Dräger Instructional CD: Mechanical Ventilation - VentWorld

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Key Idea <strong>Mechanical</strong> <strong>Ventilation</strong> Maximum compliance is found between the two inflection points. This is where tidal ventilation should occur to minimize lung damage. An inflection point is a transition from one linear compliance to another. Compliance is less below the lower inflection point due to alveolar collapse. Compliance above the upper inflection point is also less due alveolar over-distention. There is no universally accepted technique for determining the inflection points and it is often done visually. Also, inflection points are not always evident on any particular patient. Flow-Volume Loop Flow volume loops have traditionally been used in the pulmonary function lab to diagnose lung disease. Flow-volume loops generated during mechanical ventilation are different in three major respects from those obtained in the pulmonary function lab: 1. Flow volume loops obtained during mechanical ventilation are normally passive, compared to the forced vital capacity maneuver required of patients in the lab. 2. The shape of the inspiratory portion of the loop during mechanical ventilation is determined by the flow waveform generated by the ventilator. 3. The expiratory portion of the loop does reflect changes in airway resistance but does not necessarily indicate maximal flow limitation as in pulmonary function studies. One practical problem with flow-volume loop displays is that there is no consensus on how the axes should be oriented. Sometimes flow is shown on the vertical scale and sometimes on the horizontal scale. Sometimes inspiration is shown as a positive (upward) flow and sometimes it is downward. For consistency with the previous graphs, the flow-volume loops in this book will show flow on the vertical scale with inspiration in the positive, upward direction. Flow-volume loops can be used to detect autoPEEP and leaks like waveforms can, but they are most useful in the evaluation of bronchodilator response. As you will see below, examination of the expiratory portion of the flow-volume loop clearly shows a change in the shape of the curve as resistance changes. Another practical use of flow-volume waveforms is to access asynchrony during pressure controlled ventilation of neonates. If the infant breathes spontaneously, and out of phase with mandatory breaths, tidal volume delivery will be highly variable. This problem can be minimized by proper setting of the sensitivity or if SIMV is not available, setting the ventilatory frequency to hopefully entrain the patients spontaneous rate (ie, to get the patient to breathe at the same rate or some even multiple of the ventilator rate). Synchrony between the patient and ventilator and the resultant decrease in tidal volume variability is easily seen with flow-volume loops that overlap for several breaths. Low tidal volume variability is desirable not only to stabilize gas exchange but also to minimize the risk of intracranial bleeds.
Volume Control (idealized) Pressure Volume Flow A B Flow 0 5. How To Read Ventilator Displays inspiration expiration Volume Idealized waveforms (A) for volume controlled ventilation with corresponding idealized flow-volume loop (B). Note that the loop is drawn clockwise, with inspiration going to the right and down and expiration to the left and up. The dotted arrows show the correspondence between the waveform display and the loop display for the initial flow rise and tidal volume. The shape of the inspiratory portion of the flow-volume loop is dominated by the constant flow, which hides the change in volume and results in a horizontal line. During expiration, both volume and flow decrease exponentially with the same time constant (see the discussion about time constants in Chapter 4). When one exponential function is plotted against another and there is a single time constant for the system, the result is a straight line. This can be proven mathematically but you can understand it intuitively by recognizing that if volume and flow are changing at the same rate (ie, the same time constant) then the ratio of flow to volume (the slope of the expiratory portion of the flow-volume curve) must remain constant. A curve with a constant slope is a straight line. - 33 -
Page 1 and 2: Fundamentals of Mechanical Ventilat
Page 3 and 4: Fundamentals of Mechanical Ventilat
Page 5 and 6: Table of Contents 1. Introduction t
Page 7 and 8: 1. Introduction to Ventilation . Wo
Page 9 and 10: 1. Introduction to Ventilation vent
Page 11 and 12: Key Idea 1. INTRODUCTION TO VENTILA
Page 13 and 14: 1. Introduction to Ventilation all
Page 15 and 16: 2. INTRODUCTION TO MECHANICAL VENTI
Page 17 and 18: Power Transmission and Conversion 3
Page 19 and 20: Key Idea 3. How Ventilators Work Mu
Page 21 and 22: Extra for Experts “The full advan
Page 23 and 24: Key Idea 4. How to Use Modes Figure
Page 25 and 26: ACHTUNG! Das Machine ist nict fur d
Page 27 and 28: 5. How To Read Ventilator Displays
Page 33 and 34: Leak Pressure Volume Flow A B exhal
Page 35: Static Pressure-Volume Loop (compli
Page 39 and 40: Calculated Parameters 5. How To Rea
Page 41: 5. How To Read Ventilator Displays

Dräger Instructional CD: Mechanical Ventilation - VentWorld

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