Pocket Guide
44 Pages
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POCKET GUIDE
NAVA
CONTENTS Table of contents 1 2 3 4 5 6 7 8 9
Introduction Background and basic concepts Starting and running NAVA Optimizing the NAVA level Example of a weaning procedure Alarms and messages NAVA features and management tips Glossary and definitions References
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4 5 13 23 25 28 32 39 41
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INTRODUCTION Introduction This pocket guide has been produced with the aim of providing a brief and handy introduction to the mode of ventilation known as Neurally Adjusted Ventilatory Assist (NAVA). Based as it is on respiratory physiology, NAVA gives the user access to information concerning both the patient’s ability to breathe and the status of the central nervous system. The main focus of this booklet has thus been to explain in simple terms the physiological background and distinctive features of NAVA while providing easy-to-follow instructions. More information is provided elsewhere, for example in training material, while operating instructions and technical information are covered in the appropriate User or Service Manuals for the SERVO-i ventilator. While every effort has been made to keep this presentation as simple as possible, some technical terms, specialized information and unfamiliar abbreviations are unavoidable. These are explained in the definitions and glossary at the back.
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BACKGROUND AND BASIC CONCEPTS Physiological background to NAVA NAVA – Neurally Adjusted Ventilatory Assist – is an optional mode of ventilation for the SERVO-i ventilator. NAVA delivers assist in proportion to and in synchrony with the patient’s respiratory efforts, as reflected by the Edi signal. This signal represents the electrical activity of the diaphragm, the body’s principal breathing muscle. The NAVA option comprises the following parts:
1. 2. 3. 4.
NAVA software option – if not already factory-installed, the software can be installed using a PC Card. Edi Module Edi Cable Edi Catheter
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BACKGROUND AND BASIC CONCEPTS During normal respiration, a spontaneous breath starts with an impulse generated by the respiratory center in the brain. This impulse is then transmitted via the phrenic nerves and electrically activates the diaphragm (excitation), leading to a muscle contraction. The diaphragm contracts into the abdominal cavity, which leads to a descending movement, creating a negative alveolar pressure and an inflow of air.
Muscular contraction is always preceded by an electrical impulse and this electrical activation is controlled by nerve stimuli. The signal that excites the diaphragm is proportional to the integrated output of the respiratory center in the brain and thus controls the depth and cycling of the breath. When NAVA is used with the SERVO-i ventilator, the electrical discharge of the diaphragm is captured by a special catheter (the Edi Catheter) fitted with an array of electrodes. Like an ordinary feeding tube, the Edi Catheter is placed in the esophagus.
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BACKGROUND AND BASIC CONCEPTS
1.
2. 3. 4.
Edi Catheter (with electrodes marked in black – the first is the reference electrode, the others are measuring electrodes and the distance between the measuring electrodese is the Inter Electrode Distance or IED) Esophageal wall Diaphragm Stomach
The electrical activity of the diaphragm (Edi) is captured by the electrodes and relayed to the SERVO-i, which displays it and delivers assist in proportion to the measured Edi. Basically, NAVA uses the Edi signal to control the ventilator and assist the patient’s breathing in proportion to his own effort.
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BACKGROUND AND BASIC CONCEPTS Basic NAVA concepts NAVA compared with traditional mechanical ventilation The Edi signal that is picked up by the electrodes on the Edi Catheter is filtered and processed by the Edi Module. The Edi signal is measured 62.5 times per second. The processed Edi signal is relayed to the SERVO-i ventilator which will, depending on the NAVA level chosen, then deliver assist to the patient in proportion to and in synchrony with the Edi signal.
Edi (μV)
An example of an Edi curve is given in the diagram below for a single patient breath. The blue lines represent Edi signals, sampled at a rate of 62.5 times per second.
1
3
5
Time (s)
At the set trigger level, the ventilator will start to deliver assist in proportion to the Edi signal. NAVA is triggered by an increase in Edi from its lowest value, known as Edi min, rather than a specific Edi level. In the diagram below, the Edi min is 0.2 µV and the trigger level 0.5, which means that NAVA will be triggered at an absolute level of 0.7 µV.
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Edi (μV)
BACKGROUND AND BASIC CONCEPTS
Trigger level 0.5 μV above Edi min 2
0.7 0.2
1
Time (s) NAVA also employs the pneumatic trigger, based on flow or pressure, as a secondary source. In combination with the Edi trigger, this operates on a first-come-first-served basis. The ventilator will continue to “amplify” each of the subsequent measured Edi signals (the blue lines in the diagram). The pressure delivered is thus derived from the following formula: NAVA level x (Edi signal – Edi min) + PEEP The “amplification” depends on the selected NAVA level and results in a smoothly rising pressure curve to the patient. This curve follows the Edi signal pattern until the Edi signal has fallen to 70 % of its peak value, when the patient is allowed to exhale and the ventilator no longer offers any assist until the next breath is initiated and the trigger level is again reached. In this example, the result is the type of pressure curve seen in the illustration below. When presented on the user interface, these curves can be seen at the top (pressure) and bottom (Edi) of the screen shot detail below.
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BACKGROUND AND BASIC CONCEPTS P
PEEP
Edi (μV)
Time (s)
0.7 0.2
70% of Edi peak
Trigg
1
3
70 %
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5
100 %
Time (s)
BACKGROUND AND BASIC CONCEPTS The Edi signal can be monitored in all modes of ventilation, invasive and non-invasive, as well as in Standby (shown below), including values for both Edi peak and Edi min.
In Standby, the values are also trended, enabling the user to follow the Edi trend even if the patient has been extubated.
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BACKGROUND AND BASIC CONCEPTS The NAVA level The NAVA level is the factor by which the Edi signal is multiplied to adjust the amount of assist delivered to the patient. This assist is thus proportional to the patient’s Edi and as such, it follows a physiological pattern. The NAVA level varies for different patients since they require different assist levels. It may also need adjusting over time in the same patient. The NAVA level is typically set to between 1.0 and 4.0 cmH20/µV. The diagram below shows the principles behind how the Edi signal combines with the chosen NAVA level to affect the delivered pressure. P
μV 50
50
P 50
40
40
40
30 20 10
30
30
20 10
Trigg
0
20 10 0
Edi signal Edi peak 22 µV Edi min 0.2 µV
Time
Time NAVA level 1 Set PEEP 10 cmH2O Estimated P peak 31.8 cmH2O ((22-0.2)x1+10=31.8)
Time NAVA level 2 Set PEEP 10 cmH2O Estimated P peak 53.6 cmH2O ((22-0.2) x 2+10=53.6)
The NAVA level should always be adjusted and finetuned in small steps and one method of optimizing it is described in greater detail in chapter 4.
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STARTING AND RUNNING NAVA The information below covers the procedures involved in starting and running the NAVA mode. It is divided into two sections, each followed by a brief summary.
Positioning the Edi Catheter • Select the appropriate Edi Catheter for the patient, depending on weight and height. • Insert the Edi Module into the SERVO-i and connect the Edi Cable. • Perform the Edi Module function check. • Measure the distance from the bridge of the Nose (1) to the Earlobe (2) and then to the Xiphoid process (3). This is the NEX measurement. Make a note of it.
1 2
3
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STARTING AND RUNNING NAVA • Calculate the insertion distance (Y) for the Edi Catheter. This will depend on whether the Edi Catheter is inserted orally or nasally, as well as on the size of the Edi Catheter. Use the appropriate table as shown below. Insertion distance Y for nasal insertion Fr/cm
Calculation of Y
16 Fr
NEX cm x 0.9 + 18 = Y cm
12 Fr
NEX cm x 0.9 + 15 = Y cm
8 Fr 125 cm
NEX cm x 0.9 + 18 = Y cm
8 Fr 100 cm
NEX cm x 0.9 + 8 = Y cm
6 Fr 50 cm
NEX cm x 0.9 + 3.5 = Y cm
6 Fr 49 cm
NEX cm x 0.9 + 2.5 = Y cm
Insertion distance Y for oral insertion
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Fr/cm
Calculation of Y
16 Fr
NEX cm x 0.8 + 18 = Y cm
12 Fr
NEX cm x 0.8 + 15 = Y cm
8 Fr 125 cm
NEX cm x 0.8 + 18 = Y cm
8 Fr 100 cm
NEX cm x 0.8 + 8 = Y cm
6 Fr 50 cm
NEX cm x 0.8 + 3.5 = Y cm
6 Fr 49 cm
NEX cm x 0.8 + 2.5 = Y cm
STARTING AND RUNNING NAVA • Dip the Edi Catheter into water for a few seconds and insert it to the Y value calculated above. Do NOT use lubricants as this may destroy the Edi Catheter coating and interfere with the measurement of the Edi signal. • Connect the Edi Catheter to the Edi Cable. • To confirm the position of the Edi Catheter, open the “Neural access” menu and select “Edi Catheter positioning”.
• Verify the position of the Edi Catheter by analyzing the ECG waveforms. Ideally, P and QRS waves are present in the top leads, while the P waves disappear in the lower leads, where QRS amplitude also decreases. Check that the Edi scale is fixed and that it is set appropriately (greater than or equal to 5 µV). • If Edi deflections (see screen shots below) are present, observe which leads are highlighted in blue.
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STARTING AND RUNNING NAVA
1 – Edi deflections 2 – blue highlights
-
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If the second and third leads are highlighted as shown above, secure the Edi Catheter in this position after marking the Edi Catheter at its final position and making a note of the distance in centimeters.
STARTING AND RUNNING NAVA -
If the top leads are highlighted (see below), pull out the Edi Catheter in steps corresponding to the Inter Electrode Distance (IED, measured in millimeters) until the blue highlight appears in the center. Do not exceed four times the IED. Mark the Edi Catheter at its final position.
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STARTING AND RUNNING NAVA -
If the bottom leads are highlighted (see below), insert the Edi Catheter further in steps corresponding to the IED until the blue highlight appears in the center. Again, do not exceed four times the IED. Mark the Edi Catheter at its final position.
-
If the Edi signal is very low, there will be no blue highlights. If this happens, evaluate the Edi signal as described in the second point below.
• Once the position has been verified, secure the Edi Catheter in position after first checking that the marking on the Edi Catheter is in the right place and observing the ECG waveforms and their blue highlights. Make sure that the Edi Catheter is not secured to the endotracheal tube. Record the insertion length. Always follow hospital routines to check the position of the Edi Catheter when it is used as a gastric feeding tube.
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STARTING AND RUNNING NAVA • Evaluate the Edi signal. Please note that sedation, muscle relaxants, hyperventilation, excessively high PEEP and neural disorders can all result in a low or absent Edi signal, even if the Edi Catheter has been perfectly positioned. • If possible, perform an expiratory hold and verify that the positive Edi deflection coincides with a negative deflection in the pressure waveform. • Edi Catheter positioning may be reconfirmed after 1-2 hours if minor adjustments are necessary.
Edi Catheter positioning summary Select Edi Catheter and measure NEX, calculating the insertion distance, Y. Dip Edi Catheter in water and insert. Verify the position in the positioning window. Secure the Edi Catheter.
NAVA settings • To set the initial NAVA level, press “Neural access” and select “NAVA preview”.
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STARTING AND RUNNING NAVA • The gray curve then displayed on the user interface below shows the estimated pressure based on the Edi and the set NAVA level.
• Simply press “NAVA level” and use the main rotary dial to set it appropriately. Generally, the first NAVA level tried should produce the same pressure as that used in the current ventilation mode, or perhaps slightly lower. By accepting and pressing “Close”, the selected NAVA level is saved to the NAVA ventilation mode window. • To set the parameters for operating the NAVA mode, choose “NAVA” in the “Select ventilation mode” window. This opens the “Set ventilation mode” parameters window.
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STARTING AND RUNNING NAVA
• The NAVA level displayed is the initial one saved as outlined above from the “NAVA preview” window. • The other two parameters in the “Basic” column (marked with a red ring above) are PEEP (cmH2O) and oxygen concentration (%). • Trigg. Edi (marked with a blue ring above) has a default setting of 0.5 µV (the upper limit for variable background noise) and the range 0-2 µV. The value set here is the one that will trigger the ventilator to assist the patient.
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