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Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (5)

Pulse oximetry

From measurements of pressure and VT, it is possible to calculate mean airway pressure (P aw), resistance, compliance and work of breathing (Table 1). Many of the desired and deleterious effects of mechanical ventilation are determined by P aw. Factors affecting P aw are PIP, PEEP, inspiratory:expiratory ratio, respiratory rate and the inspiratory pressure waveform. Typical P aw values for passively ventilated patients are 5 to 10 cm H2O (normal), 20 to 30 cm H2O (ARDS) and 10 to 20 cm H2O (airflow obstruction). The difference between Pplat and total PEEP is determined by the combined compliance of the lung and chest wall. The VT used to calculate compliance should be corrected for the effects of volume compressed in the ventilator circuit, and PEEP should include auto-PEEP. Causes of a decrease in compliance in mechanically ventilated patients include pneumothorax, mainstem intubation, congestive heart failure, ARDS, consolidation, pneumonectomy, pleural effusion, abdominal distension and chest wall deformity. Airway resistance can be calculated from measurements of pressure and flow.

Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (4)

Evaluation of the static pressure-volume (P-V) curve may be useful for patients with acute respiratory distress syndrome (ARDS). In such patients, the P-V curve typically has a sigmoidal shape with an inflection point at low lung volume (lower Pflex) and another inflection point at high lung volume (upper Pflex) (Figure 6). PEEP should be set above the lower Pflex to avoid repeated opening and closing of lung units with each respiratory cycle. Likewise, Pplat should be set below the upper Pflex to avoid overdistension injury to the lungs. In other words, the patient should be ventilated on the linear compliant part of the P-V curve.

Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (3)

An end-expiratory pause can be used to determine autoPEEP. This method is only valid if the patient is not actively breathing and there are no system leaks (eg, circuit leak or bronchopleural fistula). For patients who are actively breathing, an esophageal balloon is needed to determine autoPEEP. During the end-expiratory pause, there is an equilibration between end-expiratory pressure (total PEEP) and proximal airway pressure. Auto-PEEP is the difference between set PEEP and total PEEP.

Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (2)

Point-of-care testing

During pressure ventilation, PIP and Pplat may be equal. This is due to the flow waveform that occurs during this mode of ventilation. With pressure ventilation, flow decreases during inspiration and is often followed by a period of zero-flow at end-inspiration. During this period of no flow, proximal airway pressure should be equal to peak alveolar pressure. It follows that PIP should be lower during pressure ventilation than during volume ventilation. With volume ventilation, PIP is greater than Pplat due to the presence of end-inspiratory flow.

Monitoring during mechanical ventilation: MECHANICS DURING MECHANICAL VENTILATION (1)

Point-of-care testing

With volume ventilation, airway pressure increases during inspiration as volume is delivered. The peak inspiratory pressure (PIP) varies directly with resistance, end-inspiratory flow, VT and elastance (ie, inversely with compliance). An end-inspiratory pause of sufficient duration (0.5 to 2.0 s) allows equilibration between proximal airway pressure and alveolar pressure. This manoeuvre should be applied on a single breath and removed immediately to prevent development of auto-PEEP. During the end-inspiratory pause, there is no flow and a pressure plateau (Pplat) develops as proximal airway pressure equilibrates with alveolar pressure. The pressure during the inspiratory pause is commonly referred to as plateau pressure and represents peak alveolar pressure. The difference between PIP and the Pplat is due to the resistive properties of the system (eg, pulmonary airways, artificial airway), and the difference between Pplat and total PEEP is due to the elastic properties of the system (ie, lung and chest wall compliance). Safe online shopping for drugs: cialis professional online to make your *drugs cheaper.

Monitoring during mechanical ventilation: INDIRECT CALORIMETRY (4)

The breath-by-breath calorimeter analyzes FiO2, fractional concentration of oxygen in expired gas (F E O2), F E CO2 and VT with each breath. This obviates the need for a mixing chamber. The system uses the same gas analysis and volume measuring devices as the open circuit calorimeter, and these systems generally have the same limitations as open circuit systems.
In patients with a pulmonary artery catheter, V O2 can be calculated from CaO2, CvO2 and cardiac output as follows:
Monitoring during mechnical
This method can only be used if a thermodilution pulmonary artery catheter is in place. Continuous 24 h indirect calorimetry will ideally produce the best estimate of resting energy expenditure (REE). However, 24 h measurements of V O2 and V CO2 are not practical unless the metabolic monitor is an integral part of the ventilator system (eg, Puritan Bennett 7250, Puritan Bennett).

Monitoring during mechanical ventilation: INDIRECT CALORIMETRY (3)

The closed circuit calorimeter uses a volumetric spirometer, a mixing chamber, a carbon dioxide analyzer and a carbon dioxide absorber. The spirometer is filled with a known volume of oxygen. As the patient rebreathes, oxygen is consumed and carbon dioxide is produced. Carbon dioxide is removed from the system by a carbon dioxide absorber. Expired gas from the patient is analyzed for the fractional concentration of carbon dioxide in expired gas (F E CO2). The volume of the spirometer is monitored to measure tidal volume (VT).

Monitoring during mechanical ventilation: INDIRECT CALORIMETRY (2)

Mixed venous oximetry

Exhaled gas from the patient is directed into a mixing chamber. At the end of the mixing chamber, a vacuum pump aspirates a small sample of gas for measurement of oxygen and carbon dioxide. The entire volume of gas then exits through a volume monitor. The analyzer periodically measures the inspired oxygen concentration. A microprocessor performs the necessary calculations. Meticulous attention to detail is required to obtain valid results using an open-circuit indirect calorimeter. The FiO2 must be stable and less than 0.60, the entire system must be leak-free, and the inspired and expired gases must be completely separated. You will always be able to find High Quality cialis super active online shopping with a trusted foreign pharmacy.

Monitoring during mechanical ventilation: INDIRECT CALORIMETRY (1)

Indirect calorimetry is the calculation of energy expenditure by the measurement of V O2 and V CO2, which are converted to energy expenditure (Kcal/day) by the Weir method:
Monitoring during mechnical

Monitoring during mechanical ventilation: MIXED VENOUS OXYGENATION (2)

C v O2 (and its components P v O2 and mixed venous oxygen saturation [Sv O2]) is decreased with decreases in CaO2 (ie, PaO2, SaO2 or hemoglobin), decreases in Q or increases in V O2. Note that an increase in V O2 with a proportional increase in Q does not affect C v O2. Also note that breathing 100% oxygen by persons with normal lung function does not affect C v O2 because increasing PaO2 affects CaO2 very little (ie, oxygen is very insoluble in blood and hemoglobin is nearly 100% saturated when breathing room air). In patients with abnormal lung function (eg, shunt), a decrease in Pv O2 may produce a decrease in PaO2.

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