. The physiological basis for these parameters relates to respiratory muscle function - the diaphragm and accessory muscles must generate sufficient negative pressure to draw air into the lungs against resistance.

Key Considerations

• Inadequate values suggest respiratory muscle weakness, which may result from critical illness myopathy, prolonged ventilation, malnutrition, or electrolyte abnormalities, all of which would predict failed extubation attempts.
• When performing these assessments, the ventilator should be set to a spontaneous breathing mode with minimal support, and measurements should be taken after the patient has rested to avoid respiratory muscle fatigue.
• The use of inspiratory pressure augmentation during spontaneous breathing trials, protocols minimizing sedation, and noninvasive ventilation immediately after extubation are also important considerations in ventilator liberation protocols, as outlined in the clinical practice guidelines 1.

Clinical Implications

• Clinicians should prioritize the use of negative inspiratory flow and negative inspiratory pressure measurements in assessing lung mechanics and readiness for ventilator liberation.
• The guidelines suggest that the initial spontaneous breathing trial be conducted with inspiratory pressure augmentation (5-8 cm H2O) rather than without, and that protocols attempting to minimize sedation be used 1.
• Patients at high risk for extubation failure who have passed a spontaneous breathing trial should be considered for extubation to preventative noninvasive ventilation, as recommended by the guidelines 1.

From the Research

Assessing Lung Mechanics and Readiness for Liberation from Mechanical Ventilation

• Negative inspiratory flow and negative inspiratory pressure play crucial roles in assessing lung mechanics and readiness for liberation from mechanical ventilation (MV) support in patients 2.
• The amount of negative pressure required to initiate inspiratory flow and peak expiratory resistance are important factors to consider when evaluating the effectiveness of mechanical ventilation systems 2.
• Systems that impose high inspiratory elastic threshold loads and expiratory flow resistive loads may have a deleterious effect on the mechanics of breathing, and thereby limit weaning success and eventually impair the recovery of certain patients in respiratory failure 2.

Ventilator-Derived Parameters

• Mechanical ventilation is a life-support system used to maintain adequate lung function in patients who are critically ill or undergoing general anesthesia 3.
• Ventilator-derived parameters, such as intrinsic PEEP, peak and plateau pressures, driving pressure, transpulmonary pressure, mechanical energy, mechanical power, and intensity, should be strictly monitored at bedside to develop a personalized approach to mechanical ventilation 3.
• During assisted mechanical ventilation, additional parameters such as the pressure generated 100 ms after onset of inspiratory effort and the pressure-time product per minute should also be evaluated 3.

Flow Curves and Respiratory Mechanics

• Inspiratory-expiratory flow curves provide valuable information about respiratory mechanics, patient effort, and patient-ventilator interactions during mechanical ventilation 4.
• Monitoring and assessing flow curves can help clinicians adjust ventilator settings and improve patient outcomes 4.
• There may be an association between inspiratory effort and flow, which can guide weaning processes and patient-ventilator synchronization 4.

Predictors of Weaning Outcome

• The Rapid Shallow Breathing Index (RSBI) is a reliable predictor of weaning outcome in critically ill patients on mechanical ventilation 5, 6.
• Ultrasonographic diaphragmatic parameters, such as diaphragmatic excursion, diaphragmatic thickening fraction, and diaphragmatic contraction velocity, can also predict weaning outcome 5.
• The effect of flow trigger on RSBI measured through the ventilator should be considered when assessing readiness for weaning from mechanical ventilation 6.