Turk Thorac J 2019; 20(2): 140-6 DOI: 10.5152/TurkThoracJ.2018.177 Invited Review Basic Concepts for Tidal Volume and Leakage Estimation in Non-Invasive Ventilation Manel Luján1,2 , Cristina Lalmolda3 , Begüm Ergan4 1 Pneumology Service, Corporació Parc Taulí, Universitat Autònoma de Barcelona, Sabadell, Spain 2 Centro de Investigación Biomédica en Red (CIBERES), Bunyola, Spain 3 Respiratory Therapist, Fundació Parc Taulí, Sabadell, Spain 4 Department of Pulmonary and Critical Care, Dokuz Eylül University School of Medicine, İzmir, Turkey Cite this article as: Luján M, Lalmolda C, Ergan B. Basic Concepts for Tidal Volume and Leakage Estimation in Non-Invasive Ventilation. Turk Thorac J 2019; 20(2): 140-6. Abstract Non-invasive ventilation (NIV) aims to maintain sufficient alveolar ventilation, improve pulmonary gas exchange, assist respiratory muscles, and decrease work of breathing. Monitoring variables such as leaks, tidal volume, and minute ventilation during therapy is cru- cial to assess the effectiveness of NIV. However, most of the time, leaks and tidal volume are not measured but estimated by NIV devices. Moreover, there are limited data for the accuracy and reliability of these estimations. Herein, we address some technical considerations for tidal volume and leakage estimation during NIV and its impact in clinical practice. KEYWORDS: Leak, non-invasive ventilation, software, tidal volume Received: 04.11.2018 Accepted: 23.11.2018 INTRODUCTION Home non-invasive ventilation (NIV) has exponentially grown since its introduction more than 30 years ago. During the last decade, high-performance portable devices are becoming ventilators and monitors at the same time, offering the clinicians important information about physiological parameters during the use of the device. Two of the most important parameters are tidal volume (TV) and leakage (that are the two faces of the same coin). Interestingly, the majority of manu- facturers do not directly measure these two parameters, but they estimate them through a built-in software. Each company offers its own software, and huge differences may be found between them not only in the presentation of variables but also in the accuracy of estimated parameters [1]. The development of algorithms for TV and leakage estimation should consider all the physical phenomena involved in the delivery of certain amount of gas to the patient by an external ventilator. Mathematically, these phenomena are sometimes a challenge, and they can substantially change when different circuits, anti-rebreathing devices, or interfaces are used. The goals of the present review are to reflect the complexity of TV and leakage estimation by these commercial algo- rithms, and to summarize the main bench studies addressing this issue. Moreover, we emphasize on potential sources of variability between manufacturers and the consequences of this variability in the clinical practice. Mathematical Basis for TV and Leakage Estimation In intensive care unit (ICU) and invasive ventilation, the TV estimation does not need advanced concepts. In fact, the di- rect measurement of flow (and volume) by the internal pneumotachograph inside the ventilator needs to be corrected only by the compressible volume, which is the amount of gas (in mL) compressed in the circuit for every cm H2O of pressure generated by the ventilator during the inspiratory phase. This volume never reaches the patient, and it usually depends on the compliance and length of the circuit (C1=ΔV/ΔP) and pressure difference. Depending on these factors, the compress- ible volume can represent up to 20% of delivered gas, but modern ICU ventilators are usually fitted with algorithms that calculate and compensate for the compressible volume [2]. If this limb configuration (double) is used in NIV, the difference between inspired and expired volume (previously cor- rected by the compressible volume) corresponds to the leakage. However, double-limb configuration, although being clearly the most exact way to estimate leaks and TV, is often too bulky for home environment. For this reason, the NIV home ventilators are usually equipped with a single-limb circuit with an expiratory safety system that prevents rebreathing of exhaled gas. This expiratory port can be an intentional and continu- Address for Correspondence: Begüm Ergan, Department of Pulmonary and Critical Care, Dokuz Eylül University School of Medicine, İzmir, Turkey E-mail: begumergan@hotmail.com ©Copyright 2019 by Turkish Thoracic Society - Available online at www.turkthoracj.org 140 Luján et al. Tidal Volume and Leakage Estimation in NIV ous leakage or an expiratory active valve, which opens only TV and leak estimation could be wrong. From a physic point during expiratory phase. In both cases, the information about of view, the intentional leakage is a function of the pressure the amount of exhaled gas is not directly measured inside difference in and out of the tubing, and can be determined the ventilator, as it happens in the double-limb configuration. following the simplified formula of the Poiseuille’s law: In the active valve circuit, a pneumatic device is pressurized F2 = P2 – Patm above pressure inside the limb, and occludes the valve during inspiration. During expiration, the pressure valve relief makes R2 sure that all the expired gas escapes through the valve, and Intentional leakage can be indirectly explored by so-called the expiratory flow signal is missing (Figure 1), thus making leak test. In this test, the ventilator automatically increases it very difficult to assess the TV values (especially if leaks pressure levels with the distal end of the mask or tubing oc- occur). In this setting, the only technical solution available cluded. Thus, pneumotachograph inside the ventilator cap- to measure TV would be to place a pneumotachograph be- tures only the flow escaping through the expiratory port, tween the valve and the interface. determining the relationship between pressure and leaks. Finally, the data are plotted in a XY graph (Figure 3), and a Nowadays, because of its simplicity not only for the patient second-degree equation can fit the amount of intentional but also for caregivers, the most preferred configuration in leak for each level of pressure with reasonable accuracy. Fig- home NIV is the single-limb with expiratory leak port, either ure 4 shows the fit for a leak test with a linear and second- in the whole mask or built as an independent piece between degree equation. These values can be subtracted from the na- the mask and the tubing. Figure 2 (a, b) schematically rep- tive graphs (F1) to obtain the true gas flow entering into and resents the flow dynamics during inspiration and expiration exiting from the patient (in absence of non-intentional leaks). when this type of circuit is used. The expiratory leak port Unfortunately, only a few models of ventilators are equipped acts as a continuous and intentional leakage (intentional F2 with this test. In their estimation, many devices do not use in Figure 2a), with a higher resistance (R2) than that imposed any information about what kind of expiratory port is used. In by the diameter of the tubing (R1). Intentional leakage flow (F2) Following Figure 2a, during inspiration: a Total gas outflow (F1) P2 (mask pressure) P1 (pressure inside the ventilator) Resistance of expiratory port (R2) Total gas outflow (F1)=F2+F3 (non-intentional leaks, if present)+F4. Conversely, during expiration (Figure 2b), the expiratory posi- tive airway pressure (EPAP) flow “washes” the circuit, and the Patient flow Ventilator F4 (TV) pneumotachograph only detects a small amount of expired Patient's R/C gas. In this case, the expiratory TV would be the sum of this small backward flow (F1) plus intentional/non-intentional Single limb with Resistance (R1) leaks (if present). Then, in the first step, the influence of this and compliance (C1) Non intentional intentional continuous leak should be determined unless the leakage (F3) Resistance of interface fitting (R3) Inspiratory phase. F1 = F2 + F3 + F4 P1 b Intentional leakage expiratory E-F1 (negative flow during expiration) flow (E-F2) P2 Patient flow (tidal P1-P2 Ventilator volume) E-F4 (TV) Flow Single limb with Resistance (R1) and compliance (C1) Non intentional leakage (E-F3) Figure 1. Difficulties in monitoring tidal volume with active valve. P1 reflects pressure inside the tubing. P2 corresponds to the pressure Expiratory phase: E-F4(TV) = E-F1 +E - F2 (+E-F3 if present) inside the pneumatic valve. P1-P2 corresponds to the difference (note than during inspiration, P2>P1, occluding the valve, and conversely Figure 2. a, b. (a) Simplified model of flow/pressure dynamics during during expiration, P1>P2, leading to valve opening). Finally, flow inspiratory phase in non-invasive ventilation with single circuit and corresponds to the pneumotachograph signal placed between the intentional leakage (see text for more details). (b) Simplified model ventilator and the valve (note that there is not expiratory flow signal). of flow/pressure dynamics during expiratory phase in non-invasive If non-intentional leakage occurs, it is very difficult to estimate the ventilation with single circuit and intentional leakage (see text for true tidal volume more details) 141 Turk Thorac J 2019; 20(2): 140-6 some modern models, the clinician can introduce the type of can be established by an equation as follows (intentional interface (nasal, oronasal, etc.) in the ventilator’s menu. If we leakage was discarded here to simplify the concepts): consider that the flow values of the expiratory ports between masks can present huge differences between the models, it F1 (known)=F4 (TV)+F3 (if present). would be easily understandable that there might be an impor- Here, F1 is total flow monitored inside the ventilator, and F4/ tant source of error in TV and leakage estimation. F3 are patient flow and leakage flow. Considering that F1 is At the same time, it should be taken into account that the continuously monitored throughout the entire cycle, the prob- conditions when the leak test is done (no TV, distal end oc- lem is how to solve a system of a single equation with two cluded) are not the same as when the ventilator is used in the unknowns. There are only two points in the cycle in which clinical practice (patient flow present). Following the same F4 (TV) value is known, for instance the transition points be- Poiseuille’s law, there is a loss of pressure across the tubing tween inspiration and expiration. In these points, patient flow directly proportional to the ventilator’s outflow. P2 can be (F4) is 0 l/min, and the entire amount of flow monitored by estimated by the following equation: the pneumotachograph inside the ventilator necessarily be- longs to non-intentional leak (Figure 5). Manufacturers usu- P2=P1-F1xR1 (Figure 2a) ally follow this principle, and estimate the leak value in the transition between expiration and inspiration, because the For this reason, some commercial ventilators are fitted with slope between inspiration and expiration is clearly steeper. a proximal pressure sensor at P2 level. Other manufacturers Choosing a wrong measure point may lead to leakage over/ developed a “pre-use test” to be performed before each clini- underestimation. cal use. The purpose of such pre-use tests (do not confound with leak test) is to calculate the compliance (C1) and resis- In fact, when applying this concept, they determine a resis- tance (R1) of the circuit, to determine compressible volume tance value (R3=EPAP/E-F3), and this value is usually applied and pressure loss respectively. Usually, C1 is measured with throughout the entire cycle to estimate the amount of leaks the completely closed distal end, and resistance is measured for every instant in the flow-time curve. with the open distal end (in both cases without intentional leakage). The absence of such algorithms (or absence of di- This approach has two main drawbacks. First, the P2 value rect P2 measurement) may lead to overestimation of P2 (and during expiration (EPAP) cannot reflect P1 during the entire leaks) and underestimation of TV [3]. expiratory phase. In other words, it is not necessarily con- stant. In fact, during expiration, there are changes in flow The second important problem during the development of leading to small changes in the P2 value. algorithms to estimate leaks and TV are the presence of even- tual non-intentional leaks around the mask. As displayed in The second problem arises when this resistance value (R3) is Figure 2, in presence of continuous leaks, the pneumotacho- not constant or is different during inspiration than during ex- graph inside the ventilator “monitors” both TV and leakage. piration. In clinical practice, it is not uncommon the finding For each instant during the ventilator cycle, the relationship of a non- intentional leak only during inspiration, or even at among total outflow, leakage, and patient flows (estimated) the end of inspiration without significant leakage during ex- piration (asymmetric leakage). In this case, the R3 values may show huge differences during a single cycle. This phenom- Pressure enon can be simulated in a bench environment with the help of a unidirectional valve that generated leaks only during in- spiration or expiration [4]. The results were that with inspira- Flow (l/sec) Figure 3. Leak test performed in the laboratory with an external pneumotachograph. In the upper panel, there is a progressive Figure 4. Plot in a Microsoft Excel sheet of the same data contained increase in pressure, with the corresponding increase in flow with in Figure 4. Observe how the correlation degree in clearly better if the distal end of the tubing occluded and an expiratory port included a second-degree polynomial model (lower plot) is used instead of a 142 in the circuit. In the lower panel, a pressure-leak plot is constructed first-degree (linear) model (upper plot) Luján et al. Tidal Volume and Leakage Estimation in NIV tory leaks, the commercial software overestimated TV, and recent study [5], if asymmetric leaks appear, the ventilator vice versa with expiratory leaks. The results obtained in this can modify its pressure support level in a different way than study (overestimation of TV in a model of inspiratory leaks expected. In this setting, it seems reasonable to recommend and underestimation in expiratory leaks) reinforced the hy- setting in the ventilator a safety lower level of pressure sup- pothesis that the approach of considering unintentional leaks port. as a linear parameter can lead to significant misestimation of TV. In the same study, an algorithm based on the assumption TV and Leak Estimation in Bench Studies of lack of proportionality between inspiratory and expiratory There is a growing evidence that bench testing is an impor- phases showed minimal deviations in the estimation of TV tant and relevant component to assess the performance of and unintentional leaks. NIV devices, although they have some limitations such as the models usually lack reflecting complex variations that occur The identification of the wrong estimation of TV and leaks during real life, such as inspiratory effort, respiratory drive, when inspiratory non-intentional leaks occur is also possible ventilatory pattern, and variability of unintentional leaks. though the examination of flow-time waveform. If the area However, a few studies addressed the accuracy of the infor- under inspiratory phase seems much greater than that in the mation provided by device software. expiratory phase, it suggests inspiratory leaks. Even more vi- sual interpretation can be the assessment through volume- Contal et al. [1] analyzed seven home NIV devices for leak time waveform (unfortunately, few built-in software displays quantification and estimation of TV with and without un- it), which in fact is the integral of flow waveform over time. intentional leaks. Each ventilator was tested for variable de- If the inspiratory (positive) area is greater than the expiratory grees of leak (0–10–24–40 and 60 L/min) at 15 and 25 cm area (e.g., in presence of inspiratory leaks), the software must H2O of inspiratory pressure and 5 cm H2O expiratory airway restart the volume-time waveform at the zero level. This re- pressure. Recordings were performed with normal respiratory start is usually seen when flow-time waveform crosses the mechanics model. When compared, it was observed that the zero level, from negative to positive flow, and it is commonly estimation of leaks by different software varied significantly seen as an abrupt “drop” in volume-time graph (Figure 6). between ventilators. The assessment of leaks was highly reli- From a clinical point of view, when the mentioned drop ap- able. It correlated well for the three ventilators, and bias was pears, the TV values in these cycles are unreliable. very close to zero. In contrast to that finding, the other four ventilators underestimated the magnitude of leaks by 6–26 Finally, a third source of variability is the presentation of leak L/min. One device estimated leaks only during the expira- information in the screen and in the built-in software. Where- tory phase. Bias for the estimation of leaks increased with as some manufacturers provide the information concerning the magnitude of unintentional leaks. Increasing leaks also leaks only as non-intentional values, others display global generated auto triggering in two ventilators. TV was underes- leaks, including values related to the expiratory port. This timated by all NIV devices, ranging between 66 and 236 mL. lack of standardization can be confusing for the clinician. The difference increased with higher IPAP level (25 cm H2O) and between 89 and 328 mL without unintentional leaks, The drawbacks of this misestimation can go beyond wrong and 97 and 404 mL with an unintentional leak of 60 L/min. In information to the clinician. It should be highlighted that in our opinion, these differences simply suggest that manufac- some new modes of NIV (the dual control or volume-targeted turers are using different algorithms to estimate TV and leaks. pressure support modes), the ventilator takes its own deci- sions based on target TV estimation. As demonstrated in a A possible explanation for underestimation of TV is lack of compensation for pressure loss within the tubing system. Luján et al. [3] evaluated the TV estimation of five NIV de- vices by comparing TV values calculated by built-in-software with TV measured by a calibrated pneumotachograph un- E-F2 level (E-F3 if present) E-F1 Figure 5. Flow measurements in single circuit with intentional leak. P=pressure. Flow A=flow directly measured on the ventilator’s exit (before intentional leakage). Flow B=flow at the entry of the patient (tidal volume). Note that in the Flow A signal, the baseline Figure 6. Differences in flow-time and volume-time waveform when level (dashed line) is displaced upwards. E-F1 corresponds to the monitoring before (upper part) and after the leaks (lower). Observe flow reaching the ventilator during expiration. E-F2 corresponds to the drop in volume-time corresponding to restart of the waveform baseline flow leakage during expiration (see text for details) 143 Turk Thorac J 2019; 20(2): 140-6 der increasing levels of the controlled leak. The main aim a T-piece was inserted after a commercial leak port that cor- of the study was to determine whether an incorporation of a responds to the continuous intentional leak used in clinical mathematical algorithm that computes leak-related pressure practice. In the inspiratory leak model, a 7.5 cm H2O PEEP losses between the proximal and distal side of the single tub- threshold valve was attached. Two levels of unintentional ing might improve the reliability of TV estimation of built-in leaks (low and high) were introduced into the system with software of the devices. The ventilators tested for calibrated calibrated holes. The study showed the addition of inspira- leak flows (35, 45, 55, 65 L/min) on 20 cm H2O of continu- tory leak resulted in a progressive decrease both in pressure ous positive airway pressure (CPAP) that corresponds to es- support and the delivered TV, falling below the targeted val- timated leak flows for the interfaces that are often used in ue. Clinically, it was shown that the mean delivered TV could clinical practice. In addition, each leak level was tested for drop by 40% in the presence of excessive inspiratory leaks. three different respiratory conditions (normal, obstructive, The delivered TV was significantly different between low and and restrictive) with three different levels of inspiratory pres- high inspiratory leak levels in all ventilators, except one. Dur- sure (5, 10, 15 cm H2O). The first important finding of this ing expiratory leak model, an active valve was used, and study was significant differences shown between displayed attached to the T-piece. It was observed that the presence of and externally measured TV in absolute values by all venti- expiratory leaks caused an increase in both pressure support lators, and all of them underestimated TV, ranging between and delivered TV. The increase in TV differed between venti- 22±9 mL and 84±25 mL. In the presence of the lowest leak lators ranging between 16% and 33%. (35 L/min on CPAP of 20 cm H2O), which corresponds to the values for a nasal mask, ventilators displayed TV in totally Carlucci et al. [6] addressed the importance of configuration different values. The second important finding was a linear of tubing in leak and TV estimations during volume-guaran- relationship observed between the magnitude of leak and teed ventilation, and showed that the ability of ventilators volume underestimation in all ventilators. The underestima- to compensate for unintentional leaks directly depends on tion reached values near 20% of the externally measured TV the configuration of the circuit used. In the bench, they have in the presence of high leaks in some ventilators. In three ven- assessed three home devices with single-limb circuits either tilators, underestimation of TV was more pronounced when with an intentional leak or with an expiratory valve. The mod- the breathing pattern was obstructive. The authors reported el consisted of three levels of leak (15, 25, and 37 mL/min) that incorporating a mathematical algorithm to the software during normal, obstructive, and restrictive respiratory pattern. for pressure loss through the tubing improved the accuracy In the absence of leaks, all ventilators, irrespective of selected of TV measurements. configuration, were able to maintain TV in all respiratory conditions. In the presence of intentional leaks, all ventilators One characteristic of unintentional leaks is that they are dy- achieved to guarantee TV by increasing inspiratory pressure. namic, which means they can change during the inspiratory However, all ventilators failed to maintain minimal TV by or expiratory phase of the respiratory cycle. Theoretically, the dropping the inspiratory pressure during unintentional leaks presence of dynamic (asymmetric) leaks is a challenge for when a single circuit used with an expiratory valve. the NIV software systems to compensate unintentional leaks. Two bench studies assessed the estimation of TV and unin- Similar to that, Khirani et al. [7] studied seven home ventila- tentional leaks during the asymmetric leak model. In the first tors with different circuit configurations. It was observed that study [4], the reliability of TV estimation was tested in a sin- among five ventilators that can be used with single-limb and gle tube bench model with random dynamic leaks (high and intentional leak, only two maintained TV at its target in the low level) during two levels of pressure support (10 cm H2O presence of unintentional leaks. In two ventilators used with and 15 cm H2O). In the basal condition with only intentional a single-limb circuit and an expiratory valve, TV fell below leaks, which corresponds to standard nasal mask leak, all four the preset value. In three ventilators that can be used with ventilators except for one underestimated TV. In the model of a double-limb circuit, only one of the ventilators overcom- inspiratory unintentional leaks, TV was overestimated rang- pensated TV during unintentional leaks, and TV was de- ing between 137 mL and 264 mL. Two ventilator softwares creased in the other two ventilators. The authors concluded underestimated leaks ranging between -11.5 and -6 L/min. that volume-guaranteed mode is effective only when a single- In the model of expiratory leaks, high level of unintentional limb circuit with an intentional leak (leak port circuit) was leaks caused auto-triggering in all ventilators. For that reason, used. Ventilators using a circuit configuration of double-limb the performance of ventilators only assessed during the low circuit or a single-limb circuit with an expiratory valve may expiratory leak. In that case, three ventilators underestimated misinterpret unintentional leaks as an increase in TV, and TV (ranging between -48 and -29 mL), and one ventilator paradoxically decrease inspiratory pressure and therefore TV. overestimated it. All ventilators overestimated unintentional leaks ranging from 2.2 to 3.1 L/min. These results show that The automatic increase in inspiratory pressure during vol- the estimation of TV in the presence of asymmetric leaks is ume-guaranteed mode in the presence of unintentional leaks erroneous, either underestimated or overestimated depend- is associated with large variations in TV [8]. Six NIV devices ing on the phase of the respiratory cycle. were evaluated for the delivery of a minimal TV in three con- ditions associated with alveolar hypoventilation: increase in In the second study, the same investigator group [5] studied airway resistance, decrease in lung compliance, and uninten- the effect of these asymmetric leaks occurring either inspira- tional leaks. The authors defined “volume guarantee failure” tion or expiration in volume-guaranteed ventilation with sin- as the inability to maintain TV of more than 500 mL and 144 gle-limb circuit. To simulate asymmetric unintentional leaks, “overshooting” as a TV>650 mL. In the model of increased Luján et al. Tidal Volume and Leakage Estimation in NIV airway resistance, five ventilators were able to maintain a the presence of a different mask/interface. Even in the condi- minimal TV of 500 mL. Of them, one ventilator overshot tion of perfect fitting, the magnitude of intentional leaks can TV. In the model of decrease in lung compliance, five ventila- change up to 20%–25% between different masks. Borel et al. tors were able to maintain a minimal TV, and no overshooting [11] showed that the capacity of ventilators to achieve and was observed. However, the maintenance of a minimal TV maintain preset IPAP is directly related to mask intentional during unintentional leak was not that sufficient. Only one leaks that were highly variable ranging between 30 and 45 ventilator was able to guarantee minimal TV with overshoot- L/min. Increasing intentional leaks (>40 L/min) because of a ing, and patient ventilator asynchrony was observed with all mask significantly impaired the capacity of all ventilators to other devices. attain and maintain a preset IPAP. Moreover, the amount of leaks from mask may produce patient ventilator synchrony. IMPLICATIONS IN CLINICAL PRACTICE It was also shown that mask with larger air leaks was shown to be associated with auto-triggering and/or decreased inspi- The main goals of the NIV therapy are to maintain sufficient ratory-trigger sensitivity [10]. Underestimation of leak flow alveolar ventilation, improve pulmonary gas exchange, as- would delay expiratory trigger time and therefore increase in- sist respiratory muscles, and decrease work of breathing. spiratory time and TV; whereas overestimation of leak would The ability of an NIV device to maintain a stable TV is re- shorten it and decrease both parameters. Another important lated not only to its pressurization capacity but also to its technical point to consider is the circuit configuration. So, far, accurate assessment of leaks and TV. Ideally, a ventilator studies showed that the best option for NIV configuration for should be able to rapidly respond to changes for effective compensation of unintentional leaks and accurate estimation TV with and without leaks and, changes in respiratory sys- of TV is single-limb circuit with an intentional leak [6,7]. In tem impedance. Insufficient support because of inaccurate the presence of a true expiratory valve, most NIV devices fail measurement of leaks and therefore TV inevitably would to perform efficiently. cause ineffective assisted ventilation and, therefore, persis- tent hypercapnia. Today, many centers use telemonitoring for the management of NIV support. With new developments in technology, re- Recently, built-in software has been available for clinicians to mote adjustment of ventilatory settings by the clinician ac- monitor and download data in the NIV devices. The NIV set- cording to electronically transferred data from the patient’s tings are usually adjusted according to the parameters shown device can be an option [12-14]. Although electronically by the ventilator software. Clinicians should keep in mind transferred data from built-in software may provide important that each NIV device is different, and the reliability and the information in a very feasible way, clinicians should be very accuracy of leak and TV estimation are highly variable from cautious for the heterogeneity and inaccuracy of parameters one device to another. Underestimation or overestimation of from online downloaded data provided by the different soft- leaks and TV by the software can cause wrong adjustments. ware systems of NIV devices. The lack of validation of these Moreover, because of underestimation of TV, clinicians usual- software and monitored parameters by independent authori- ly tend to increase inspiratory pressures to maintain adequate ties is one of the main limiting factors of its wider use. ventilation. This would eventually increase the possibility of unintentional leaks and/or hyperinflation. If there is a marked CONCLUSION increase in the inspiratory flow during compensation, several Non-invasive ventilation is accepted as an effective therapy other side effects such as increased leakage, impaired mask for acute and chronic respiratory failure. The success of NIV seal, eye irritation, and increased aerophagia and gastric dis- is directly related to monitoring therapy optimally. Today’s tention may occur. The consequences of these problems re- technology gives us the opportunity to monitor, even re- sult in patient discomfort and non-compliance to NIV. This motely, many parameters by built-in software systems of NIV might be particularly more problematic in volume-targeted devices. However, the results from bench studies showed modes as these modes change IPAP depending on estimated that the accuracy and reliability of the information provided TV. The presence of unintentional asymmetric leaks could by the software systems are still questionable, and external lead to clinically significant hypoventilation because of a validation of these systems is an urgent need. For now, we decrease in inspiratory pressure. An overshooting of TV dur- should keep in mind that data from NIV software are helpful, ing volume-targeted ventilation may cause hyperventilation, but they should only be used in conjunction to other moni- patient ventilator asynchrony, decrease in patient’s respira- toring parameters such as the clinical status of the patient, tory effort, and arterial carbon dioxide levels lower than the pulse oximetry, arterial blood gas analysis, and polysomnog- apnea threshold [8,9]. raphy when needed. Finally, medical societies should ask the manufacturers for homogeneity in their algorithms estimating Bench studies showed that the underestimation of TV is di- physiological parameters in the ventilated patient. rectly related to the magnitude of leaks [3]. The degree of intentional leak is dependent not only on the pressure but also on the interface used. The mask determines the level of intentional leak, but estimation of this leak and therefore vari- Peer-review: Externally peer-reviewed. ations in inspiratory time and TV is determined by the ven- Author Contributions: Concept – M.L. ;Design – M.L.; Supervision – tilator [10]. Many manufacturers validate the performance M.L., B.E.; Resources – M.L., C.L., B.E.; Materials – M.L., C.L., B.E.; of their NIV device only with their own mask. Hence, each Data Collection and/or Processing – M.L., C.L., B.E.; Analysis and/or built-in software would erroneously estimate leaks and TV in Interpretation – M.L., C.L., B.E.; Literature Search – M.L., C.L., B.E.; 145 Turk Thorac J 2019; 20(2): 140-6 Writing Manuscript – M.L., C.L., B.E.; Critical Review – M.L., C.L., intentional leaks during volume-targeted ventilation. Intensive B.E.; Other – M.L., C.L., B.E. Care Med 2013; 39: 59-65. [CrossRef] 7. Khirani S, Louis B, Lreoux K, et al. 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