Waveform of the week: Inspiratory time too short

Inappropriate termination during breath delivery may be the result of an inspiratory time which is too short for the patients inspiratory time constant, when utilizing PC-CMV or PC-IMV.

The above image demonstrates a I-time set too short as evidence of the inspiratory flow waveform not fully decelerating to baseline (shown by red the arrow).

Ideally the operator should titrate I-time to allow for full deceleration of the inspiratory flow waveform. When flow is allowed to fully decelerated, alveolar recruitment, alveolar ventilation, & mean airway pressure is maximized.

The below video clip shows the titration of I-time to allow for a fully decelerating flow waveform, observe the increase in Vt even when the pressure, compliance, & resistance remains constant.

Waveform of the week: Rise time too fast

Setting the rise time too fast, especially in combination with a small E.T. tube or high airway resistance may result in noticeable pressure over-shooting during the early stage of inspiration. This pressure over-shooting may lead to the inappropriate termination of inspiration by setting off the pressure limitation threshold.

Waveform of the week: Rise time too slow

Rise time setting too slow as evidence by the sloping pressure waveform.

A slow rise time may cause increased work of breathing for patients with a high inspiratory drive, decreases mean airway pressure & alveolar time which may decrease the delivered tidal volume.

The following video clip demonstrates the increase in tidal volume with the speeding up of rise time to achieve the optimal rectangular pressure waveform.

Waveform of the week: Flow Mismatch

W

Flow Mismatch as evidence by the scooping in the pressure waveform, a indication of inappropriate inspiratory flow to meet patient demands.


Flow Mismatch

Flow mismatch happens during volume ventilation (VC-CMV or VC-IMV) as a patients respiratory drive increases the fixed/set flow rate does not provide enough assist for the patients demands.
Flow mismatch is easy to identify by recognizing pressure scooping in the pressure waveform. Additionally, the practitioner can evaluate P0.1 to quantify excessive effort.
To correct or prevent flow mismatch the operator can simply adjust the flow rate to match patient demands. However, spontaneous decreases in ventilatory demand will result in unnecessarily higher than average ventilator assist which may result in respiratory muscle disuse, a lower PaCO2 set-point, and delay weaning.
Other actions include; switching from a constant flow pattern to a decelerating flow pattern
, this provides a high initial peak flow, which provides lower peak airway pressures, improved gas exchange, and less patient work.
The operator may also switch to an "Adaptive Pressure Control" mode, pressure control (PC-CMV, PC-IMV) , knowledge based control, or to "Proportional Assist Ventilation".
Lastly, consider increasing sedation if patient demand & or tidal volume exceed clinical goals.

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Waveform of the week: Auto-PEEP

Auto-PEEP indicated by the expiratory flow (red arrow) waveform not returning to baseline (pink arrow).

Auto-PEEP/Intrinsic PEEP (PEEP i)

PEEP-i is actually expiratory asynchrony however, it has a effect on patient triggering. 

The pressure associated with the trapped volume acts as a inspiratory load to be over come by the patient during spontaneous breathing. 

Increased Risks of PEEP-i

• Minute ventilation greater than 15 lpm.
• Airway resistance greater than 15 cmH2O.
• Evidence of expiratory flow obstruction (e.g. expiratory time constant > 1.2 seconds).

Strategies to Reduce PEEP i

• Increase expiratory time.
• decrease respiratory rate.
• Decrease expiratory resistance. 

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Waveform of the week: Ineffective efforts.

Weaning: APRV optimization of settings.

When utilizing certain ventilators there is no need to switch to a different mode of ventilation to wean the patient from APRV. Weaning is accomplished simply by decreasing the CPAP level (p-High) while simultaneously increasing the CPAP time (t-High). 

Decreasing the p-High in increments of 1-2 cmH2O, while increasing the t-High by 0.5 seconds per cmH2O reduction in p-High.

When p-High is at an acceptable CPAP level, the patient may be considered for extubation. Additionally, spontaneous breaths may be supported with automatic tube compensated to elevate work of breathing associated with the artificial airway.

Decreasing PaCO2: APRV optimization of settings.

• First, assess the patients level of sedation, if sedation is used it should be titrated so the patient is easily awakened with light stimuli, and spontaneous breathing is promoted. 

• Second, reassess expiratory flow make sure that T-PEFR is within 50-75% . If T-PEFR is greater than or equal to 75% and oxygenation is acceptable, consider increasing t-Low by 0.05-0.1 increments to achieve a 50% T-PEFR.

• Third, if not contraindicated increase minute ventilation by increasing p-High or p-High and t-High.

• Lastly, if oxygenation is acceptable and paCO2 is a concern the practitioner may increase minute ventilation by decreasing t-High and increasing p-High simultaneously. 

note- decreasing t-High will increase frequency however, mean airway pressure is sacrificed and less end expiratory lung volume is generated. The t-Low should be reassessed & titrated to allow for appropriate release time. 

Additionally, t-Low should not be extended solely to allow for paCO2 removal, increasing the t-Low may lead to alveolar derecruitment.

Improving Oxygenation: APRV optimization of settings.

Improving Oxygenation: titrating t-Low

To improve oxygenation one of the first goals when utilizing APRV is to maximize end-expiratory lung volume.

To do this assess the T-PEFR; if the T-PEFR is less than 50% decrease the release time until a T-PEFR of 75% is obtained. 

*the above image shows a measured PEFR of 50%, even if the calculated value is 50% or greater the operator can still adjust the t-Low setting to obtain a T-PEFR of up to 75% to maximize lung recruitment. 

Improving Oxygenation: increasing p-High or p-High & t-High

Another way to improve oxygenation during APRV is to increase mean airway pressure. 

One way is to increase p-High to recruit aveoli by achieving threshold opening pressure, p-High should be adjusted at only 2-5 cmH2O increments while monitoring the patients hemodynamic status. 

Furthermore, t-High can be lengthened, this increases gas mixing & recruits alveoli with longer time constants.

note- always assess hemodynamics if increasing p-High or t-High. 

If increasing settings is limited due to decreased cardiac output or hypotension, consider therapeutics which increase cardiac output & blood pressure.

Optimal t-Low settings

Optimal t-Low settings are displayed in the images to the left.

 T-PEFR at 50, 60, & 75 percent of the peak expiratory flow rate. 

Notice these precise adjustments of t-Low. 

Waveform of the week : the Ventilator Cockpit

Many people fear technology is replacing them and not enhancing us. This is not true for the 

 ventilator cockpit or "Ventilation Cockpit" . 

The Ventilation Cockpit is a tool on Hamilton Medicals C2 & G5 ventilators, which makes the operator better at recognizing changes in patients ventilatory status & frees up time, for more one on one patient care. The cockpit integrates various data (lung mechanics, settings, and spontaneous activity) into informative graphics. 

The cockpit displays two graphics:

One is the "Vent Status" pictured below.

With this graphic the user is able to identify the amount of ventilatory support needed, oxygenation status, CO2 elimination, patient activity, & if the patient meets indications for weaning or liberation from the mechanical ventilator.

The second graphic provides real-time information on the patients lung mechanics, EtCO2 measurement, & spontaneous effort. 

The Cockpit is a very useful tool, allowing the operator to visualize multiple processes without going to various screens or pages. 

*for more information visit the Hamilton Medical website (Hyper-linked at the top of the page) or my YouTube page for a overview.

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t-Low: APRV initial settings

Initial t-Low settings are 0.2-0.8 seconds in adult & pediatric patients, and 0.2-0.4 seconds in neonates. 

This short time interval is needed to minimize lung de-recruitment, a longer release phase may interfere with oxygenation. 

note- this cannot be accomplished when using Bi-level on the PB 840 ventilator, where t-Low is not directly set. However, one can manipulate the frequency & "TH" settings to set a t-Low. 

Since atelectasis can develop rapidly when peak airway pressure drops below the opening pressure of the injured lung, t-Low should always be titrated based on analyzing the 'Peak Expiratory Flow Rate' (PEFR) and the 'Peak Expiratory Flow Rate Termination' (T-PEFR) point, utilizing the flow waveform pattern. 

*The above image identifies the PEFR & the T-PEFR in the flow waveform pattern (purple). 

Titration of t-Low

t-Low should be titrated to obtain a T-PEFR at 50-75 percent of measured PEFR.

Example: the above image shows a measured PEFR of -1050 milliliters/second (blue arrow) and a T-PEFR of -350 milliliters/second (yellow arrow). 

This equals 33% of the PEFR. 

In this example the t-Low would have to be set lower (shorter time) to achieve the 50-75% range to allow for optimal paCO2 removal and maintain enough expiratory lung volume to prevent alveolar de-recruitment.

Observation of the flow waveform should be paired with patient ventilator assessments to identify any need for t-Low adjustments. Changes in pulmonary dynamics result in the need to adjust t-Low settings.

 

p-Low: APRV initial settings

To maximize PaCO2 removal p-Low must be set at zero cmH2O. This allows for a rapid peak expiratory flow, which creates a venturi effect that help drafts paCO2 removal. 

Additionally, the high peak expiratory flow rate promptly ends the t-Low phase and allows for the p-High phase to be resumed earlier to optimize alveolar recruitment and exposure time. 

If p-Low is set greater than zero, the peak expiratory flow rate will be decreased and delayed.

Furthermore, a p-Low setting higher than zero creates additional expiratory resistance during the release phase resulting in a more turbulent expiratory flow pattern and decreases the venturi effect.

note- even though p-Low is set at zero lung derecruitment is not a issue, for t-Low is titrated to maintain expiratory lung volume. 

Notice the above image showing that during p-Low there is still pressure in the airways above zero, even when the ventilator is set with a p-Low at 0 cmH2O. 

t-High: APRV initial settings

• The t-High setting allows for sustained lung recruitment, improving gas exchange by increasing alveolar surface area.
• To maintain maximal recruitment the t-High should be set at ~ 85% of the total cycle time.

Notice the above image with an appropriate set prolonged t-High, thus allowing for the majority of the breath phase to be devoted to alveolar recruitment. 

Time Setting

• The recommended initial t-High setting in adults is 4-6 seconds.
• Pediatrics from 3-5 seconds.
• Neonates 2-3 seconds. 

p-High: APRV initial settings

Adults

• Set p-High at the desired or target plateau pressure, typically around 20-30 cmH2O.
• If transitioning from conventional ventilation the measured plateau pressure may be used as long as it is <>
• Setting p-High 2-4 cmH2O above desired mean airway pressure may be used to initially set p-High. 

Pediatrics

• Set p-High at the desired or target plateau pressure, typically around 20-30 cmH2O.
  
• If transitioning from conventional ventilation the measured plateau pressure may be used as long as it is < 30. 
• Setting p-High 2-4 cmH2O above desired mean airway pressure may be used to initially set p-High.
  

Neonates

• Set p-High at the desired or target plateau pressure, typically around 10-25 cmH2O.
  
• If transitioning from conventional ventilation the measured plateau pressure may be used as long as it is < 25. 
• Setting p-High 1-2 cmH2O above desired mean airway pressure may be used to initially set p-High. 

Mean Airway Pressure & PaO2/FiO2 Ratio

The below table can be used to help set the initial p-High. Example if a patients calculated P/F ratio is only 160, then it will take a mean airway pressure of at least ~ 25 cmH2O for adequate oxygenation. 

Waveform of the week: Double-triggering

Double triggering is when a breath is initiated before the previous breath has time to end. 

Double triggering results from vigorous inspiratory demand or effort exceeding the volume or flow delivery settings on the ventilator. 

Double triggering highlighted in yellow, notice how another breath is initiated before the previous breath has time to end. 

Conditions which contribute to double-triggering:

• Sighs.
• Coughing.
• Volume or flow settings set inappropriately low. 
• Inadequate sedation.
• ARDS low Vt strategy.
• High ventilatory demand:

-High metabolic rate.

-High PaCO2.

-Reduced ventilatory assistance. 

Double triggering is not really a issue with trigger threshold/sensitivity it has more to due with inspiratory flow synchrony.

So any effort to reduce double-triggering should focus on matching inspiratory flow to the patients demands.

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What is t-Low?: APRV terminology

The t-Low sets the time interval for the low pressure/CPAP phase (p-Low).

• Allows for intermittent release in airway pressure, providing paCO2 removal.
• Partially unloads the patients work of breathing associated with pure CPAP breathing.
• The name "t-Low" is used in both Draeger and Hamilton ventilators. 

note- t-Low should not be considered "expiratory time" as the patient may exhale throughout the entire inspiratory phase. 

On the Servo-i ventilator "T-PEEP" is the t-Low setting when using Bi-Vent to mimic APRV.

t-Low on the Draeger, Evita 4 ventilator. 

When using Bi-Level on the PB 840 there is no setting to set the low pressure interval. The operator must change frequency & "TH" to set a t-Low. This can become problematic when trying to precisely set a t-Low interval. 

What is p-Low?: APRV terminology.

The p-Low setting, sets the lower level of CPAP during the release phase.

The term "p-Low" is used in Draeger & Hamilton medical ventilators. 

The above image shows the p-Low setting on the Draeger, Evita XL.

The highlighted setting "PEEP" is the p-Low setting in Bi-Level, on the Servo-i ventilator. 

The "PEEP-L" setting is used in Bi-level (PB 840) to mimic p-Low.

Waveform of the week: Trigger Delay

Trigger delay or inspiratory time delay is when there is a lag between the patient effort (pressure or flow deviation) and the rise of inspiratory flow.

A common cause of trigger delay is the result of a decreased respiratory drive, which can be present during:

• Sleep.
• Sedation.
• High levels of assisted ventilation.
• Hypocapnea.

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What is t-High?: APRV terminology

• t-High is the set time for the upper CPAP phase (p-High).
• Allows for sustained recruitment allowing for improved gas exchange by increasing alveolar surface area. 

The above image shows the t-High Phase in a Hamilton Galileo ventilator. 

t-High setting on the Evita XL


t-High setting on the Servo-I, Bi-Vent mode.

PB 840 Bi-Level mode "T-H" is the t-High setting. 

What is p-High?

• p-High is the upper CPAP or pressure setting when utilizing APRV.
• p-High regulates end-inspiratory lung volume & is analogous with mean airway pressure

The above image shows the p-High phase during APRV. 

p-High setting in Bi-Vent on the Servo-I ventilator. 

Bi-Level in the PB 840 ventilator "PEEP-H" is the p-High setting.

pHigh setting on the Draeger Evita XL ventilator.

APRV: Mode Classification

APRV has been classified as a time-triggered, pressure-limited, time-cycled ventilation mode.

Which provides two levels of CPAP:

1. p-High (high pressure).
2. p-Low (low pressure).

During two set time periods:

1. t-High (time @ the high pressure).
2. t-Low (time @ the low pressure).

Furthermore the ventilators release valve allows for unhindered spontaneous breathing during both the t-High & t-Low phases (aka. Bi-phasic ventilation).

Breathing Pattern

When classifying APRV based on the breathing pattern, one would classify APRV as Pressure Control-Intermittent Mandatory Ventilation (PC-IMV) with an active exhalation valve. 

(Click on the picture for a larger image)

This image shows APRV with the t-High setting shorter to demonstrate that spontaneous breaths are allowed between mandatory breaths, distinguishing a IMV modality.

(Click on the picture for a larger image)

This photo shows spontaneous breathing within the mandatory breaths, this is evidence of a active exhalation valve.