Inspiratory
Pause---
It’s use in
clinical practice…..
If indicated and beneficial, I like to use a pause setting when ventilating patients in a volume ventilation mode. At many hospitals, and with many fellow RCP’s, this one ventilator option seems to bring out a reaction one would expect had I reached into a black bag of ancient alchemy and witchcraft remedies. For some strange reason, an inspiratory pause in pressure control ventilation elicits no such reaction.
While some may feel that this is a somewhat silly issue, it actually touches on almost all of the reasons for why we do what we do with the ventilator. If that’s silly call me Curly.......
Like many maneuvers we perform with ventilating a patient, an inspiratory pause is a double-edged sword. One question, or concern, is whether it belongs in the realm of flowrates and waveforms (i.e. square vs. tapered, which are the settings we adjust on a timely basis), or whether or not an inspiratory pause fits more within the realm of Peep settings, settings which presently require a physician order to adjust/apply in routine clinical practice.
I strongly feel the former—that we should be able to set a pause without an order. Sure you could further flow starve a patient, but we can do that already with too low of a flowrate. Sure you could worsen auto-peep, but again you could do that anyway with too low of a flow and too high a minute ventilation. And you may further worsen hemodynamics by compormising preload, but again that could also be done by too low of a flowrate and too high of a mean airway pressure and Peep.. In short, yes this double-edged sword will allow you to do even more damage if you aren’t watching out for the above anyway.
So if you are already not evaluating the flow needs of the patient, checking for auto-peep, and evaluating the hemodynamic effects of the ventilator, then please don’t use a pause even with an order because you will probably do more harm than good. And while you are at it, ask yourself why it is that you are responsible for the application of pressure to the cardio-pulmonary system of critically ill patients if you can’t evaluate it’s effects in the manner I’ve described above.
Let’s not let this lowest common denominator approach affect the rest of us, or limit the benefits we may bestow on patient care!
Another problem with relying on an order for the pause is that then we could be forced to maintain the pause in the face of worsening hemodynamics or increased patient demand for ventilation.
Picture this----a patient comes back from OR fluid overloaded, but otherwise in fair shape. As he needs 100% O2 you obtain an order for .4 seconds insp. Pause. 10 hours later it’s 3am and now the patient has begun to get a septic look. His SVR is down to 500, blood pressure is 90/45, he is awake and tachypneic with a set RR of 12, but he is actively assisting for a total rate of 22 with marginal ABGs on 80% O2—ph 7.35 PaCO2 38 PaO2 80. You look at the pressure and volume waveforms and notice that the Peak pressure is occuring as the patient is attempting to exhale against the set insp pause. So the pause should be taken off and the flow rate increased to meet patient demand, even considering a square waveform. Do you call the MD to ask if the pause may be removed? Do you leave the pause on “because it’s ordered”?
When the ramp waveform is utilized (PB 7200) flow tapers down to 5 L/M at the end of the delivered VT . So with a flow rate of say 50 L/M on a ramp waveform the flow is so low, for so long, that you have essentially set a pause anyway.
Of particular interest is the Consensus Statement on Mechanical Ventilation---the most important target pressure to monitor is the end inspiratory occlusion pressure, keeping it less than 35 cmH2O over the peep. An added benefit of routinely setting a small pause on a ventilator is that this provides us with routine monitoring of the most important number we should be looking at---the plateau pressure.
“The underlying pathophysiology of various disease states varies with time, and thus the mode, settings, and intensity of ventilation should be repeatedly re-assesed.”
And with that in mind…..
·
I
would like to discuss why an inspiratory pause setting may be desirable,
·
what
problems or concerns may arise with inspiratory pauses, and
·
why
only
using
them with an MD order would be just as potentially dangerous clinically
as
having to rely on MD orders to set
flowrates.
In
essence relying an an MD
order
would function as either ban on them, or may act to prevent the pause
from
being removed when warranted.(i.e “you can’t do that without an order!”
or
“you can’t take that pause off without an order”)
1.
to
raise mean airway pressure and increase FRC and improve oxygenation
2.
to
enhance the distribution of ventilation
3.
to
achieve the same beneficial ventilatory pattern as seen in pressure control
and
pressure support. By combining high
initial insp flowrates with the
decelerating
waveform AND adding an insp pause of .3 to .4 seconds (or even longer in severe
ARDS when using the Open Lung Approach)one can
mimic
the beneficial pattern of pressure control and at the same time maintain
the
volume guarantee to the patient.
Often
the lungs are only recruited at end inspiration---a pause may aid in
furthering
the recruitment
What
are the potential drawbacks/dangers of an insp pause? Basically there
are
three----
1.
hemodynamic
compromise as seen with too much Peep relative to
preload,
2.
flow
asynchrony( the patient is attempting to breathe out as the
pause
is being maintained), and
3.
any
contribution to auto-peep by futher
decreasing
time for exhalation. Let’s look at those...
·
Hemodynamic
Compromise---basically if the arterial waveform dampens when a
mechanical
tidal volume is delivered, that signifies that the preload is
insufficient
relative to the ventilatory gauntlet we are imposing.
(See
the book Mechanical Ventilatory Support by Perel and Stock pages 61-65) Learn
to always look at the relationship between arterial waveform changes and
delivered VT. This is dynamite
stuff.
Another way to rapidly assess at the bedside the balance between the patient’s preload and the ventilatory gauntlet that we are imposing is to compare the CVP pressure and the mean airway pressure. Bear in mind that the CVP is in mmHg and the mean airway pressure is in cwp. If, for instance, the CVP is 14 mmHg and the mean airway pressure is 24 then the patient doesn’t have enough preload/vascular volume to effectively traverse the pressures from mechanical ventilation (take about a third off the cwp to compare to mmHg). Adding a pause will raise the mean airway pressure.
I look at all of this anyway, and often, if the patient is relatively hypovolemic, increase flowrate, turn down the minute ventilation, inquire if anything is being down to increase vascular volume, etc. In short---we can do the same compromise by using the decelerating waveform with a low inspiratory flowrate, and with the use of Peep. Perhaps we need to ban the decelerating waveform and flowrates less than 60 l/m? Or Peep itself? (just kidding).
Flow Asynchrony---Once again, with a low flowrate in
the decelerating waveform you are just as likely to have the same problem. In the decelerating waveform flow ramps down
to 5 l/m by the end of the breath, with the mean flowrate being very slightly
over the peak (i.e 40 l/m peak would have a mean flowrate of 20 l/m). So the inspired gas may not be totally still
(paused) by the end of the breath, but to the patient who is trying to exhale,
that is a very academic difference. ( In fact one of the disadvantages of
pressure support is that sometimes COPDers need to actively exhale to cycle the
breath into exhalation.) Consider the patient getting 800ccs VT with a
decelerating waveform of 40 l/m without any pause. Then consider turning up the flowrate to 100 L/M and adding an
insp pause of .3 seconds. The insp time is probably about the same in each
setting, but now you are better guaranteed to have flow synchrony at the
beginning of the breath, where it counts,
and get the positive pressure effect advantage at the end of the breath
,where it counts. And yes, your peak insp pressure now will be higher as the
higher initial flowrate slams into the ET tube. But it is essentially a meaningless number.
·
Regarding
auto-peep, you can get auto-peep anyway without a pause. Certainly
You should be looking for it. And if you do find auto-peep, then do whatever is warranted to lessen or
eliminate it—decrease minute volume, crank up the flowrates, remove
any
pause, or “pause
effect” (switch
to square instead of ramp
waveform).
Actually one problem of using the pause, is if the next
therapist doesn’t notice/recognize that the pause is on, and then, in an
attempt to reduce the PIP, turns down the flowrate. If hemodynamic effect,flow synchrony, or auto-peep aren’t being
looked at, then the patient will be worse off.
But again, the
same damage could be done anyway without any pause.
Part
of our job is to “sculpt” the breathe to meet the variations that occur in
patient demand and requirements. Flowrate setting, waveform settings, pressure
vs. volume ventilation, and insp pause can all come into play. It would be unfortunate if we don’t all look
at and consider another tool we may use.
·