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- Pressure and flow waveforms reveal a wealth of information regarding the patient's physiologic derangement.
- Distinguishing the contributions of resistive and elastic pressures allows tailoring and monitoring of therapy.
- Auto-PEEP should be sought in all mechanically ventilated patients.
- Ventilator waveforms show how adequately the physician has accommodated the ventilator to the patient.
- Patient effort confounds interpretation of pressures and flows.
- Attention to ventilator waveforms can improve the accuracy of hemodynamic interpretation.
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Intensive care ventilators generate tidal ventilation by applying to the endotracheal tube or mask a pressure higher than the alveolar pressure. This is true whether the mode of ventilation is volume-preset (volume assist-control ventilation [ACV] and synchronized intermittent mandatory ventilation [SIMV]); pressure-preset (pressure-support ventilation [PSV] and pressure-control ventilation [PCV]); or more complex modes (pressure-regulated volume control [PRVC], proportional assist ventilation [PAV], airway pressure release ventilation [APRV], and volume-support ventilation [VSV]). The capability to display waveforms turns modern ventilators into sophisticated probes of patient respiratory mechanics and of patient-ventilator interaction. Respiratory system mechanics and waveform analysis should be integrated into routine ventilator management of the critically ill patient. The fundamental aims are to (1) determine the nature of the mechanical derangement of the respiratory system; (2)~assay the response to therapy; (3) reveal intrinsic positive end-expiratory pressure (auto-PEEP); and (4) determine the patient-ventilator interaction to guide adjustment of ventilator settings. In addition, respiratory muscle activity must be considered when measuring hemodynamic pressures such as the pulmonary artery occlusion pressure (pulmonary wedge pressure, Ppw) or the right atrial pressure (Pra), since these pressures are determined at end-expiration. The time point of end-expiration, as well as the presence of inspiratory and expiratory effort (both of which can greatly confound interpretation of hemodynamic pressures) can be readily discerned by analyzing ventilator waveforms.
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It is easiest to derive clinically useful information about the patient's respiratory system when volume-preset modes such as ACV or SIMV are used. At least when the patient is passive, the pressure at the airway opening (Pao) and the pressure vs. time waveform reflect the mechanical properties of the respiratory system, yielding valuable clinical information. During pressure-preset modes, such as PSV and PCV, some information can be derived from the flow vs. time waveform, but this information is generally less readily interpreted than that obtained during volume-preset ventilation. Below we review the determinants of the pressure and flow versus time waveforms during volume-preset, then pressure-preset ventilation, including how to recognize and quantitate auto-PEEP as well as a method for using this information to adjust the ventilator. Volume-pressure loops are reviewed in terms of how they may aid management of the patient with acute lung injury (ALI) or acute respiratory distress syndrome (ARDS). The potentially confounding effect of patient effort on the pressure and flow waveforms is discussed. Finally, examples of problems revealed through careful interpretation of waveforms are presented.
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Gas is driven to and from the lung by a pressure difference between alveolus and airway ...