Below is the the full text of an article I wrote a while back. As Rod Sterling used to say, "for your submission..."

To really look at why SIMV is pretty much a worthless mode, we first have to review the whole concept of "weaning" patients from the ventilator.
A term currently gaining some usage is "liberating" the patient from the ventilator. While I feel this is a step in the right direction, as the term "liberate" at least doesn't falsely describe what occurs. But the term "liberate from the ventilator" seems to conjure up an image of a group of SWAT RT's breaking down the ICU doors commando style. And I cringe inside as I imagine "Liberation Trials", Liberation Modes, and "Liberation Studies". The term "liberate" describes the end goal of removing the patient from the ventilator. It does not, however, provide a mental construct of the process from full support to "liberation".

I suggest another term

"Fledge" To raise a young bird until it is read to fly (Random House Collgege Dictionary).

Doesn't the term "fledging" realistically describe the process? What if instead of "weaning" we just simply fledged our patients in the ICU "nest". And we checked daily if they were ready to fly.

The term fledge encapsulates both the raising of the bird ; resolving the patient's disease ,preventing complications, assuring ventilatory support which is both full and comfortable, as well as doing daily "flight checks" when the bird looks ready. Meanwhile they can move around the nest and move their wings at will (ala currently possible spontaneous breathing on A/C). Our focusing on that first five minutes of an SBT, perhaps keeping the patient on the vent (CPAP or Spont Mode) to assure alarms available, is all akin to watching out for cats.

To the tune of "Lyin' Eyes" by The Eagles
He wonders how it ever got this crazy
He thinks about a mode he learned in school
Did the vents get better or did patients just get more lazy?
We've come so full circle, some feel just like a fool
My, oh my, you sure know how to arrange things
We've got Automode now and MMV
Ain't it funny how these new modes didn't change things
Still the same old bird you used to be

We've heard it all before at conferences, we've read it in so many articles. Patients on mechanical ventilation occupy a large part of the ICU population. Trying to get patients removed from mechanical ventilation takes up a lot of clinician time and energy.

In this light the default mental constructs of what we are doing can make a huge difference.

Then there is the Consensus Statement in this regard..
If one goes to www.rcjournal.com and then clicks on Clinical Practice Guidelines/Weaning and Discontinuing Ventilator Support ..one will find the guidelines.
Evidence-Based Guidelines for Weaning and Discontinuing Ventilatory Support
A Collective Task Force Facilitated by the American College of Chest Physicians, the American Association for Respiratory Care, and the American College of Critical Care Medicine (italics new)
(and at the bottom a link to a PDF of the entire article)
The ramifications for the Respiratory Therapy profession are profound. The goal is to advance our theory and practice such that patients whom can be removed from mechanical ventilation are do so expeditiously AND that patients not ready to come off are optimally supported until they are able to do so.
But adopting the Consensus Statement for many clinicians, be they RT's, MD's or RN's, involves far more than just implementing point-by-point the individual guidelines. If one looks at the underlying message of what is "new under the sun" I believe it's that we should instill in ourselves the mental construct that what we are attempting to do is more akin to the "fledging" of birds in a nest -- as opposed to the "weaning" of a baby from breastmilk.
Unfortunately a deep-rooted misconception is rife among many Respiratory Therapists. The misconception is that our quest to get patients off the ventilator is best described as one of "weaning". There are "weaning modes", "weaning trials", "weaning parameters" and so on
Usually I find exercises or debates in semantic correctness to be (removed "such") a bore. But in this case our thinking AND resultant practice is actually corrupted, both by the use of the term as well as the acceptance of it's definition. (text removed) And often our care of patients is way less than what it could be.. Further, for years around this long -standing misconception, there has arisen a whole cottage industry. We have essentially useless "weaning modes" like SIMV(1), MMV and Automode. Many of us seem to have a psychological pride or professional stake in the concept that getting the patient off the ventilator has more to do with our knob twirling skills than with the fact that when/if the patient recovers sufficiently from their disease process they will come off the ventilator on their own. Like a bird which is ready to found one day to be ready to fly from the nest.

Yes the Guidelines do use the word "weaning". But what is the definition of weaning?
When one looks at the definition of the word "wean" one finds....

· 1: to accustom (as a child) to take food otherwise than by nursing
2 : to detach from a source of dependence <being weaned off the medication> <wean the bears from human food -- Sports Illus.>; also : to free from a usually unwholesome habit or interest <wean him off his excessive drinking> <settling his soldiers on the land..., weaning them from habits of violence -- Geoffrey Carnall> Merriam's Online Webster Dictionary1 (trim definition?)

· (I just have to comment.."wean them from habits of violence"? "You can only hit your little sister three times today, twice tomorrow, and only once next week").
If we are "weaning" our patients off the ventilator that seems to imply that there is some physiologic adjustment or process underway as the patient breathes ever so gradually more on their own. If one is trying to assist a patient with nicotine withdrawal then certainly the term weaning matches the physiologic events as nicotine addiction is being treated. If a baby is making the transition from breast milk to whole food, then certainly the term "weaning" applies, as a gradual physiologic adjustment is in play. But imagine if a baby were to be on breastmilk exclusively until an advanced age and then, if under dire circumatances, forced to undergo an immediate conversion to whole food. The baby would be able to survive the transition and really wouldn't need any "weaning". Thus it is often with some of our patients as they recover and advance.

I drink anywhere from 2-4 cups of coffee a day. Should I ever decide to "go off" coffee I would probably be wise to avoid those headaches by gradually reducing the number of cups I drink (lessening the acute effects of rebound cerebral vasodilation). I would be weaning myself off in order to allow physiologic adjustment to occur gradually.

Ditto with giving small amounts of IV alcohol to patients in order to avoid the onset of delirium tremens. If the patient is in the hospital long enough we may have "weaned" them off their physiologic dependence.

Along with this "weaning" mindset comes the concept that the ventilator itself has fostered dependence, even laziness. The Toxic Ventilator! And I've so often heard RT's, RN's, and MD's make "weaning"-related comments as if the patient has gotten lazy and needs to be "pushed". I sarcastically refer to this as "The Judeo/Christian Weaning Ethic" I always conjur up images of Cheerleaders and coaches( and those obnoxious parents at Little League games). While in some patients (particularly long termers) the psychological RECONDITIONING aspects of ventilator withdrawal may somewhat justify the term "weaning", just as if one was weaning Bears in Yellowstone off human food, in both cases a direct approach is still probably better. (see the footnote below for a case description of "Psychological Weaning"(2).

But the majority of patients are usually ready for extubation after a brief spontaneous breathing trial. But what about those whom do not pass their first, or even subsequent, spontaneous breathing trials (SBT)? The Consensus Guidelines recommend thereafter only once-a day repeats of the spontaneous breathing trial, and these only for those patients continuing tomeet the SBT entry criteria. And full support in between. What logical role would any SIMV rate reductions play in such a gameplan?! Given that a failed SBT provides at least enough of a conditioning signal, is this best described as "weaning"? Does not the continued use of the term "weaning" tend to tempt us to forgo both the letter and spirit of the Consensus Guidelines and return to our former ways?

True the "long term" patients need to have gradually increasing times of spontaneous breathing. But even this is best not termed "weaning" either. Rather it is a mix of two things..One is simple re-conditioning, a re-conditioning which will only occur provided the patient gets compensatory rest and nutrition. .The other aspect to long term "weaning" is that we are simply doing a sloppy and lengthy trial, one in which we hope that the patient's angle of recovery intersects well with our angle of ventilatory withdrawal. Those patients may look fine for one or even eight hours on a T-tube. But their long-term energy stores and strength may not suffice for the long haul. Again, it's not weaning which is occurring but rather reconditioning and recovery. We're still just testing the waters and the patient will come off when they are ready.

By way of example: If in 6 months I were to be running regularly in 26 mile marathons it would be because I had reconditioned myself. Not because I "weaned" myself off the TV, the couch and the computer, and reading journals, although that might be a secondary aspect. After all, I could forgo all those things and substitute by merely spend time at the local bar. Just as we could theoretically "wean" our patients off the ventilator and merely place them on ECMO.
Another "contaminant" in our thinking seems to be a migration of anesthesia concepts regarding sedation and paralysis. If a patient needs sedation or paralysis, there are good clinical grounds for only giving as much as needed-- and not a drop more. But this notion seems to be expanded to also include that of ventilatory support ala "The Toxic Ventilator" concept. This notion can be harmful and I often see it lead to various modes and routines"”ones which seem to induce a chronic state of fatigue as clinicians try to fine-tune the level of ventilatory support to the bare minimal. Often this fatigue is both masked by partial support and plagued with intermittent wild dashes towards some imaginary finish line. If, on an imaginary and increasing scale of ventilatory support of 1-10, the patient fails outright at level 4, then the goal seems to keep them at level 5 for as long as possible, punctuated with regular attempts to "dip" into level 4. If "weaning" were truly what we are doing, then this perspective would make sense. But by such "fine tuning" we risk much and gain nothing. When the patient is ready to come off the ventilator they will, irrespective of any knob twirling or precision tuning we do. Just as a baby bird reared well in a nest will fly away when ready.
In fact, by not focusing on full support, by not assuring the patient can rest and recover between daily spontaneous breathing trials, we may actually be working against the potential day ever coming to pass that the patient passes the trial.

Going back to the Guidelines, if one considers the types of practices, ones still rampant, which go against the guidelines they would include...
· Instead of a daily SBT the patient is kept on some adjustable level of partial ventilatory support such as SIMV or PS. The level is "weaned" off. Perhaps this is done in parallel with daily SBT's, or perhaps not.
· When a patient fails an SBT, instead of returning to full support the patient is instead placed on SIMV or PS. The level or rate may be increased. Or not.
· Instead of doing only a daily SBT, the patient is "weaned" several times a day.
· Even though a decision has been made not to extubate a patient irregardless of how an SBT turns out, the SBT is still done in order to "work them".

So how did things ever get so crazy? To understand the current theoretical mess we seem to find ourselves in, I think going back to describe the origin of how some of the ventilator modes and concepts arose is in order....(This history is also succinctly described in the book Clinical Practice in Respiratory Care by James Fink and Gerald Hunt Lippincott Williams & Wilkins See the chapter on Ventilator Weaning)

WAY BACK WHEN........

I first entered the profession of Respiratory Therapy back in 1982. In that era the ventilators I almost exclusively used in practice were Puritan Bennett MA-1's and the Bear Ventialtors. The MA-1 was factory designed to function only in Assist Control mode. A volume breath was triggered by either the patient's inspiratory effort, a timed setting, or by an overly sensitive trigger setting. A bellows then pushed air into the patient. Flow was square. It was crude but effective. However, the patient often had to be sedated to tolerate the resultant less than stellar form of patient-ventilator synchrony"”one which would occur with spontaneous breathing on the patient's side meeting the ventilator.. Imagine what it would be like to wake up in an ICU, intubated, scared, perhaps with a high temperature or pain with each inspiration, ..and to also be tethered to such a device! If patient ventilator synchrony can be compared to two dancers ,this was having dance partners composed of one 20 year old Salsa dancer and an 80 year old ballroom dancer. (Debbie, have at a better analogy)

To then see if the patient were ready to be extubated one had to remove the ventilator from the patient and place the patient on a T-tube. And you (or the nurse) had to stay with the patient for the entire time. Often the patient would look fine during the T-tube trial and go on to be extubated. But imagine if the patient became dyspneic, RR say 35 or higher with high inspiratory demands, clearly needing continued ventilatory support. The only way to reconcile returning the patient to a ventilator such as the MA-1 would be to sedate, even paralyze, the patient. And especially so after a failed T-tube trial.

A patient could breathe spontaneously on an MA-1 in Assist Control Mode. But one had to set both the flow rate and tidal volume high enough to overshoot the patient's spontaneous flow and volume demands. If one failed to do so the patient would be sucking negative into the circuit and a vicious cycle of patient-ventilator asynchrony would ensue. I recall setting VT's of 1.2 liters as a matter of course, even on small individuals. The sigh volume became the set tidal volume. And even with such aggressive flow and volume settings after suctioning or bed changes the patient would still often have to be hand-ventilated for a time before returning them to the ventilator.

Then Intermittent Mandator Ventilation (IMV) was invented. The concept was that the patient could breath spontaneously between mandatory breaths. We would take over some of the work of breathing with the ventilator, the patient could share in the load. Ventilator-induced muscular atrophy would be avoided, patients could become more easily extubated, etc.

But to configure the MA-1 Ventilator to function in IMV mode one had to disable the sensitivity function----note IMV without the "S"(-IMV) for synchronization. One would run flow from a blender into a bag affixed via an H valve to the cascade humidifier. As A/C needed to be disabled, the only way to do that was to make the mandatory breaths arise from time only. It was postulated, and often did seem to happen, that the patient would "learn" to synchronize with the mandatory rate. (Reminds me of the old saying about Death and Taxes.) (keep part in parenthesis?)

So crude unsynchronized volume breaths would be interspersed with spontaneous breaths in which the patient would be moving flow along the circuit---there was no pressure support with these breaths.

Then along came the Bear Ventilator, one which had an actual SIMV mode. Now the mandatory breaths would attempt to synchronize with the patient's spontaneous breaths. And the patient would have to activate a sluggish demand valve. It seemed that the patient would open the demand valve and a rush of air would ensue a bit later in the breath cycle.
A brief aside about the Bear Ventilator. In addition to SIMV mode there was both an Assist/Control mode and a Controlled Mandatory Ventilation mode, or CMV. In CMV the patient was locked out of the ventilator. The patient could only get the mandatory times modes. Any spontaneous breaths would not open any demand valve, nor would they trigger a mandatory breath. Suffocation within the circuit was how it was described. To this day I don't know why that mode was even placed on the ventilator. The salient point is that ever after the mode "CMV" was tainted with the reality of that original Bear Ventilator configuration. When trying to initiate CMV ventilation on the PB 7200 or the Drager Evita 4 I have run into those biases from MD's, RN's, and fellow RT's.

Then PB came out with the MA2+2, which had an SIMV mode. Let me just say that this ventilator had the most sluggish demand valve I have ever witnessed. I would actually see patient's pull the manometer way negative during the spontaneous breaths.
What all this meant originally is that a patient needing full ventilatory support often had to be sedated such that prolonged ventilatory support often did entail significant atrophy and de-conditioning. (text omtted). Therefore a desire ensued for the patient to begin some spontaneous breathing while still receiving substantial ventilatory assistance. If one has an uncooperative and sluggish dance partner, perhaps one would be better off dancing alone.

The crude form of A/C then in existence begat IMV. The asynchrony of IMV begat SIMV. The sluggish demand valves of the early SIMV ventilators then begat Pressure Support. Pressure Support hasn't exactly been a panacea though. The Flow termination criteria on the PB 7200 was set at a constant 5L/M. This often meant that COPDers would have to actively exhale to cycle into the expiratory phase. The 5% flow termination criteria of the Servo 300A meant, and especially when combined with the set MV target, could lead to outright asyncronny and increased auto-peep (3)
The economic reimbursement aspect of this also begs to be mentioned. In the "old days" of fee-for-service reimbursement the hospital was actually quite profitably reimbursed for ventilator hours. A surgical patient recovered on an MA-1 might quickly come off the ventilator after a T-tube trial. But now hours more profit could be reaped --by having a patient on a ventilator longer as the SIMV rate was "weaned". In so many ways we continue to pay the price for that era!

Meanwhile as we went from IMV to SIMV to PS, throwing in MMV and Automode along the way a curious thing has happened.

A/C (or labeled CMV on some ventilators) got better. It became a good dance partner. Not perfect, but consider

In the new hybrid modes such as PRVC/Autoflow/VC+ the mode is essentially volume targeted Pressure Control. The ventilator is in a volume-targeted and adjustable PC mode, the exhalation valve is "active" meaning that the time of inspiration is akin to a CPAP level. The patient can now breath spontaneously AND get full support. There are really only three potential problems..

1) The time of inspiration may not match that of the patient. This problem is touted to be solved by switching the patient to Pressure Support (or it's volume targeted variant, Volume Support). But Pressure Support breaths are far from perfect in this regard as well.
2) If the patient is truly in need of robust full support and has a tidal volume dialed in lower than what the patient is demanding, a vicious cycle may ensue. The more the patient draws in the lower the subsequent pressure control levels. This problem would also exist if the patient were to be in volume control mode, but the resultant starkly evident patient-ventilator asynchrony would mandate an immediate response and mode change/adjustment. With the hybrid modes the onset can be more insidious. The VT either needs to be directly increased, indirectly increased via a switch to PC, or the patient sedated. It is not the "fault" of the mode, just clinicians playing catch-up with the new sights and sounds arising from an old problem now wrapped in new technology. It will be interesting to see if new ventilator modes (PAV or the "P0.1 Controller") solve this occasional but serious problem).
3) Some of the hybrid PC/VC modes adjust the PC level via volume leaving the ventilator, some by the volume returning, one by a mix of both. Without getting into the various situations whereby each could be a plus or minus, I would just say that the RT's in attendance need to be aware of what algorithm their specific ventilator follows and the scenarios where it could be problematic (chest tube leak, running external flow for continuous meds, cuff leak, etc.).

So a quest for a Holy Grail of ventilation and weaning was begun many years ago. It was a quest based on the inherent technical problems of the original A/C mode. As we, and our patients, traversed the path from crude A/C to SIMV, adding PS, throwing in MMV, Automode, etc., the original problems were resolved. Now a patient can be in a volume targeted A/C mode and get the benefits of both pressure and volume ventilation. We used to bear witness to a cacophony of the sights and sounds of patient-ventilator asynchrony. A patient would be pulling in to activate a sluggish trigger sensitivity, then a crude piston volume push would ensue, or perhaps a sluggish demand valve. Then, if the patient were ready to exhale before the inspiratory flow was finished, pressure would build to a peak and then dump and crash. These problems have all been replaced with their respective solutions.

We didn't come up with better "weaning" modes, we came up with a better fledging mode. The T-tube concept still works fine for the spontaneous breathing trial, as does the new improved forms of A/C work best when the patient needs to be returned to a "stable, non-fatiguing form of support". To some it may seem that we've come full circle, but we could never accomplish what we can today if we had that old technology.

Perhaps the only thing now needing to be weaned is our old concept in regards to weaning itself?

Jeff Whitnack, RRT/RPFT



(1) My comments about SIMV are meant to apply to it's use as THE default support mode, as opposed to A/C. It is not meant to be a criticism of IMV as an adjunct to augment venous return where high levels of Peep are used in ARDS. But again, technology marches forward and now APRV fills that gap. But that era from Florida begat the MA2+2 I recall.
(2)In fact I recall one such case a couple years ago. A patient was sent to us from another facility for rehab and follow-up. One lung had been replaced due to severe COPD. The other lung had then had Lung Volume Reduction Surgery to keep it from expanding onto the "good lung". He was trached and hadn't been off a vent in about two years. So this patient was being ventilated via Bipap to the trache. The "weaning" was to be done by daily reducing the Bipap level from something like 12 or 15 down to 5 or 8. The patient was very nervous and would get very upset about his PS level being reduced. Some therapists found that they could "sneak in" and turn down the PS without the patient knowing it, purportedly facilitating the weaning method. This may have "worked" for awhile, but then backfired as the patient eventually caught on. So one day I had this patient and was to turn down his PS. This was still after the Ventilator Discontinuation Guidelines had come out. I was both appalled and disgusted that they weren't even being discussed or debated by the MD's or RT's. So after suctioning and giving the usual bronchodilator medication, I had this patient lay back and did some relaxation drills with him. Then I removed him from the Bipap, uncuffed his trache and affixed a Passy Muir valve to his trache. He was in a step down ICU and I sat there talking with him for a full 30 minutes. His RR never climbed over 25, no labored breathing or SOB, and on room air his SpO2 was around 93-96 the entire time. During our talk I informed him that, while he was breathing OK while relaxing it would be my guess that should he get excited or do a lot of activity the resultant load on his lung might lead to a need to return to ventilatory support.
That patient hadn't talked in years and the next day was able to talk to his wife. We eventually were able to send him back to his home, and he only had a ventilator at night.

I bring this up to point out that even psychological "weaning" is sometimes best done ala the full support/SBT recipe.

3) Patient-Ventilator Interactions during Volume-Support Ventilation: Asynchrony and Tidal Volume Instability -- A Report of Three Cases
Thierry M Sottiaux MD
During pressure-support ventilation, tidal volume (VT) can vary according to the level of the patient's respiratory effort and modifications of the thoraco-pulmonary mechanics. To keep VT as constant as possible, the Siemens Servo 300 ventilator proposes an original modification of pressure-support ventilation, called volume-support ventilation (VSV). VSV is a pressure-limited mode of ventilation that uses VT as a feedback control: the pressure support level is continuously adjusted to deliver a preset VT. Thus, the ventilator adapts the inspiratory pressure level, breath by breath, to changes in the patient's inspiratory effort and the mechanical thoraco-pulmonary properties. The clinician sets VT and respiratory frequency, and the ventilator calculates a preset minute volume. It has been shown that ineffective respiratory efforts can occur during pressure-support ventilation. A mismatch between the neural (ie, patient) and mechanical (ie, ventilator) timings is the main cause of missing breaths occurring while the ventilator is in the inspiratory phase: the reason is that the ventilator does not cycle from inspiration to expiration until the inspiratory flow decreases to a threshold value (5% of the peak inspiratory flow). The patient's ineffective efforts can also occur during the expiratory phase of the ventilator: in that situation, the inspiratory effort occurs before complete lung emptying and is not high enough to trigger the ventilator. The risk of the patient making ineffective efforts is increased by the algorithm included in the VSV mode. If the patient makes numerous ineffective efforts, the frequency of effective efforts (recorded by the ventilator) can be lower than the set frequency, in which case a new target VT is calculated by the ventilator to achieve the preset minute volume. As VT increases, the mismatch between the neural and mechanical timings also increases. I report 3 clinical observations showing numerous patient respiratory efforts not sensed by the ventilator and inducing VT instability during VSV. These ineffective efforts can occur during inspiratory and expiratory phases. The mechanisms are discussed. [Respir Care 2001;46(3):255-262] Key words: asynchrony, dyssynchrony, patient-ventilator interactions, volume-support ventilation, pressure-support ventilation, tidal volume, inspiratory effort, ventilator triggering, mechanical ventilation.
Introduction
Pressure-support ventilation (PSV) is a patient-triggered, pressure-limited, flow-cycled mode of ventilation. PSV provides a constant level of positive pressure during spontaneous ventilation. Breaths are pressure-triggered or flow-triggered. During PSV, tidal volume (VT) can vary according to the patient's inspiratory efforts and thoraco-pulmonary mechanics. The Siemens Servo 300 ventilator offers a ventilation mode called volume-support ventilation (VSV). The goal of VSV is to ensure a constant, preset VT during PSV. Using a closed-loop control system, the Siemens Servo 300 adapts the level of inspiratory pressure support (PS) to deliver a preset VT. Delivered VT is used as a feedback control for continuous adjustment of the PS level.1 The ventilator automatically adapts the PS level to changes in the mechanical thoraco-pulmonary properties and the patient's inspiratory effort. To initiate VSV the clinician sets the target VT and respiratory frequency. VSV includes a specific algorithm. On the basis of the preset respiratory frequency and VT, the ventilator calculates a "minimum minute ventilation" (respiratory frequency multiplied by VT).1 If the patient's breathing frequency is lower than the preset frequency, the minimum minute volume cannot be reached. In that situation (minute volume lower than that calculated on the basis of the preset parameters), the ventilator calculates a new target VT as a reference for regulation of PS. The maximum new calculated VT may be up to 150% of the preset VT. For example, with a preset VT of 500 mL and a preset frequency of 10 breaths/min, the minimum minute volume is 5 L/min. If the patient's frequency drops below 10 breaths/min, the new calculated VT increases to reach the calculated minimum minute volume and the maximum new calculated VT will be 750 mL.
See The Related Editorial on Page 232
During both pressure-triggered and flow-triggered VSV, a bias flow of 2 L/min is delivered into the circuit during expiration. When the ventilator is set to "flow-triggering," the ventilator can be triggered when a flow of 0.7-2 L/min is inhaled by the patient from this bias flow. Flow-triggering is set by moving the trigger button into the green area (lower sensitivity) or red area (higher sensitivity, with the risk of auto-triggering). In this study, we set the trigger button directly between the green area and the red area.
During VSV, the Siemens Servo 300 cycles from inspiration to expiration when the inspiratory flow reaches 5% of the peak inspiratory flow. A safety mechanism limits the inspiratory time (TI) to a value of 80% of the preset respiratory cycle duration.
Ineffective respiratory efforts may occur during the patient-triggered modes of ventilation, inducing VT variability.2 Nava et al3 and Jubran et al4 have shown that many patients suffering chronic obstructive pulmonary disease make ineffective efforts during PSV and appear to struggle against the ventilator. The potential for dyssynchronous interaction during VSV has not been evaluated in the literature.
Patient-ventilator asynchrony may escape routine clinical survey. Nevertheless, careful analysis of available respiratory waveforms provided by new ventilators allows the clinician to detect the wasted efforts occurring during the gas delivery phase of the ventilator or during the expiratory phase. Available graphic displays include scalars (waveform plotting pressure or flow or volume vs time) and loops (simultaneous plotting of two respiratory variables).5,6 Because active use of respiratory muscles may affect the patterns of the curves, patient and ventilator frequencies can be determined by examination of the displayed curves. "1:1 interaction" means that all the patient's breathing efforts trigger the ventilator and induce lung inflations applied by the ventilator. Conversely, "non-1:1 interaction" indicates the presence of active spontaneous inspirations that are not assisted by mechanical inflation.
This report illustrates some examples of patient-ventilator interaction during VSV with the Siemens Servo 300. We observe that patient-ventilator dyssynchrony during VSV can induce marked VT instability.
The entire text of this article is available in the March 2001 issue of RESPIRATORY CARE.

Hi Jeff, finally someone who understands me!!
I agreed with your article and was pleased gain more fire-power to hopefully change the world here at the VA. Interested in coming back??I'd like to add just a thought I feel is important to consider when ventilating and weaning (opps I said that word). Why not rest on PCV?? Make the transition to PSV easier, I mean the waveforms and mechanics are essentially the same, minus a few variables and if tolerated set the PC level to the PS level, switch the rate off and in the morning your flying solo again! That way we take that bleepin Autoflow option out of the picture, just tell them it doesn't any longer, hey?? And whats this MMV mode?? Is that not the most useless mode?? it doesn't belong in the ICU. (don't tell Brad, he loves it). It belongs in the PACU. That way the nurses don't have to get up from the snack table. I have several other theories on why PCV has the potential to prove very useful, give me a call when your bored next time.By the way what's an MA1, I graduated in 99', maybe i missed class that day, hangover!!

Hey Tony,

Hey let's talk shop here in front of all!

PCV might be a better option when CMV/Autoflow, A/C PRVC, or A/C VC+ is on a patient and....

the patient keeps demanding more than the set tidal volume. The ventilator's PC/VC hybrid keeps giving less as the patient demands more. In such a case it MIGHT be better to switch to pure PC. The only problem is if one is trying to follow the ARDSnet style or protocol then the distending pressure might be too great. Some recent studies seem to show that having too high a tidal volume is also harmful even in patients at risk of developing ALI/ARDS. And what ventilated patients are totally out of the risk.

So another strategy is to just switch to pure flow controlled volume control, using a high flow rate. And, of course, optimal sedation. But if sedation isn't an option (BP considerations) then PC is the way to go.

See

Richard H. Kallet, James A. Alonso, John M. Luce, and Michael A. Matthay
Exacerbation of Acute Pulmonary Edema During Assisted Mechanical Ventilation Using a Low-Tidal Volume, Lung-Protective Ventilator Strategy*
Chest, Dec 1999; 116: 1826 - 1832.


But usually I think using a volume targeted pressure control is the best way to unload both the elastic and resistive loads, and allow the patient to breath spontaneously--the full support before and after the SBT. But Tony, try putting a patient on CMV/Autoflow and see how long it lasts. "CMV?! Aren't we locking them out?" "Why are they on CMV?"..."Have we given up on the patient?".

I don't see how putting the patient on PCV will prepare them for PS? The PS should be zero anyway, cept for minimal ATC(or guessing surrogate), just enough to compensate for ET tube (and Tobin would argue with even that).

An MA-1 was a crude volume ventilator.

I don't blame Servo for the Automode, Drager for MMV. They are just catering to a market in need of catching up.

whitnack@pacbell.net

Tony, you should have seen me one day on ICU rounds. There was this patient---pretty much your standard train wreck. Put him on low PS and everything would rapidly go to hell---BP up, HR up, RR up, etc. So on rounds the young intern says "there's nothing else to do but adjust the ventilator". I turn into a demented Sam Kenison version of an RT ("It's a desert, nothing grows here") "The ventilator isn't broken, the patient is. Fix the patient and then he can begin to come off the ventilator".