In recent years there has been growing interest in monitor-ing non-invasive mechanical ventilation, extending beyond physiopathological knowledge of interactions between the patient and the ventilator. Evidence is beginning to appear in literature that associates the quality of non-invasive home ventilation with survival1,6.There is a wide spectrum of tech-nical possibilities for acquiring data about ventilation qual-ity, ranging from the simple practice of pulse-oximetry to obtaining user information by means of downloading data stored in the ventilators’ internal memory (built in software) as well as more complex polygraphy or polysomnography configurations from sleep laboratories. In reality these con-stitute the gold standard for monitoring2 but are sometimes difficult to conduct in clinical practice, because they use up so many resources. Frequently this means that they are only used for patients if readings obtained using simpler techniques are unable to provide sufficient information
Indications and limitations of built in software
Monitoring using built in software offers an integrated view of the ventilator use by the patient. Not only is it able to de-termine compliance with prescribed therapy, but in modern ventilators it also offers the possibility of downloading infor-mation about flow and pressure-time waveforms. It offers a detail which provides cycle-by-cycle analysis and in some models, as shown with the Vivo 50 (see figure 1), also in-corporates biological signals, such as pulse-oximetry .By us-ing algorithms to process signals, the software is also able to make a distinction, with an acceptable degree of accuracy, between whether or not the volume delivered by the venti-lator corresponds to the current volume reaching the patient or whether, on the contrary, it corresponds to leak flow3.
Fig 1. Example of monitoring with Vivo 50 PC software, with cy-cle-to-cycle information, and also incorporating a pulse-oximetry signal.
However, there are circumstances in clinical practice in which monitoring via software may be insufficient for NIV titration or for diagnosing certain types of patient-ventilator asynchrony. A common example are the upper airway ob-struction events6. During these events, the flow delivered by the ventilator against a closed upper respiratory airway is virtually non-existent. This usually makes it impossible to determine using software analysis of the graphs wheth-er or not the patient is making any effort in the course of this event. Determining effort in this case has implications relating to treatment, as a distinction can be made between events requiring effort, on account of an insufficient level of expiratory pressure, and events involving no effort because of, for example, the glotic response to increases in pressure or volume7.
Likewise, in a situation of events involving patient-ventilator asynchrony, knowledge of the patient’s ventilatory pattern may be important in order to diagnose them with certainty, For example, if the belts have sufficient resolution, they are able to detect the movement associated with ineffective ef-forts. They can also help to clearly delimit the point at which the patient’s inspiration begins and ends. This information may be difficult to gauge simply using basic curves in very common specific situations, such as occurrence of leaks.
What are effort belts?
Effort belts are based on the technique of inductance pleth-ysmography. They consist of two elastic belts which are fixed to the patient’s ribcage and abdomen. Inside they have a conductor wire or loop. A weak low voltage current circulates through this conductor wire creating a small magnetic field. As the band is stretched and relaxed by the patient’s breath-ing the cross-sectional area within the band changes slightly. This change in cross-section produces a slight change in the magnetic field that results in a change in the frequency of the current. This change can be measured and converted to a voltage output that creates the waveform. The key concept is that the stretching and relaxing of the band can be mea-sured accurately and depicted as a waveform.
If the system is accurately calibrated, this technique is also reliable for determining the patient’s tidal volume, both overall and separated into its thoracic and abdominal com-ponents. It is used relatively frequently in intensive care medicine for the monitoring of the ventilatory pattern in a sedated and paralyzed patient undergoing invasive ven-tilation5. However, as sleep medicine and monitoring of non-invasive mechanical ventilation involves patients who are not sedated and who produce spontaneous move-ments during the monitoring period, it is usually used as a non-calibrated signal and, therefore, offers mainly quali-tative information about the patient’s ventilatory pattern4.
How to interpret the signal?
In sleep medicine the inductive plethysmography signal tra-ditionally indicates the patient’s active effort. In the case of ventilation, in addition to reflecting the patient’s effort it may also indicate passive displacement during passive insuffla-tion (for example, during a controlled cycle).
In order to provide the clinician with useful information the signal from thoraco-abdominal belts must be interpreted fol-lowing several basic concepts from, which can be summa-rized as follows:
1. Synchronous or asynchronous start of inspiration with the ventilator cycle: inspiration begins (if the polarity is positive) when the signal rises above the baseline. The end of inspiration corresponds to the maximum value previous to its return to baseline. In a way, it is a signal which retains sym-metry with that of the patient’s inspired tidal volume. If these concepts are applied to a patient under non-invasive venti-lation, the start of inspiration must coincide with the start of pressurization of the ventilator. This synchrony can be seen in figure 2A, while figure 2B clearly shows that the patient’s inspiration starts after the ventilator’s controlled cycle has begun. It must be pointed out that this change is difficult to diagnose in the trace created by the built in software with-out help provided by information from the belts (figure 2 C)
Fig 2 A. Respiratory polygraphy in a patient under non-invasive ventilation and adequate synchrony between the signals from the belt and the ventila-tor. Note that in each cycle the start of the patient’s inspiration detected by the belts is in synchrony with the start of ventilation pressurization.
Fig 2 B. The same patient as in the previous figure during another part of the registration. Note that inspiration, detected by the belts, begins after the start of the ventilator cycle (which is controlled and not demanded by the patient).This explains the distortion observed in the flow wave (in red)
Fig 2 C. Registration of software (Vivo 50) without the belt signal. Distor-tion of the flow wave can be seen. This does not present the characteristic of descending morphology, although without the assistance of belts the diagnosis of dysynchrony, which can be seen in the previous figure, could be difficult for the clinician.
2. Signal amplitude: Increases or reductions in signal am-plitude from the belts are shown by changes in tidal volume, and indirectly may correspond to a reduction or increase in the patient’s ventilatory control. Figure 3 shows that the de-saturations correspond to prior fluctuations of the flow wave which are parallel to that of the belts, suggesting a reduction of ventilatory control. Just as in obstructive events, desatura-tion takes place between 20 and 30 seconds after the event.
Fig 3. Example of reduction of the ventilatory control. Note the fluctuations in the flow wave, with variable amplitude movements on the belts, which are otherwise in phase, in the absence of any obstruction of the upper airway.
3. Thoraco-abdominal synchrony: under normal circum-stances, the signal from the chest and abdominal belts must be of equal polarity and the start and end of inspiration and expiration must coincide. When this synchrony occurs, it is said that the phase angle (rho) between the belts is 0º, and the belts are “in phase”. As this angle increases, thoraco-ab-dominal asynchrony also increases, and in situations of total asynchrony completely opposite polarity may occur (rho= 180º or “belts in phase opposition”). The cause of phase op-position or belt time lag is always an effort. Most of the time due to partial or total closure of the upper airway. It is to be noted that, if the effort is produced with accessory muscles, for example in diaphragmatic paresis, the phase opposition will be on the other side. Figure 4 A shows a total time lag, typical of an obstructive event (in this case, without any ventilator controlled cycles, as there is backup rate set), and fig-ure 4 B shows a similar example but in this case a backup rate is set and mandatory controlled cycles can be observed. On the other hand, figure 5 shows that during the obstructive event (absence of flow) there is no fluctuation of signal from the belts (obstructive event without effort).
Figure 4 A. Obstructive event with patient’s accompanying effort. Note that the pressure and flow wave signals are completely flat during the event, in the absence of any programmed backup rate. It is important to note the phase inversion of the chest and abdominal belts (continuous line) during the ob-struction, in contrast to the situation when the airway is open (dotted line)
Figure 4 B: Obstructive event with accompanying effort in ventilatory mode with a set backup rate. Note the movement of the belts when the flow drops and the controlled cycles begin (*). It should be pointed out that when these controlled cycles begin, the number of efforts measured by the belts is always greater than the number of controlled cycles, indicating dissocia-tion between the patient and the ventilator, secondary in this case to the presence of the obstructive event. Courtesy of Dr. J. Sayas (Madrid)
Figure 5. Obstructive event without accompanying effort (*). Note the ab-sence of chest and abdominal movements during the event, unlike the ex-amples shown in figures 4 A and 4 B.
When to use thoracic and abdominal effort belts during NIV?Thoraco-abdominal belts may be helpful in the following sit-uations:
1. For patients with residual obstructive events of the upper airway, in order to document whether or not this is accom-panied by any effort from the patient (see examples in fig-ures 4 and 5 for the differential diagnosis)
2. For patients with complex primary asynchronies, for ex-ample auto-triggering, double triggering, reverse triggering or patient-ventilator uncoupling, in which it is important to establish the synchrony between the cycles provided by the ventilator and the patient’s respiratory movements. An exam-ple can be seen in figure 6. There are different cycles in which there is no displacement of thoraco-abdominal belts, which means it is fairly probable that auto-triggering is involved
Figure 6. Example of auto-trigger. Note that during the entry of cycles at a respiratory rate greater than the base level there is no belt movement, con-cluding that these cycles are not demanded by the patient, and are there-fore suggestive of auto-triggering. Courtesy of Dr. M. González (Santander)
Recommended “Flow Chart” for monitoring of patients under NIV
The recommendation about how we must proceed to mon-itor patients under NIV could be split into two situations, the process for starting the NIV and that of systematically con-trolling (follow-up) the patient under NIV.
1. During the process of starting NIV, adaptation usually takes place in sessions of progressive duration in order to increase tolerance and to ensure that the respiratory insufficiency that was the reason for the indication is corrected. In many cen-tres, before definitively discharging the patient to continue with domiciliary ventilation, a complete polygraphy under ventilation is performed. This includes synchronized signals of flow-pressure versus time, together with a biological vari-able (SpO2 or transcutaneous CO2) and a signal from the thoraco-abdominal belts. The process is shown in diagram-matical form in figure 7
Fig 7. Algorithm for monitoring during initial steps of NIV titration
2. During the follow-up process, the decision algorithm is the one shown in figure 8. As indicated in this figure, in patients whose respiratory insufficiency, pathological pulse-oximetry or sub optimum compliance has not been properly correct-ed, a staggered analysis of the situation must be performed: with the first stage involving leak evaluation, the second the existence and assessment of upper airway events and the third analysis of primary asynchrony. The information pro-vided by the thoraco-abdominal belts may be useful in steps 2 and 3, and is also essential for diagnosing the underlying problem responsible for the poor quality ventilation.
Fig 8. Algorithm for management of monitorization in NIV patients
Information from the thoraco-abdominal belts may be useful in various circumstances associated with non-invasive home mechanical ventilation, basically for classifying upper airway events and as a help in diagnosing (and therefore, handling) patients with complex asynchrony problems during ventila-tion. In addition to knowledge of the main problems, inter-pretation requires careful correlation with the flow and pres-sure versus time graphs.
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