Article Figures & Data
Figures
- Fig. 1.
Relationship of alveolar ventilation to alveolar PO2 and PCO2 and to the ultimate SaO2 and PaCO2 with an assumed oxygen consumption of 250 mL/min and CO2 production of 200 mL/min. The shaded area represents a normal alveolar ventilation. Note that as alveolar ventilation increases, alveolar PCO2 decreases exponentially, and PaCO2 follows. In contrast, as alveolar ventilation increases, the alveolar PO2 increases and approaches the inspired PO2. However, because hemoglobin saturation is virtually complete at PO2 values > 70–80 mm Hg, arterial oxygen content rises little at alveolar PO2 above this. Data from Reference 10.
- Fig. 2.
Pressure-volume plots illustrating loading and unloading ventilatory muscle with positive-pressure breaths. Pressure is on the horizontal axis, and volume is on the vertical axis. The solid diagonal line represents the passive respiratory system compliance. Pressures to the left of this line are patient-generated, and pressures to the right of this line are machine-generated. The area of the pressure volume plot represents work (shaded for patient work, open for machine work). A: Represents a normal subject with normal work. B: Represents a diseased patient with excessive work. C: Represents the same diseased patient being totally unloaded by a machine breath (ie, no patient work). D: Represents the patient only doing enough work to trigger a machine breath that provides most of the work to breath. E: Represents the patient and the ventilator interacting to share the work in a fashion that resembles the normal subject work pattern in A. F: Represents a poorly interactive breath in which inadequate unloading by the machine produces excessive loading of the patient. Conceptually, E would seem ideal. From Reference 41, with permission.
- Fig. 3.
Noninvasive ventilation (NIV) has effects on the ventilatory control center (VCC) in the brainstem. The VCC has an intrinsic pattern generator with important inputs from gas exchange sensors and from mechanical load sensors. The output from the VCC controls ventilatory muscle inspiratory muscle intensity and timing. Importantly, this output can be modulated by cortical influences and drugs, an effect sometimes referred to as a loop gain. Positive-pressure NIV can affect the VCC in a variety of ways that include beneficial (and sometimes harmful) effects on gas exchange, ventilatory muscle loading, and even cortical influences that are affected by the sense of dyspnea/anxiety. CRS = compliance of the respiratory system; Raw = airway resistance.
