Principles of Critical Care > Part IV. Pulmonary Disorders > Chapter 32. Ventilator Waveforms: Clinical Interpretation Gregory A. Schmidt

Key Points

"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..."