7200

1. Waveforms
   Identify pressure and flow waveforms of volume controlled ventilation

   Rectangular Flow Waveform Highest Peak Pressure Lowest Mean Airway Pressure

   Descending Ramp Flow Waveform Lowest Peak Pressure Highest Mean Airway Pressure

   Sine Flow Waveform Peak pressure less than rectangular waveform and higher than descending. Mean Airway Pressure is higher than rectangular waveform but lower than descending.

   Rectangular Flow Waveforms Comparing Fast and Slow Space Ventilation Inspiratory flow wave remains constant. Expiratory flow takes longer to return to baseline with slow space. Plateau pressures are greater with fast space. The peak minus plateau pressure difference is greater in slow space due to the increased airway resistance and normal or increased compliance.

   Ramp Flow Waveforms Comparing Fast and Slow Space Ventilation Changing to a ramp flow waveform lengthens inspiration and may cause air-trapping in slow space lung units. Expiratory flow takes longer to return to baseline during slow space ventilation. Peak and plateau pressures essentially equal each other in a ramp waveform since inspiratory flow has dropped to zero at end inspiration. Airway pressures are higher in fast space ventilation. The increase in airway pressure from high airway resistance in slow space ventilation is minimized with a ramp flow waveform.

   Sine Flow Waveforms Comparing Fast and Slow Space Ventilation Changing to a sine flow waveform lengthens inspiration and may cause air-trapping in slow space lung units. Expiratory flow takes longer to return to baseline in slow space. Peak pressures are less than a rectangular waveform but more than a descending ramp. The peak minus plateau pressure difference is less than a rectangular waveform but more than a descending ramp in slow space ventilation due to the lower flowrates at the beginning and end of inspiration.

   Certain conditions may be easier to detect viewing volume waveforms. The first graphic shows flow/time and volume/time waveforms of a volume controlled breath during normal function. The second graphic shows the same breath with a pause added.

   The next graphic is an example of what a leak in the circuit would look like in a volume waveform. The last set shows a volume waveform during a patient disconnection.

   Pressure waveforms can be used to adjust Flow-by. In Flow-by, the patient's inspiratory effort creates a drop in flow, rather than a drop in pressure, to trigger a breath. If the flow sensitivity and base flow are adjusted properly, there should be little, if any, drop in pressure when the patient triggers a breath.

   Interpret pressure/volume loops of volume controlled ventilation

   Set the volume and pressure ranges appropriately. The volume axis should touch the pressure axis at the pressure baseline. The portion of the curve to the right (positive pressure side) of the volume line represents the work performed by the ventilator. Low compliance will decrease the slope and flatten the loop. The bottom part of the curve is inspiration and the upper part is exhalation.

   An assisted volume controlled breath shows the patient's inspiratory effort to the left of the volume axis. The area of the loop to the left is displayed as Inspiratory Area in the upper left portion of the Graphics screen. The inspiratory area is the patient's work of breathing in joules/liter. Normal work of breathing is 0.3 to 0.8 joules/liter. Respiratory muscle fatigue is likely to occur if the work of breathing exceeds 1.5 j/l.

   Pressure/volume loops can also be used to determine the appropriate PEEP level. If an inflection point appears on the inspiratory limb, the PEEP should be set at that level of pressure. The inflection point represents an improvement in compliance from alveolar recruitment. The dashed lines represent the change in slope (compliance) on either side of the arrow (inflection point).

   Flow/volume loops help in evaluating whether airway obstruction lessens after bronchodilator therapy. Improvement would result in a higher peak expiratory flow (top half). Keep in mind that these breaths are not forced exhalations. If there are higher expiratory flows from a reduction in airway obstruction, there would be less of a scooped appearance during mid to end exhalation.
   Remember that exhalation and the following inspiration are from different breaths. An exhalation may be from a ventilator breath followed by a spontaneous inspiration if the patient is in SIMV mode.

   Interpret pressure and flow waveforms of pressure controlled ventilation

   In pressure controlled ventilation, volume delivery can be drastically different at any given level of pressure control, depending on the patient's compliance and resistance.

   When ventilating fast spaces with pressure control, lengthening inspiratory time will not increase tidal volume if inspiratory flow returns to baseline as indicated by the arrow. In this case the tidal volume can only be increased with an increase in the level of pressure control.

   Since expiratory flow quickly returns to baseline in fast space, much higher respiratory rates can be tolerated without air-trapping. If air-trapping is desired as in inverse ratio ventilation, inspiratory time should be lengthened while observing flow waveforms. It is better to fix the I:E ratio rather than the inspiratory time when providing inverse ratio ventilation. Otherwise, it is preferable to fix the Inspiratory Time to minimize fluctuations in volume delivery.

   When ventilating slow spaces, inspiration typically ends before inspiratory flow has returned to baseline. Lengthening inspiratory time will increase volume delivery. However, greater inspiratory volumes will require more time for exhalation and lower respiratory rates are necessary to prevent unwanted air-trapping.

   In CMV mode, slight increases in the respiratory rate may lead to air-trapping in cases of predominantly slow space lung units as in patients with COPD. Note in all the air-trapping examples, that whether or not the airway pressure in the pressure/time waveform returns to baseline at end-exhalation is NOT important. The airway pressure in this case IS NOT a measurement of Auto-PEEP. There needs to be an end-expiratory pause in order for airway pressure to equal alveolar pressure and measure Auto-PEEP.

   No. The arrow indicates that the inspiratory flow has returned to baseline and maximal volume delivery at that level of pressure control has occurred. Lengthening inspiratory time or going to a larger I:E ratio will not increase the tidal volume.

   Increasing inspiratory time will increase tidal volume if flow waveform truncated at end of inspiration.

   Can observe actual expiratory time to estimate time constant and determine whether tc fast or slow (normal tc = .5 seconds). Fast space occurs when compliance is decreased and airway resistance is normal or decreased as in ARDS. Slow space occurs when the airway resistance is increased and compliance is normal or increased as in COPD. 3 seconds/ 5 = 0.6 for one tc. Slightly slower.
   The pressure/volume loop on the right is in pressure control ventilation. The straight vertical line represents the inspiratory pressure level.
   When compliance decreases, the tidal volume will decrease. The pv loop changes as shown in red. Since inspiratory pressure is controlled, there will be no over-inflation spikes or flattening of the curve.

   Go back to ventilator table.

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