Resp Care
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Acute lung injury/acute respiratory distress syndrome is a syndrome of low respiratory compliance. However, longstanding knowledge of applied respiratory mechanics and refined imaging techniques have shown that this is clearly an oversimplified view. ⋯ These basic measurements, once the domain of applied physiologists only, are now available to aid clinicians to choose the appropriate ventilator settings to promote lung recruitment and avoid injury during lung-protective ventilatory strategies. High-resolution lung imaging and bedside recording of physiologic variables are important tools for clinicians who want to deliver specialized care to improve the outcome of critically ill patients in acute respiratory failure.
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The interaction of a mechanical ventilator and the human cardiovascular system is complex. One of the most important effects of positive-pressure ventilation (PPV) is that it can decrease venous return. PPV also alters right- and left-ventricular ejection. ⋯ Understanding and managing these complex and often opposing interactions in critically ill patients is facilitated by analysis of hemodynamic and ventilator waveforms at the bedside. The relationship of PPV to changes in the arterial pressure waveform gives important information regarding appropriate fluid and vasopressor treatment. This article focuses on effects of respiratory pressures on hemodynamics and considers how cardiac pressures can be transmitted to the airway and cause ventilator malfunction.
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Obstruction of the large and small airways occurs in several diseases, including asthma, chronic obstructive pulmonary disease, cystic fibrosis, bronchiectasis, and bronchiolitis. This article discusses the role of ventilator waveforms in the context of factors that contribute to the development of respiratory failure and acute respiratory distress in patients with obstructive lung disease. Displays of pressure, flow, and volume, flow-volume loops, and pressure-volume loops are available on most modern ventilators. ⋯ Presence of flow limitation during expiration and excessive airway secretions can be determined from flow-volume loops. Abnormalities in pressure-volume loops occur when the trigger sensitivity is inadequate, with alterations in respiratory compliance, or during patient-ventilator asynchrony. Thus, ventilator waveforms play an important role in management of mechanically-ventilated patients with obstructive lung disease.
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Patient-ventilator interaction can be described as the relationship between 2 respiratory pumps: (1) the patient's pulmonary system, which is controlled by the neuromuscular system and influenced by the mechanical characteristics of the lungs and thorax, and (2) the ventilator, which is controlled by the ventilator settings and the function of the flow valve. When the 2 pumps function in synchrony, every phase of the breath is perfectly matched. Anything that upsets the harmony between the 2 pumps results in asynchrony and causes patient discomfort and unnecessarily increases work of breathing. ⋯ During pressure-controlled or pressure-support ventilation we can adjust variables that affect when the inspiration terminates (eg, inspiratory time, expiratory sensitivity). (4) Expiratory phase. Patients with obstructive lung disease are particularly prone to developing intrinsic positive end-expiratory pressure (auto-PEEP) and therefore have difficulty triggering the ventilator. Bedside evaluation for the presence of auto-PEEP should be routinely performed and corrective adjustments made when appropriate.
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Pressure support ventilation (PSV) is a commonly used mode. It is patient-triggered, pressure-limited, and (normally) flow-cycled. Triggering difficulty occurring during PSV is usually due to intrinsic positive end-expiratory pressure. ⋯ Patient-ventilator dyssynchrony may occur during PSV if the flow at which the ventilator cycles to exhalation does not coincide with the termination of neural inspiration. The newer generation ventilators offer clinician-adjustable flow-termination during PSV. Ventilator waveforms may be useful to appropriately adjust the ventilator during PSV.