https://pubmed.ncbi.nlm.nih.gov/32844110/?s=09

Understanding Flow Waveforms and Their Impact on Mechanical Ventilation

• Importance of Inspiratory-Expiratory Flow Curves:
  • Flow curves chronologically show the velocity and direction of inspiration and expiration and are influenced by the respiratory mechanics, the patient’s effort, and the mode of ventilation and its settings.
  • The information provided by the flow curves during mechanical ventilation, such as respiratory mechanics, the patient’s effort, and patient–ventilator interactions, are very helpful when adjusting the ventilator setting.
• Association between Inspiratory Effort and Flow:
  • There may be association between inspiratory effort and flow, and this may further guide clinicians, especially in the weaning process and when patients are not synchronizing with the ventilator.
• Flow Waveforms and Their Utilization:
  • Summarizing the different flow waveforms utilized in commonly used ventilator modes with their advantages and disadvantages.
  • Flow waveforms shapes and patterns are very beneficial for the management of patients undergoing mechanical ventilatory support.
• Importance of Attention to Flow Graphics:
  • Flow sensors not only measure flow but also act as trigger sensors for the patient’s spontaneous breathing, integrate the flow signals to tidal volume, and send feedback for ventilator adjustments in closed-loop systems.
  • Disequilibrium between the tidal volume delivered to the patient and the one measured inside the ventilator occurs due to the pneumatic behavior of the breathing circuit compliance, which can lead to differences in flow and tidal volume among different ventilators.
• Expiratory Flow and its Implications:
  • Expiratory flow is dependent on the applied airway pressure (driving pressure) and tidal volume.
  • Auto-PEEP, caused by insufficient expiratory time for the new breath to start, can lead to hyperinflation, worsened oxygenation, increased ventilation–perfusion mismatch, increased work of breathing, and adverse hemodynamic effects.
• Effects of Lung Diseases on Expiratory Flow:
  • Chronic obstructive airway disease (COPD) results in reduced peak expiratory flow (PEF) and prolonged expiratory time constant, leading to hyperinflation and adverse effects on oxygenation and ventilation.
  • Restrictive lung disease has very low time constant, resulting in rapid return to functional residual capacity (FRC).
• Use of Pneumotachometer in Ventilated Infants:
  • A study investigating the use of pneumotachometer in ventilated infants showed differences between expiratory tidal volume measured by ventilator and that measured with pneumotachometer at the airway or inside the trachea.
  • Equations were developed to compensate for the circuit compliance and the differences in flow and tidal volume among different ventilators.
• Ventilator Display of Graphics and Waveforms:
  • Most ventilators display the graphics of flow (L/min), pressure (cmHO), and tidal volume (mL) against time, and can also display those graphics as loops against each other.
  • Different ventilator manufacturers use different commercial flow sensors and place those sensors either proximal to the patient or to the ventilator.
• Expiratory flow limitation (EFL):
  • EFL is a common phenomenon among critically ill patients, especially in COPD patients, obesity, heat failure, and Acute Respiratory Distress Syndrome (ARDS) patients.
  • Diagnosis of EFL involves altering the expiratory driving pressure and comparing the difference of flow in different conditions at the same lung volume.
• Flow-controlled ventilation:
  • A new ventilator mode called flow-controlled ventilation has shown enhancement of lung aeration and improved gas exchange in animal studies.
  • It uses linear inspiratory and expiratory flow in contrast to exponential expiratory flow in volume- and pressure-controlled modes.
• Types of inspiratory waveforms:
  • There are five different inspiratory flow waveforms used by different modes of ventilators: constant, descending, sinusoidal, decelerating, and ascending.
  • Waveform type can affect peak inspiratory airway pressure, mean airway pressure, and inspiratory time, subsequently affecting ventilation and oxygenation.
• Factors affecting waveforms:
  • Multiple factors including respiratory rate, respiratory system mechanics, rise time, and patients’ muscle efforts affect inspiratory flow shape and peak inspiratory flow.
  • In volume-controlled modes, peak inspiratory flow remains constant with a fixed flow rate and varies according to inspiratory time and peak inspiratory pressure based on respiratory mechanics.
• Effect of patients’ muscle efforts:
  • Higher patient muscle efforts result in higher peak inspiratory flow and mean inspiratory flow.
  • The effect of patients’ Pmus on flow is not well studied or reported.
• Effects of different flows:
  • The effects of different inspiratory flow patterns on respiratory mechanics, oxygenation, ventilation, and work of breathing have been controversial and nonconclusive.
  • Decelerating waveforms achieve higher mean airway pressures and alveolar recruitment with lower peak inspiratory flow, while constant airflow ramp produces shorter inspiratory time with higher peak inspiratory flow.
• Effects of flows on ventilator-induced lung injury (VILI):
  • High peak inspiratory flow was suggested to cause accelerated VILI in animal models using very high tidal volumes.
  • The effects of high peak inspiratory flow on VILI in human studies are unclear due to ethical constraints.
• Clinical use:
  • Modern ventilators do not have a spirometer built-in to calculate tidal volumes.
  • The calculation of tidal volumes is essential in the clinical setting for patient management.