REVIEW URRENT C OPINION
Functional residual capacity and absolute lung volume Diederik Gommers
Purpose of review To discuss the role of measuring functional residual capacity (FRC) during mechanical ventilation to improve patient ventilator settings in order to prevent ventilator-induced lung injury. Recent findings Nowadays, FRC can be measured without the use of tracer gases and without disconnection from the ventilator. It is shown that FRC can provide additional information to optimize the ventilator setting; for example, FRC measurements can differentiate between responders and nonresponders after a recruitment maneuver, and in combination with dynamic compliance one can differentiate between recruitment and overdistention during a positive end-expiratory pressure trial. In addition, FRC measurements enable not only to estimate stress and strain at the bedside, but also to estimate ventilation inhomogeneity. Summary In conclusion, measuring FRC could be extremely valuable during mechanical ventilation, but clinical studies are needed to prove whether this technique will improve outcome. Keywords EELV, functional residual capacity, homogeneity, mechanical ventilation, strain, stress
INTRODUCTION The measurement of functional residual capacity (FRC) has been advocated for years as a valuable tool for optimizing respiratory settings in mechanical ventilation. FRC is the lung volume during breathing at the end of expiration and is regularly determined at pulmonary function laboratories. FRC measurement techniques are based on the dilution of tracer gases, like sulfur hexafluoride washout, closed-circuit helium dilution, or opencircuit multibreath nitrogen washout. All these techniques still need expensive and complex instrumentation, and are in general not suitable for routine FRC measurements in the ICU. FRC can also be measured with computed tomography (CT), but this technique is not available for routine measurements at the bedside. An alternative is the simplified helium dilution method, using a re-breathing bag with a helium mixture. However, an important disadvantage of this technique is the interruption of mechanical ventilation for a short period of time. Recently, Stenqvist and colleagues [1] have introduced a novel method to measure FRC without interruption of mechanical ventilation, based on a simplified and modified nitrogen multiple breath washout (NMBW) technique, which is nowadays
integrated in a commercially available ICU ventilator. This method requires a step change in the inspired oxygen fraction (FiO2), without the need for supplementary tracer gases or specialized additional monitoring equipment.
BACKGROUND It has become clear that mechanical ventilation itself can cause damage to the lung in critically ill patients, also known as ventilator-induced lung injury (VILI). Insight into the mechanisms of VILI has shown that ventilation should, on one hand, prevent overdistention or ‘volutrauma’ which is the result of excessive stress at the end-inflation and, on the other hand, prevent atelectrauma, which is a kind of injury induced by the repetitive opening and Department of Adult Intensive Care, Erasmus MC, Rotterdam, The Netherlands Correspondence to Diederik Gommers, MD, PhD, Department of Adult Intensive Care, Room H623, Erasmus MC’s, Gravendijkwal 230, 3015 CE, Rotterdam, The Netherlands. Tel: +31 10 7040704; fax: +31 10 7032874; e-mail: [email protected] Curr Opin Crit Care 2014, 20:347–351 DOI:10.1097/MCC.0000000000000099
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Cardiopulmonary monitoring
KEY POINTS Bedside FRC measurement may help clinicians to improve ventilator settings in order to diminish ventilator-induced lung injury (VILI). FRC measurements can be used to calculate stress and strain at the bedside. Lung homogeneity is of special importance in preventing VILI and can be estimated from the washout curve during FRC measurements.
closing of alveolar units at the end exhalation, presumably because of inadequate levels of positive end-expiratory pressure (PEEP) to prevent derecruitment. The first part is achieved by applying low tidal volume ventilation with low plateau pressures, as applied in the acute respiratory distress syndrome (ARDS) network trial [2], and this is nowadays called ‘protective ventilation’. The second part is achieved by open-lung ventilation that uses a recruitment maneuver to open up collapsed lung units and applying sufficient level of PEEP to keep these lung units open. The protective ventilation strategy is applied in combination with a low level of PEEP, and therefore alveolar collapse or atelectasis will become predominant in the lung. The collapsed lung units will cause impaired arterial oxygenation because of shunt, and mechanical stress because of repeated opening and closing of alveoli. However, extended experimental research has shown that the use of higher levels of PEEP can reduce and prevent lung damage. Different studies in ventilated patients showed an improvement in secondary endpoints with the use of higher PEEP levels, but were unable to show a significant decreased mortality rate. A meta-analysis combined these studies and showed significantly decreased hospital mortality when applying higher levels of PEEP in the more severe patients with acute respiratory failure [3]. Therefore, it has been advocated to use higher PEEP levels tailored to the individual patient, with adequate monitoring to minimize the risk for overstretching of the ‘healthy’ alveoli. The method to reliably obtain PEEP levels for each individual patient remains elusive despite years of extensive clinical and laboratory research. A possible method of identifying optimal PEEP could be the measurement of absolute lung volume. FRC is the lung volume at the end of expiration during spontaneous breathing. In critically ill patients receiving mechanical ventilation, the level of PEEP determines FRC and therefore it is better to speak of end-expiratory lung volume (EELV). PEEP is usually 348
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adjusted to increase FRC with the aim of achieving adequate arterial oxygenation.
TECHNIQUES OF FUNCTIONAL RESIDUAL CAPACITY MEASUREMENTS CT studies are the gold standard for FRC determination in research ventilation studies, but transporting patients repeatedly to radiography and the risk of cumulative radiation exposure limit the use of serial CT imaging for this purpose. Traditionally, FRC measurement techniques are based on either closed-circuit indicator gas dilution or open-circuit indicator gas washout tests. For this latter, external tracer gases like helium or sulfur hexafluoride (SF6) are used and an intelligent dispersing device is needed to deliver a constant concentration adjusting to the flow pattern. The use of oxygen eliminates the need for external tracer gases, but the washout curve has to be meticulously corrected for oxygen consumption and using accurate oxygen sensors. Conventional paramagnetic oxygen sensors have response times above seconds and cannot be used for a detailed intrabreath washout curve. Weismann et al. [4] used a rapid oxygen sensor, with a response time less than 200 ms to measure oxygen washout curves and developed an algorithm to obtain accurate EELV measurements in critically ill patients (Draeger, Germany). Stenqvist and colleagues [1] introduced a simplified and modified NMBW technique, which is integrated in the ¨ m ventilator. The calculation of nitrogen Engstro breath-by-breath is based on the values of carbon dioxide production (VCO2) end-tidal oxygen concentration (EtO2), and end-tidal carbon dioxide concentration (EtCO2). With this method, there is no need to use supplementary gases, but the breathing pattern has to be constant in order to achieve a valid VCO2. Chiumello et al. [5] compared this method with the gold standard CT and found that it correlates well and may be easily used in clinical practice. Maisch et al. [6 ] performed an incremental and decremental PEEP trial in 20 anesthetized patients undergoing elective surgery. They measured EELV and arterial oxygenation, and performed dynamic compliance and dead space calculations. It was shown that EELV increased after each incremental PEEP step, with the highest value at the highest PEEP level used. In addition, EELV was higher during the decremental PEEP trial, compared with equal PEEP levels during the incremental PEEP trial, because of a recruitment maneuver performed in between. It was concluded that EELV and PaO2 changes were less sensitive, and that dynamic compliance and dead space calculations were the most reliable parameters to define ‘best’ PEEP at the bedside. We [7] reported &
Volume 20 Number 3 June 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Functional residual capacity and absolute lung volume Gommers
EELV measurements during a decremental PEEP trial (15–10–5 cm H2O) in 45 mechanically ventilated patients divided into three groups related to pulmonary condition: normal lungs (N), primary lung disorder (P), or secondary lung disorder (S). At 5 cm H2O of PEEP, EELV was 31, 20, and 17 ml/kg predicted body weight in groups N, P, and S, respectively. These measured values were only 66, 42, and 34% of the predicted sitting FRC. Only in patients with secondary lung disorders, PEEP-induced EELV changes were the result of derecruitment. Dellamonica et al. [8] created a bedside monitoring tool to calculate the alveolar recruitability by using EELV measurements. The baseline FRC was defined at zero PEEP (ZEEP), and thereafter they increased PEEP to 5 and 15 cm H2O, respectively. By calculating the DEELV/FRC ratio, they defined low recruiters and high recruiters at a cutoff point of at least 73%. FRC is normally measured in the sitting or standing position, and is sex, height, and age dependent. Clearly, FRC in ventilated patients cannot be compared to FRC predicted values obtained from pulmonary function testing. The following formula, recommended by the European Respiratory Society [9], provides a good estimate of the FRC in healthy sitting individuals: female : 2:24 height ðmetersÞ þ 0:001
to optimize the ventilator setting, for example, EELV measurements can differentiate between responders and nonresponders after a recruitment maneuver, and in combination with compliance one can differentiate between recruitment and overdistention. Recruitment leads to increase of compliance, whereas overdistention will reduce compliance.
STRESS AND STRAIN In an experimental study, Protti et al. [11] showed that ventilation with high tidal volumes, resulting in an expiratory volume of 1.5 times FRC (¼strain of 1.5), caused severe lung edema; all their study animals died within the observation period of 54 h. Recently, in a second study [12 ], the authors ventilated all animals with a strain of 2.5 and showed that high tidal volumes with a low level of PEEP damaged the lungs and increased mortality, whereas high PEEP levels together with low tidal volume, but with the same strain of 2.5, did not result in edema and all animals in this group survived. With the current techniques, FRC can be measured to calculate stress and strain. Strain describes the relation between end-inspiratory volume (i.e. tidal volume and PEEP volume) and FRC, and is calculated using the formula: &&
age ðyearsÞ 1:00 male : 2:34 height ðmetersÞ þ 0:022 age ðyearsÞ 1:23 Ibanez and Raurich [10] showed that FRC decreased by 25% after changing the patient’s position from sitting to supine during spontaneous breathing in healthy volunteers. If one assumes that ventilation of a ‘healthy’ lung at a PEEP of 5 cm H2O occurs approximately at FRC level, then we found a reduction of 34% in patients mechanically ventilated but without lung disorders [7]. This extra reduction of EELV (34 vs. 25%) is probably because of loss of muscle tension attributed to the use of sedation in our ICU patients. In addition, we found a reduction of 58% in patients with ARDS, and thus the predicted FRC was only 42%. Dellamonica et al. [8] measured a mean FRC of 31% of predicted in ARDS patients. In patients with moderate-to-severe ARDS, an FRC of less than 1.0 l can be expected. Even after applying a recruitment in combination with a PEEP of 15 cm H2O, the FRC marginally exceeds 1.0 l in some of these patients. In patients with low potential for recruitment, the resulting overdistention associated with PEEP is largely predominant. Therefore, EELV can provide additional information
Strainglobal ¼
VT þ VPEEP FRC
Strain has a static (PEEP) and a dynamic (tidal volume) component. Therefore, static strain and dynamic strain are calculated according the following formulas: Strainstatic ¼
VPEEP FRC
Straindynamic ¼
VT FRC
Stress is calculated using the following formula: Stress ¼ Elastance Strain &&
Recently, Stenqvist et al. [13 ] introduced a simplified method to calculate elastance based on the following formula that can also be estimated by FRC measurements at two different PEEP levels: Elastance ¼
DPEEP DVPEEP
Although lung stress and strain are frequently used in experimental studies describing VILI, there is a limited amount of clinical studies describing these two parameters. Chiumello et al. [14] used the
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Cardiopulmonary monitoring
helium technique to estimate strain at PEEP 5 and 15 cm H2O, and stress was calculated based on the esophagus pressure measurement. Both Gonza´lezLo´pez et al. [15] and Dellamonica et al. [8] used the ¨ m ventilator to measure EELV at the differEngstro ent PEEP levels, but EELV at ZEEP (¼FRC) was not directly measured. FRC was estimated from EELV at 5 PEEP minus the volume difference between EELV at 5 and 10 PEEP, taking into account a linear relationship. We also calculated stress and strain in 29 mechanically ventilated patients admitted to the ICU during a decremental PEEP trial. We found that global stress and strain mainly exist of the static component (PEEP), whereas dynamic component was negligible despite the use of relative high tidal volumes (8–10 ml/kg) in these patients.
CONCLUSION
HOMOGENEITY
Acknowledgements None.
In 1970, Mead et al. [16] estimated that forces acting on lung tissue increase with a factor 4.5 when lungs are inhomogeneously ventilated. This was recently confirmed by Rausch et al. [17], who performed X-ray tomographic microscopy (generating detailed three-dimensional alveolar geometries) in rat lungs and found local strain values of four times the global strain. This inhomogeneity raises stress and increases the risk to develop VILI. Therefore, the use of an inhomogeneity index as a target for ventilation strategies would be very beneficial in this context. Although inhomogeneity indices are often used in pulmonary function laboratories and improve after application of PEEP in pediatric anesthesia, their use in the ICU is limited by the need of specialized equipment and tracer gases. Several studies have worked on the development of inhomogeneity indices and indicator gas injectors based on SF6 for critically ill patients, but implementation remained difficult because of the need of specialized equipment and gas containers at the bedside. The availability of a routine method to quantify homogeneous alveolar ventilation at the bedside is expected to help to optimize ventilator settings in individual patients to achieve optimal gas exchange. Recently, we [18] described the assessment and integration of a ventilatory inhomogeneity index based on a rapid oxygen sensor incorporated into FRC equipment (Draeger, Germany) without the need for external tracer gases. To our knowledge, this is the first study to describe indices of alveolar inhomogeneity measured by medical grade oxygen sensors and conventional breathing systems, and applicable during mechanical ventilation. Studies are underway to evaluate the indices in critically ill patients. 350
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Blood gases are frequently used to monitor the patient’s lung function during mechanical ventilation. It is known that variation in gas exchange is not always sufficient to assess anatomical lung recruitment or derecruitment in patients with ARDS. Therefore, it seems reasonable to monitor absolute lung volume changes in these patients in order to optimize ventilator settings. Nowadays, FRC can be measured without the use of tracer gases and without disconnection from the ventilator. Also, stress and strain can easily be calculated by measuring FRC. Therefore, it seems reasonable that ICU ventilators are equipped with a technique to measure FRC in patients with severe respiratory failure.
Conflicts of interest The author received consulting fees from GE Healthcare, as being a member of its critical care advisory board between 2009 and 2012, and travel expenses and fees for giving oral presentations for GE Healthcare and Draeger.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Olegard C, Sondergaard S, Houltz E, et al. Estimation of functional residual capacity at the bedside using standard monitoring equipment: a modified nitrogen washout/washin technique requiring a small change of the inspired oxygen fraction. Anesth Analg 2005; 101:206–212. 2. 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. 3. Putensen C, Theuerkauf N, Zinserling J, et al. Meta-analysis: ventilation strategies and outcomes of the acute respiratory distress syndrome and acute lung injury. Ann Intern Med 2009; 151:566–576. 4. Weismann D, Reissmann H, Maisch S, et al. Monitoring of functional residual capacity by an oxygen washin/washout; technical description and evaluation. J Clin Monit Comput 2006; 20:251–260. 5. Chiumello D, Cressoni M, Chierichetti M, et al. Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume. Crit Care 2008; 12:R150. 6. Maisch S, Reissmann H, Fuellekrug B, et al. Compliance and dead space & fraction indicate an optimal level of positive end-expiratory pressure after recruitment in anesthetized patients. Anesth Analg 2008; 106: 175–181. All parameters are measured during a standardized incremental and decremental PEEP trial in surgery patients with healthy lungs in order to find the best parameter to describe the optimal PEEP. 7. Bikker IG, van Bommel J, Reis MD, et al. End-expiratory lung volume during mechanical ventilation: a comparison with reference values and the effect of positive end-expiratory pressure in intensive care unit patients with different lung conditions. Crit Care 2008; 12:R145. 8. Dellamonica J, Lerolle N, Sargentini C, et al. PEEP-induced changes in lung volume in acute respiratory distress syndrome. Two methods to estimate alveolar recruitment. Intensive Care Med 2011; 37:1595–1604. 9. Stocks J, Quanjer PH. Reference values for residual volume, functional residual capacity and total volume measurements. Official Statement of The European Respiratory Society. Eur Respir J 1995; 8:492–506.
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Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Functional residual capacity and absolute lung volume Gommers 10. Ibanez J, Raurich JM. Normal values of functional residual capacity in the sitting and supine positions. Intensive Care Med 1982; 8:173–177. 11. Protti A, Cressoni M, Santini A, et al. Lung stress and strain during mechanical ventilation: any safe threshold? Am J Respir Crit Care Med 2011; 183:1354– 1362. 12. Protti A, Andreis DT, Monti M, et al. Lung stress and strain during mechanical && ventilation: any difference between statics and dynamics? Crit Care Med 2013; 41:1046–1055. A very nice experimental study in which static and dynamic strain are changed, and it was shown that lung injury is mainly caused by dynamic strain and not by static strain. 13. Stenqvist O, Grivans C, Andersson B, et al. Lung elastance and transpul&& monary pressure can be determined without using oesophageal pressure measurements. Acta Anaesthesiol Scand 2012; 56:738–747. An alternative method has been found for measuring elastance.
14. Chiumello D, Carlesso E, Cadringher P, et al. Lung stress and strain during mechanical ventilation for acute respiratory distress syndrome. Am J Respir Crit Care Med 2008; 178:346–355. 15. Gonza´lez-Lo´pez A, Garcı´a-Prieto E, Batalla-Solı´s E, et al. Lung strain and biological response in mechanically ventilated patients. Intensive Care Med 2012; 38:240–247. 16. Mead J, Takishima T, Leith D. Stress distribution in lungs: a model of pulmonary elasticity. J Appl Physiol 1970; 28:596–608. 17. Rausch SM, Haberthur D, Stampanoni M, et al. Local strain distribution in real three-dimensional alveolar geometries. Ann Biomed Eng 2011; 39:2835– 2843. 18. Bikker IG, Holland W, Specht P, et al. Assessment of ventilation inhomogeneity during mechanical ventilation using a rapid response oxygen sensor based oxygen washout method. Intensive Care Med Exp (in press).
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Functional residual capacity and absolute lung volume Diederik Gommers
Purpose of review To discuss the role of measuring functional residual capacity (FRC) during mechanical ventilation to improve patient ventilator settings in order to prevent ventilator-induced lung injury. Recent findings Nowadays, FRC can be measured without the use of tracer gases and without disconnection from the ventilator. It is shown that FRC can provide additional information to optimize the ventilator setting; for example, FRC measurements can differentiate between responders and nonresponders after a recruitment maneuver, and in combination with dynamic compliance one can differentiate between recruitment and overdistention during a positive end-expiratory pressure trial. In addition, FRC measurements enable not only to estimate stress and strain at the bedside, but also to estimate ventilation inhomogeneity. Summary In conclusion, measuring FRC could be extremely valuable during mechanical ventilation, but clinical studies are needed to prove whether this technique will improve outcome. Keywords EELV, functional residual capacity, homogeneity, mechanical ventilation, strain, stress
INTRODUCTION The measurement of functional residual capacity (FRC) has been advocated for years as a valuable tool for optimizing respiratory settings in mechanical ventilation. FRC is the lung volume during breathing at the end of expiration and is regularly determined at pulmonary function laboratories. FRC measurement techniques are based on the dilution of tracer gases, like sulfur hexafluoride washout, closed-circuit helium dilution, or opencircuit multibreath nitrogen washout. All these techniques still need expensive and complex instrumentation, and are in general not suitable for routine FRC measurements in the ICU. FRC can also be measured with computed tomography (CT), but this technique is not available for routine measurements at the bedside. An alternative is the simplified helium dilution method, using a re-breathing bag with a helium mixture. However, an important disadvantage of this technique is the interruption of mechanical ventilation for a short period of time. Recently, Stenqvist and colleagues [1] have introduced a novel method to measure FRC without interruption of mechanical ventilation, based on a simplified and modified nitrogen multiple breath washout (NMBW) technique, which is nowadays
integrated in a commercially available ICU ventilator. This method requires a step change in the inspired oxygen fraction (FiO2), without the need for supplementary tracer gases or specialized additional monitoring equipment.
BACKGROUND It has become clear that mechanical ventilation itself can cause damage to the lung in critically ill patients, also known as ventilator-induced lung injury (VILI). Insight into the mechanisms of VILI has shown that ventilation should, on one hand, prevent overdistention or ‘volutrauma’ which is the result of excessive stress at the end-inflation and, on the other hand, prevent atelectrauma, which is a kind of injury induced by the repetitive opening and Department of Adult Intensive Care, Erasmus MC, Rotterdam, The Netherlands Correspondence to Diederik Gommers, MD, PhD, Department of Adult Intensive Care, Room H623, Erasmus MC’s, Gravendijkwal 230, 3015 CE, Rotterdam, The Netherlands. Tel: +31 10 7040704; fax: +31 10 7032874; e-mail: [email protected] Curr Opin Crit Care 2014, 20:347–351 DOI:10.1097/MCC.0000000000000099
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Cardiopulmonary monitoring
KEY POINTS Bedside FRC measurement may help clinicians to improve ventilator settings in order to diminish ventilator-induced lung injury (VILI). FRC measurements can be used to calculate stress and strain at the bedside. Lung homogeneity is of special importance in preventing VILI and can be estimated from the washout curve during FRC measurements.
closing of alveolar units at the end exhalation, presumably because of inadequate levels of positive end-expiratory pressure (PEEP) to prevent derecruitment. The first part is achieved by applying low tidal volume ventilation with low plateau pressures, as applied in the acute respiratory distress syndrome (ARDS) network trial [2], and this is nowadays called ‘protective ventilation’. The second part is achieved by open-lung ventilation that uses a recruitment maneuver to open up collapsed lung units and applying sufficient level of PEEP to keep these lung units open. The protective ventilation strategy is applied in combination with a low level of PEEP, and therefore alveolar collapse or atelectasis will become predominant in the lung. The collapsed lung units will cause impaired arterial oxygenation because of shunt, and mechanical stress because of repeated opening and closing of alveoli. However, extended experimental research has shown that the use of higher levels of PEEP can reduce and prevent lung damage. Different studies in ventilated patients showed an improvement in secondary endpoints with the use of higher PEEP levels, but were unable to show a significant decreased mortality rate. A meta-analysis combined these studies and showed significantly decreased hospital mortality when applying higher levels of PEEP in the more severe patients with acute respiratory failure [3]. Therefore, it has been advocated to use higher PEEP levels tailored to the individual patient, with adequate monitoring to minimize the risk for overstretching of the ‘healthy’ alveoli. The method to reliably obtain PEEP levels for each individual patient remains elusive despite years of extensive clinical and laboratory research. A possible method of identifying optimal PEEP could be the measurement of absolute lung volume. FRC is the lung volume at the end of expiration during spontaneous breathing. In critically ill patients receiving mechanical ventilation, the level of PEEP determines FRC and therefore it is better to speak of end-expiratory lung volume (EELV). PEEP is usually 348
www.co-criticalcare.com
adjusted to increase FRC with the aim of achieving adequate arterial oxygenation.
TECHNIQUES OF FUNCTIONAL RESIDUAL CAPACITY MEASUREMENTS CT studies are the gold standard for FRC determination in research ventilation studies, but transporting patients repeatedly to radiography and the risk of cumulative radiation exposure limit the use of serial CT imaging for this purpose. Traditionally, FRC measurement techniques are based on either closed-circuit indicator gas dilution or open-circuit indicator gas washout tests. For this latter, external tracer gases like helium or sulfur hexafluoride (SF6) are used and an intelligent dispersing device is needed to deliver a constant concentration adjusting to the flow pattern. The use of oxygen eliminates the need for external tracer gases, but the washout curve has to be meticulously corrected for oxygen consumption and using accurate oxygen sensors. Conventional paramagnetic oxygen sensors have response times above seconds and cannot be used for a detailed intrabreath washout curve. Weismann et al. [4] used a rapid oxygen sensor, with a response time less than 200 ms to measure oxygen washout curves and developed an algorithm to obtain accurate EELV measurements in critically ill patients (Draeger, Germany). Stenqvist and colleagues [1] introduced a simplified and modified NMBW technique, which is integrated in the ¨ m ventilator. The calculation of nitrogen Engstro breath-by-breath is based on the values of carbon dioxide production (VCO2) end-tidal oxygen concentration (EtO2), and end-tidal carbon dioxide concentration (EtCO2). With this method, there is no need to use supplementary gases, but the breathing pattern has to be constant in order to achieve a valid VCO2. Chiumello et al. [5] compared this method with the gold standard CT and found that it correlates well and may be easily used in clinical practice. Maisch et al. [6 ] performed an incremental and decremental PEEP trial in 20 anesthetized patients undergoing elective surgery. They measured EELV and arterial oxygenation, and performed dynamic compliance and dead space calculations. It was shown that EELV increased after each incremental PEEP step, with the highest value at the highest PEEP level used. In addition, EELV was higher during the decremental PEEP trial, compared with equal PEEP levels during the incremental PEEP trial, because of a recruitment maneuver performed in between. It was concluded that EELV and PaO2 changes were less sensitive, and that dynamic compliance and dead space calculations were the most reliable parameters to define ‘best’ PEEP at the bedside. We [7] reported &
Volume 20 Number 3 June 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Functional residual capacity and absolute lung volume Gommers
EELV measurements during a decremental PEEP trial (15–10–5 cm H2O) in 45 mechanically ventilated patients divided into three groups related to pulmonary condition: normal lungs (N), primary lung disorder (P), or secondary lung disorder (S). At 5 cm H2O of PEEP, EELV was 31, 20, and 17 ml/kg predicted body weight in groups N, P, and S, respectively. These measured values were only 66, 42, and 34% of the predicted sitting FRC. Only in patients with secondary lung disorders, PEEP-induced EELV changes were the result of derecruitment. Dellamonica et al. [8] created a bedside monitoring tool to calculate the alveolar recruitability by using EELV measurements. The baseline FRC was defined at zero PEEP (ZEEP), and thereafter they increased PEEP to 5 and 15 cm H2O, respectively. By calculating the DEELV/FRC ratio, they defined low recruiters and high recruiters at a cutoff point of at least 73%. FRC is normally measured in the sitting or standing position, and is sex, height, and age dependent. Clearly, FRC in ventilated patients cannot be compared to FRC predicted values obtained from pulmonary function testing. The following formula, recommended by the European Respiratory Society [9], provides a good estimate of the FRC in healthy sitting individuals: female : 2:24 height ðmetersÞ þ 0:001
to optimize the ventilator setting, for example, EELV measurements can differentiate between responders and nonresponders after a recruitment maneuver, and in combination with compliance one can differentiate between recruitment and overdistention. Recruitment leads to increase of compliance, whereas overdistention will reduce compliance.
STRESS AND STRAIN In an experimental study, Protti et al. [11] showed that ventilation with high tidal volumes, resulting in an expiratory volume of 1.5 times FRC (¼strain of 1.5), caused severe lung edema; all their study animals died within the observation period of 54 h. Recently, in a second study [12 ], the authors ventilated all animals with a strain of 2.5 and showed that high tidal volumes with a low level of PEEP damaged the lungs and increased mortality, whereas high PEEP levels together with low tidal volume, but with the same strain of 2.5, did not result in edema and all animals in this group survived. With the current techniques, FRC can be measured to calculate stress and strain. Strain describes the relation between end-inspiratory volume (i.e. tidal volume and PEEP volume) and FRC, and is calculated using the formula: &&
age ðyearsÞ 1:00 male : 2:34 height ðmetersÞ þ 0:022 age ðyearsÞ 1:23 Ibanez and Raurich [10] showed that FRC decreased by 25% after changing the patient’s position from sitting to supine during spontaneous breathing in healthy volunteers. If one assumes that ventilation of a ‘healthy’ lung at a PEEP of 5 cm H2O occurs approximately at FRC level, then we found a reduction of 34% in patients mechanically ventilated but without lung disorders [7]. This extra reduction of EELV (34 vs. 25%) is probably because of loss of muscle tension attributed to the use of sedation in our ICU patients. In addition, we found a reduction of 58% in patients with ARDS, and thus the predicted FRC was only 42%. Dellamonica et al. [8] measured a mean FRC of 31% of predicted in ARDS patients. In patients with moderate-to-severe ARDS, an FRC of less than 1.0 l can be expected. Even after applying a recruitment in combination with a PEEP of 15 cm H2O, the FRC marginally exceeds 1.0 l in some of these patients. In patients with low potential for recruitment, the resulting overdistention associated with PEEP is largely predominant. Therefore, EELV can provide additional information
Strainglobal ¼
VT þ VPEEP FRC
Strain has a static (PEEP) and a dynamic (tidal volume) component. Therefore, static strain and dynamic strain are calculated according the following formulas: Strainstatic ¼
VPEEP FRC
Straindynamic ¼
VT FRC
Stress is calculated using the following formula: Stress ¼ Elastance Strain &&
Recently, Stenqvist et al. [13 ] introduced a simplified method to calculate elastance based on the following formula that can also be estimated by FRC measurements at two different PEEP levels: Elastance ¼
DPEEP DVPEEP
Although lung stress and strain are frequently used in experimental studies describing VILI, there is a limited amount of clinical studies describing these two parameters. Chiumello et al. [14] used the
1070-5295 ß 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins
www.co-criticalcare.com
349
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Cardiopulmonary monitoring
helium technique to estimate strain at PEEP 5 and 15 cm H2O, and stress was calculated based on the esophagus pressure measurement. Both Gonza´lezLo´pez et al. [15] and Dellamonica et al. [8] used the ¨ m ventilator to measure EELV at the differEngstro ent PEEP levels, but EELV at ZEEP (¼FRC) was not directly measured. FRC was estimated from EELV at 5 PEEP minus the volume difference between EELV at 5 and 10 PEEP, taking into account a linear relationship. We also calculated stress and strain in 29 mechanically ventilated patients admitted to the ICU during a decremental PEEP trial. We found that global stress and strain mainly exist of the static component (PEEP), whereas dynamic component was negligible despite the use of relative high tidal volumes (8–10 ml/kg) in these patients.
CONCLUSION
HOMOGENEITY
Acknowledgements None.
In 1970, Mead et al. [16] estimated that forces acting on lung tissue increase with a factor 4.5 when lungs are inhomogeneously ventilated. This was recently confirmed by Rausch et al. [17], who performed X-ray tomographic microscopy (generating detailed three-dimensional alveolar geometries) in rat lungs and found local strain values of four times the global strain. This inhomogeneity raises stress and increases the risk to develop VILI. Therefore, the use of an inhomogeneity index as a target for ventilation strategies would be very beneficial in this context. Although inhomogeneity indices are often used in pulmonary function laboratories and improve after application of PEEP in pediatric anesthesia, their use in the ICU is limited by the need of specialized equipment and tracer gases. Several studies have worked on the development of inhomogeneity indices and indicator gas injectors based on SF6 for critically ill patients, but implementation remained difficult because of the need of specialized equipment and gas containers at the bedside. The availability of a routine method to quantify homogeneous alveolar ventilation at the bedside is expected to help to optimize ventilator settings in individual patients to achieve optimal gas exchange. Recently, we [18] described the assessment and integration of a ventilatory inhomogeneity index based on a rapid oxygen sensor incorporated into FRC equipment (Draeger, Germany) without the need for external tracer gases. To our knowledge, this is the first study to describe indices of alveolar inhomogeneity measured by medical grade oxygen sensors and conventional breathing systems, and applicable during mechanical ventilation. Studies are underway to evaluate the indices in critically ill patients. 350
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Blood gases are frequently used to monitor the patient’s lung function during mechanical ventilation. It is known that variation in gas exchange is not always sufficient to assess anatomical lung recruitment or derecruitment in patients with ARDS. Therefore, it seems reasonable to monitor absolute lung volume changes in these patients in order to optimize ventilator settings. Nowadays, FRC can be measured without the use of tracer gases and without disconnection from the ventilator. Also, stress and strain can easily be calculated by measuring FRC. Therefore, it seems reasonable that ICU ventilators are equipped with a technique to measure FRC in patients with severe respiratory failure.
Conflicts of interest The author received consulting fees from GE Healthcare, as being a member of its critical care advisory board between 2009 and 2012, and travel expenses and fees for giving oral presentations for GE Healthcare and Draeger.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Olegard C, Sondergaard S, Houltz E, et al. Estimation of functional residual capacity at the bedside using standard monitoring equipment: a modified nitrogen washout/washin technique requiring a small change of the inspired oxygen fraction. Anesth Analg 2005; 101:206–212. 2. 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. 3. Putensen C, Theuerkauf N, Zinserling J, et al. Meta-analysis: ventilation strategies and outcomes of the acute respiratory distress syndrome and acute lung injury. Ann Intern Med 2009; 151:566–576. 4. Weismann D, Reissmann H, Maisch S, et al. Monitoring of functional residual capacity by an oxygen washin/washout; technical description and evaluation. J Clin Monit Comput 2006; 20:251–260. 5. Chiumello D, Cressoni M, Chierichetti M, et al. Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume. Crit Care 2008; 12:R150. 6. Maisch S, Reissmann H, Fuellekrug B, et al. Compliance and dead space & fraction indicate an optimal level of positive end-expiratory pressure after recruitment in anesthetized patients. Anesth Analg 2008; 106: 175–181. All parameters are measured during a standardized incremental and decremental PEEP trial in surgery patients with healthy lungs in order to find the best parameter to describe the optimal PEEP. 7. Bikker IG, van Bommel J, Reis MD, et al. End-expiratory lung volume during mechanical ventilation: a comparison with reference values and the effect of positive end-expiratory pressure in intensive care unit patients with different lung conditions. Crit Care 2008; 12:R145. 8. Dellamonica J, Lerolle N, Sargentini C, et al. PEEP-induced changes in lung volume in acute respiratory distress syndrome. Two methods to estimate alveolar recruitment. Intensive Care Med 2011; 37:1595–1604. 9. Stocks J, Quanjer PH. Reference values for residual volume, functional residual capacity and total volume measurements. Official Statement of The European Respiratory Society. Eur Respir J 1995; 8:492–506.
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Functional residual capacity and absolute lung volume Gommers 10. Ibanez J, Raurich JM. Normal values of functional residual capacity in the sitting and supine positions. Intensive Care Med 1982; 8:173–177. 11. Protti A, Cressoni M, Santini A, et al. Lung stress and strain during mechanical ventilation: any safe threshold? Am J Respir Crit Care Med 2011; 183:1354– 1362. 12. Protti A, Andreis DT, Monti M, et al. Lung stress and strain during mechanical && ventilation: any difference between statics and dynamics? Crit Care Med 2013; 41:1046–1055. A very nice experimental study in which static and dynamic strain are changed, and it was shown that lung injury is mainly caused by dynamic strain and not by static strain. 13. Stenqvist O, Grivans C, Andersson B, et al. Lung elastance and transpul&& monary pressure can be determined without using oesophageal pressure measurements. Acta Anaesthesiol Scand 2012; 56:738–747. An alternative method has been found for measuring elastance.
14. Chiumello D, Carlesso E, Cadringher P, et al. Lung stress and strain during mechanical ventilation for acute respiratory distress syndrome. Am J Respir Crit Care Med 2008; 178:346–355. 15. Gonza´lez-Lo´pez A, Garcı´a-Prieto E, Batalla-Solı´s E, et al. Lung strain and biological response in mechanically ventilated patients. Intensive Care Med 2012; 38:240–247. 16. Mead J, Takishima T, Leith D. Stress distribution in lungs: a model of pulmonary elasticity. J Appl Physiol 1970; 28:596–608. 17. Rausch SM, Haberthur D, Stampanoni M, et al. Local strain distribution in real three-dimensional alveolar geometries. Ann Biomed Eng 2011; 39:2835– 2843. 18. Bikker IG, Holland W, Specht P, et al. Assessment of ventilation inhomogeneity during mechanical ventilation using a rapid response oxygen sensor based oxygen washout method. Intensive Care Med Exp (in press).
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