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Transitioning to Volume Target Ventilation in the NICU
A Short Primer
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Dave Lockwood, RRT, Clinical Account Manager, Hamilton Medical, Inc.
Research has shown that ventilator induced lung injury involves not only barotrauma, but also volutrauma. A majority of research has been in the field of adult ventilation, but research has shown the same to be true for neonatal ventilation and lung injury. Early ventilators, designed for neonates, lacked the ability to deliver or monitor small volumes accurately. Machines of that time could control pressures more accurately, which neonatal research suggested was the main causative factor of acute and chronic lung injury at that time.
With the advent of the computer age and more precise measuring transducers, ventilators could deliver small volume more accurately. Consequently, research started to demonstrate that volutrauma was just as damaging as barotrauma. The key to the delivery of an accurate small tidal volume was the ability to measure the actual volume without the influence of the circuit compressible volume. This was achieved by placing the flow sensor outside the body of the ventilator in front of the circuit wye, otherwise known as proximal monitoring.
Many neonatal researchers will stress that the flow sensor should not only monitor parameters, but also be able to regulate the breath from the flow sensor at the wye. Now that there are several infant ventilators on the market that can function in this capacity, many neonatal intensive care units are moving towards volume targeted ventilation and seeing improvements in patient outcomes ranging from decreased ventilator length of stay to lower BPD (bronchopulmonary dysplasia) rates. If your facility wants to switch to volume targeted ventilation, here are a few evidence based suggestions to keep in mind as you become more familiar with this mode of ventilation.
(Editorial note, 'volume targeted' ventilation is most commonly implemented as volume targeted pressure control mode or adaptive pressure control, see section in this ENEWS on modes)
Different volume targets for different disease states:
When first starting volume targeted ventilation, many primary care givers want to use a small tidal volume (VT) on every patient or disease process. Studies have shown that the VT target needs to be within a range to accommodate individual disease states.
It seems logical to use very low VT’s in very low birth weight infants (VLBW), but studies have shown that too small of a VT causes the infant to be “air hungry”. Infants with BPD will also feel and exhibit signs of “air hunger” if the ml/kg VT is too low.
Based upon several well documented studies, the following tidal volume ranges (1, 2) were cited the most often:
Pre-term infant w/RDS <700g 5-6ml/kg(a)
Pre-term infants w/RDS 700-1500g 4-5ml/kg
Pre-term infants w/RDS >1500g 4-4.5ml/kg
Term infant w/BPD 5-7ml/kg
Term infant w/MAS 5-7ml/kg
Term infant with CDH 4ml/kg
Term infant with pneumonia 4ml/kg
(a) Smaller infants require higher VT to compensate for flow sensor deadspace (2)
Infants that were subjected to prolonged ventilation tended to suffer from tracheal dilatation, which increases deadspace and as a result may need higher VTs. (3) The other aspect of volume targeted ventilation that some primary care givers initially struggle to comprehend is that this mode of ventilation is targeted and self-weaning.(2) What this means is that the PiP automatically decreases as the infants lung function improves. This is the same as using PC ventilation but manually adjusting the pressure control setting, as the VT increases, you would manually decrease the pressure control setting. With volume targeted ventilation, the ventilator automatically does it for you. Most ventilators will wean inspiratory pressure down to 3 - 5 cm above set PEEP. If the infant is consistently breathing, PiP is low, and the infant is considered stable, consider extubation.
(Editor's note, the peak inspiratory pressure drops due to the pressure control setting dropping)
Volume targeted ventilation generally works fine with ETT leaks up to ~ 50% (4) as PiP will automatically increase to adjust the VT. As the ETT leak increases above 50%, expired VT’s are less accurate due leakage around the ETT and most vents will underestimate the expired VT. If the infant is unstable with leaks approaching or exceeding 50%, consider switching to pressure control mode or assessing the need, risk/benefits to switching to a large ETT.
References:
- Kezler et al. Early Human Development, 2012
- Klingenberg et al. A Practical Guide to Neonatal Volume Guarantee Ventilation, J Perinatol. 2011;(9): 575-585
- Bhutani et al. Am J Dis Child 1986; 140: 449-452
- Keszler M et al. Volume Guarantee Ventilation. Clin Perinatol 2007; 34: 107 - 116
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"The Tape Measure", A New Tool In Lung Protective Ventilation
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John Newton, RRT-NPS, Clinical Account Manager, Hamilton Medical, Inc.
Timely initiation of lung protective ventilation using low tidal volumes in ARDs has been shown to reduce mortality2. Many institutions develop “aviation style” check lists and written protocols to facilitate appropriate lung protective tidal volumes (LPV) when clinically indicated1. A recent study by Han S, Martin GS, et al, evaluated the percentage of ARDS patients that receive LPV within 48 hours of diagnosis in seven teaching hospitals between 2002 and 2008. The study looked at 421 sepsis-related ARDS subjects in 7 different medical and surgical intensive care units. They found that only 53% of the included patients received LPV. Women received LPV less frequently than men (46% versus 59%, P < 0.001). 307 (73%) subjects were categorized as receiving LPV based on actual body weight (ABW), without gender difference (75% and 72% for women and men, respectively, P = 0.416)(3). This study also showed “the ventilatory care in intubated ARDS patients was different by gender, likely from their height difference. On average, women are shorter than men and thus their calculated tidal volumes in mls would be smaller than men's, based on PBW derived from height and gender. If this factor is not considered in the ventilatory care of women with ARDS and consequently women receive higher tidal volumes than men, this difference in treatment may contribute to the higher mortality seen in mechanically ventilated women compared with men.”3
Some mechanical ventilators allow the clinician to set predicted body weight (aka Ideal Body Weight) and one would think that this would eliminate the error in calculating appropriate LPV tidal volumes. However, the mistake is often made to enter the actual body weight is entered when the predicted body weight is needed leading to the larger tidal volumes. The Hamilton ventilator platform however requires both gender and height to be entered. The ventilator utilizes a look up table that will calculate the predicted body weight allowing for the correct calculation of lung protective tidal volumes4.
As a result of the above described common errors in using ABW versus PBW to determine lung protective tidal volumes, short stature individuals (commonly women) are therefore less likely to receive the correct volumes.
As stated in the recent study: “In conclusion, women are less likely to receive LPV compared to men. However, after adjustment for height and severity of illness, there is no difference between men and women in exposure to LPV. This is most likely from the differences in height, leading to the inaccurate selection of tidal volumes for women. Strategies to standardize LPV delivery, independent of differences in severity of illness and height, are necessary.”3
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A simple tool such as a disposable medical tape measure may have a significant impact on the number of patients that indeed receive LPV in a timely matter. Using the patient height and gender to determine the predicted body weight will allow for an accurate determination of appropriate tidal volume in order to truly provide LPV.
References:
- Mazer M, Pancoast T, Bangley C, Wu Q: A simple method to enhance use of lower tidal volume mechanical ventilation. Crit Care Med 2010, 38:A367.
- Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and the acute respiratory distress syndrome. The Acute Respiratory Distress Syndrome Network. N Engl J Med 2000, 342:1301-1308.
- Han S, Martin GS, et al. Short Women With Severe Sepsis-Related Acute Lung Injury Receive Lung Protective Ventilation Less Frequently: An Observational Cohort Study. Crit Care; 2011;15 (November 1): R262.
- The IBW, based on Pennsylvania Medical Center (adults) and Traub SL.
Am J Hosp Pharm 1980 (pediatric patients), is calculated as:
Pediatric: IBW (kg) = 0.0033 x Patient height 2 (cm) + 0.3237 x Patient
Height (cm) + 14.386 cm x kg/cm
Adult male: IBW (kg) = 0.9079 x Patient height (cm) - 88.022 cm x kg/cm
Adult female: IBW (kg) = 0.9049 x Patient height (cm) - 92.006 cm x kg/cm
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Modes and Breath Types: Having a Better Understanding
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April Frost, RRT, Account Manager, Hamilton Medical, Inc.
Paul Garbarini, MS, RRT, VP R&D, Hamilton Medical, Inc.
When addressing the best approach to ventilation, often times there is confusion regarding which mode and breath type is most beneficial for your patient. Part of this uncertainty may stem from some clinicians utilizing one particular mode of ventilation for the majority of their patients. Also, the traditional breath types of volume control ventilation and pressure control ventilation have been the predominant modes used. With the advent of microprocessors and new modes of ventilation, clinicians have an assortment of options available to them. Addressing these options may help assist clinicians when selecting a breath type and mode of ventilation in the future. A comprehensive discussion of the taxonomy or classification of modes of ventilation is well beyond the scope of an ‘ENEWS’ article. The authors would recommend readers to review the ‘Chatburn’ taxonomy of ventilation.
This article reflects an attempt to adhere to the Chatburn taxonomy but also reflects the author's personal empiric experience, noting that there is no definitive evidence proving one mode is ‘better’ than another. In this article we will limit content to the most commonly used modes of ventilation and provide some understanding of Hamilton’s Adaptive Support closed loop mode of ventilation.
“Volume control” ventilation seems to be a common mode of ventilation for intubated patients. We will consider ‘volume control ventilation to be VC-CMV or VC-SIMV in which flow is controlled during inspiration and pressure varies (is the dependent variable), some may still refer to this mode as ‘AC- assist control’ mode. In VC-CMV (continuous mandatory ventilation), there are machine time triggered delivered breaths and the patient can trigger additional breathes. All breaths are are time cycled off. The clinician sets the tidal volume, respiratory rate, and flow rate (depending on the ventilator configuration, the user may set an inspiratory time or I:E ratio instead of flow rate. In these cases flow rate is set indirectly.) In VC-SIMV (synchronized intermittent mandatory ventilation) , if the patient triggers a breath outside the ‘timing window’ for the set rate, resulting in a total rate above the set rate, these additional breaths are spontaneous breaths, which are triggered by the patient and cycled off by the patient.
A potential advantage to volume controlled ventilation is being able to better control PaCO2 since there is a guaranteed rate and tidal volume, minute ventilation is constant for the patient. However, in the face of decreasing lung compliance, peak inspiratory pressures increase to deliver the preset tidal volume. Therefore, patients who have a restrictive disease process such as severe pneumonia or adult respiratory distress syndrome (ARDS) may have high peak airway pressures to achieve the preset tidal volume. The same increases in pressure can occur with obstructive disease processes such as asthma due to increased airway resistance.
One way to monitor for potential ventilator induced lung injury is by monitoring plateau pressured. Plateau pressure is measured during by inflation hold or pause at end inspiration. With a hold or pause maneuver, flow is zero and pressure equilibrates between the ventilators pressure sensor and the alveoli. Therefore, plateau pressure estimates alveolar pressure which is a better assessment of potential lung injury than peak airway pressure. This is because the peak airway pressure includes the pressure needed to overcome both airway resistance and lung compliance. If flow is not occurring, the airway resistance component of peak airway pressure is not present. This is why in volume control the plateau pressure is most often less than the peak airway pressure as volume control delivers breaths with a constant flow rate. In pressure controlled breaths, flow is variable and peak pressure may be the same as plateau pressure if the inspiratory flow reaches 0 at end inspiration. This is often the case with restrictive diseases. The clinician should assess the pressure waveform during the hold maneuver; the pressure should remain constant. If the pressure is dropping during the hold maneuver and there is a leak, the plateau pressure will be falsely low. The goal is to keep plateau pressures less than 30 cmH20. If the plateau pressures are elevated (≥ 30cmH20), this may lead to ventilator induced lung injury. The potential for Lung injury increases when the pressure applied between the alveoli and the plural space , called the trans-pulmonary pressure is ~25cm or higher. Since the plateau pressure approximates the alveolar pressure, it is used as an estimate of transpulmonary pressure. However if the pleural pressure (pressure outside the lung) is high for example due to obesity, the plateau pressure may be overestimating the transpulmonary pressure as the pleural pressure decreases how much plateau (alveolar) pressure is transmitted across the lung. This is when measuring esophageal pressure (which correlates with pleural pressure) may be useful.
When using volume control ventilation, as noted above, the clinician sets an inspiratory flow rate. When flow rate fails to meet the patient’s inspiratory demand, asynchrony may occur. This may cause an increase in work of breathing (WOB) , which can then increase the patient’s oxygen consumption. To mitigate this, clinicians may routinely give sedation to control the patient’s respiratory drive, decrease oxygen consumption and improve synchrony. However, administering high levels of sedation also can create a greater length of ventilator days for the patient.2 Before sedation is considered, the clinician should attempt to adjust the inspiratory flowrate and/or pattern to better achieve synchrony. Alternatively Pressure Controlled modes have variable flow which may enhance synchrony in some patients.
In the past, when a patient’s airway pressures were high due to lowered pulmonary compliance, pressure control ventilation (PCV) was often the next mode of choice. As noted with VC-CMV, PCV can be can be in the form of PC-CMV or PC-SIMV. In PCV modes, pressure is controlled/constant during inspiration and flow is variable (the dependent variable).
Often the peak airway pressure drops as compared to volume control mode for the same tidal volume achieved. However, it is erroneous to assume the patient is at less risk for lung injury just because the peak pressure decreased. If tidal volume and PEEP are the same, plateau pressures will be same regardless of whether the breath is a volume controlled or pressure controlled breath. The peak pressure may decrease when switching from volume control to pressure control because of the difference in flow rates. Also, when a patient is difficult to oxygenate, controlling pressure is preferred over volume.3 This was in part due to the constant inspiratory pressure delivered during inspiration along with the decelerating flow pattern, which may result in a more even distribution of ventilation than with volume control ventilation.4, but it also may be due to an increase in mean airway pressure due to reaching higher pressures earlier in the breath or if the Inspiratory time was extended.
In PC modes, the clinician sets a driving pressure above PEEP, respiratory rate and inspiratory time. A drawback to this breath type is that the exhaled tidal volumes vary from breath-to-breath with changes in compliance and resistance. Monitoring tidal volume delivery to prevent hypoventilation and increasing PaCO2 levels or too high tidal volumes is critical when using PCV. On the other hand, in the context of more recent knowledge regarding ventilator induced lung injury, some might prefer to see tidal volume reduced and tolerate acceptable elevations in CO2 (“permissive hypercapnia”) and not have pressure rise with decreases in lung compliance or increases in airway resistance.
One way to achieve a target tidal volume as with volume control ventilation, and limit pressure as with PCV would be to use a PCV mode in which a target tidal volume is set and pressure is automatically titrated to achieve the target tidal volume while flow remains variable as in pressure control modes. These modes are sometimes referred to as Dual Control Modes, and they have a variety of names. More recent ventilator mode taxonomy would describe this type of mode as ‘adaptive pressure control’ or ‘volume targeted pressure control ventilation’. The pressure regulated volume targeted breaths are time cycled off and machine or patient triggered. As with VC and PC modes these volume targeted pressure control modes can employ a breath pattern as CMV or SIMV., Brand names for these modes are ‘PRVC’ (pressure regulated volume control), ‘autoflow’, ‘Adaptive Pressure Ventilation’, ‘Volume +’, ‘CMV+ etc. With the regulation of pressure, the target tidal volume is delivered at the lowest possible pressure depending on the patient’s lung mechanics. The ventilator delivers several test breathes initially to determine the patient’s resistance and compliance and the minimal amount of pressure required to deliver the target tidal volume. Each ventilator manufacturer may have a different preset algorithm for this volume targeted pressure control mode of ventilation..
Hamilton Medical calls their volume targeted pressure control mode, Adaptive Pressure Ventilation (APV), and the principles of operation are as follows:
- The ventilator first determines the patient’s volume/pressure response by three initial test breaths.
- The device then determines what the lowest possible pressure is to achieve the target tidal volume based on those test breaths.
- If the patient’s tidal volume is equal to the target tidal volume, the pressure stays constant. If the tidal volume is higher or lower than the target tidal volume, the inspiratory pressure is adjusted up or down by up to 2 cmH20 per breath to achieve the target tidal volume.
- The maximum allowable pressure is 10 cmH20 below the high pressure alarm limit setting. If the inspiratory pressure reaches this threshold of 10 cmH20 below the high pressure alarm limit, a pressure limitation alarm occurs, and the ventilator can no longer continue to ramp the inspiratory pressure up to try to achieve the target tidal volume. The set inspiratory time is met, breaths are delivered, but the target tidal volume may no longer be achieved unless the high pressure alarm limit is increased, which will increase the pressure limitation along with it. This allows the ventilator to then ramp the pressure up until the target tidal volume is again being delivered to the patient.
Tidal volume are continuously assessed to guarantee the target tidal volume is delivered to the patient at the least possible pressure. Some ventilators titrate pressure based on the exhaled tidal volume, others inspired tidal volume and some provide feedback based on the average of inspiratory and expiratory volumes. It is important for the clinician to know the volumes the ventilator is using to base its pressure adjustment on. Hypoventilation is prevented by delivering the target tidal volumes (along with a set rate) to the patient just as with volume control ventilation.
One potentially undesirable effect of volume targeted pressure ventilation is that if a patient’s demand and effort increases significantly, higher tidal volumes may be delivered to the patient, causing the ventilator to reduce the driving pressure (level of support) being delivered to the patient. This may create a scenario in which the patient is under supported. The decrease in pressures can also lead to a decrease in mean airway pressure (MAP), making hypoxemia a potential issue.
This is a glass half full, half empty scenario. The vast majority of the time using the least amount of pressure to achieve the target pressure is desired and as the patients' lung mechanics improve and/or spontaneous drive improves, the pressure is ‘weaned’, progressing the patient towards liberation from the ventilator. As noted above however, in cases where the patient is still in severe respiratory failure or other conditions in which the patient has a high respiratory drive, the clinician should set the low pressure alarm to a level where they will be notified if pressure drops to an undesired level. The options in managing this scenario would include treating the underlying cause of increased patient minute volume demand, sedation, increasing the target volume and/ or rate setting to that that the patient is demanding. Some might consider switching back to volume control modes with constant flow and this may be helpful in individual patients. But if the set tidal volume in VC mode is less than the patient desired volume and/or as previously described, the flow does not meet patient demand, increased work of breathing and asynchrony may occur.
Adaptive Support Ventilation (ASV) is a mode of ventilation which is proprietary to Hamilton Medical, Inc. Basic settings in this mode of ventilation are as follows: a high pressure limit, patient height and gender, and target minute ventilation. A mathematical model (the Otis least work of breathing equation) is used to find the best respiratory rate and tidal volume combination within safety limits that is the least WOB for the patient to achieve the target minute volume. The best tidal volume/rate pattern is chosen based on the breath by breath measurement of the expiratory time constant, which is dependent on the patients static compliance and resistance. The machine then calculates the anatomical dead space for the patient based on their IBW and sets a minimum allowable tidal volume to avoid dead space ventilation. (the clinical must adjust the minute volume target as needed to account for physiologic dead space by assessing PaCO2 as with other modes). ASV continuously interacts with the patient and adapts to breathing pattern changes. This mode can provide full support to passive patients and transition to intermittent support and to just spontaneous breaths. Patient’s requiring full support will receive pressure- limited, time cycled, volume targeted breathes as described previously. As the patient starts actively breathing, any patient triggered breaths are volume targeted, pressure-limited, flow cycled breaths. These are pressure support breaths with a minimum target tidal volume that is also determined by the ASV algorithm. Pressure support will decrease and/or increase to achieve the target tidal volume, and if necessary, mandatory breaths will be added to ensure that the minute ventilation set by the clinician is maintained. This allows the patient to transition back and forth from full support to spontaneous breathing as the patient’s drive changes, without the clinician making mode changes as they would normally have to do with other modes. Because ASV takes into account the patient’s pulmonary mechanics, RR and VT will adapt to change in pulmonary mechanics. For instance, if the patient has poor compliance such as with ARDS, the mode will automatically employ a lung protective low tidal volume strategy. On the opposite end of the spectrum, a patient with high compliance lungs such as with emphysema, or high resistance such as acute asthma, ASV will target a larger tidal volume and lower rate to allow for lung emptying and to avoid autopeep. As with other modes of ventilation, arterial blood gas monitoring is utilized to asses acid-base balance. If the patient has a respiratory acidosis or increased WOB, the minute volume setting should be increased by the clinician to eliminate PaCO2 or reduce WOB. If the patient has a respiratory alkalosis, the minute ventilation should be decreased to allow for an increase in PaCO2. Instead of making a decision to change RR and VT as with traditional modes of ventilation, the one parameter change will make the best possible decision for the patient based on their lung mechanics.
ASV should be used with caution on patients with erratic breathing patterns or those with large leaks as with a bronchopleural fistula. In cases with abnormal breathing patterns such as cheyne-stoke breathing, ASV is not recommended. Also, ASV is only approved for patients ≥ 3kg.
**Outside the USA, some countries have available ‘Intelligent-ASV’ in which the ASV target minute volume is automatically adjusted.**
Every patient encounter brings its own unique set of problems and circumstances. Therefore, it is important to remember that simply one mode of ventilation will not work for every patient. Having an in-depth knowledge of how each mode of ventilation works can assist in making the best decision for the patient. The ultimate goals are to protect the lungs, improve ventilation to perfusion matching, provide patient comfort by promoting synchrony and liberate from mechanical ventilation as soon as possible.
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