Measurement of end-expiratory lung volume by oxygen washin–washout in controlled and assisted mechanically ventilated patients

Monitoring Lung Injury Severity and Ventilation Intensity during Mechanical Ventilation

Acute respiratory distress syndrome (ARDS) is a severe form of respiratory failure burden by high hospital mortality. No specific pharmacologic treatment is currently available and its ventilatory management is a key strategy to allow reparative and regenerative lung tissue processes. Unfortunately, a poor management of mechanical ventilation can induce ventilation induced lung injury (VILI) caused by physical and biological forces which are at play. Different parameters have been described over the years to assess lung injury severity and facilitate optimization of mechanical ventilation. Indices of lung injury severity include variables related to gas exchange abnormalities, ventilatory setting and respiratory mechanics, ventilation intensity, and the presence of lung hyperinflation versus derecruitment. Recently, specific indexes have been proposed to quantify the stress and the strain released over time using more comprehensive algorithms of calculation such as the mechanical power, and the interaction between driving pressure (DP) and respiratory rate (RR) in the novel DP multiplied by four plus RR [(4 × DP) + RR] index. These new parameters introduce the concept of ventilation intensity as contributing factor of VILI. Ventilation intensity should be taken into account to optimize protective mechanical ventilation strategies, with the aim to reduce intensity to the lowest level required to maintain gas exchange to reduce the potential for VILI. This is further gaining relevance in the current era of phenotyping and enrichment strategies in ARDS.

Single-breath oxygen dilution for the measurement of total lung capacity: technical description and preliminary results in healthy subjects

Objective. Total lung capacity (TLC) assessment outside of a research laboratory is challenging. We describe a novel method for measuring TLC that is both simple and based only on portable equipment, and report preliminary data in healthy subjects.Approach. We developed an open circuit system to administer a known amount of oxygen to a subject in a single maximal inspiratory maneuver. Oxygen fraction, expired and inspired flows were continuously monitored to allow a precise computation of the mass balance. Values of TLC and functional residual capacity (FRC) were compared with standard methods (body plethysmography and multiple-breath helium dilution). Twenty healthy subjects participated to the study, eleven of which performed the maneuver twice to assess test-retest reliability.Main results.There was high agreement in TLC between the proposed method and the two standard methods (R² > 0.98, bias not different from 0, and 95% limits of agreements <± 0.4 l for both). Test-retest reliability was high (intraclass correlation coefficient >0.99 and no bias). Results were similar for FRC, with a slightly higher variability due its sensitivity to changes in posture or breathing pattern.Significance.Single-breath oxygen dilution is accurate and reliable in assessing TLC and FRC in healthy subjects. The technique is appealing for time- or resource-limited settings, such as field physiological research expeditions or mass screenings.

Accuracy of the end-expiratory lung volume measured by the modified nitrogen washout/washin technique: a bench study

Background

The functional residual capacity (FRC) determines the oxygenating capacity of the lung and is heavily affected in the clinical context of the acute respiratory distress syndrome. Nitrogen-wash-in/wash-out methods have been used to measure FRC. These methods have rarely been validated against exactly known volumes. The aim of the study was to assess the accuracy and precision of the N₂ washout/washin method in measuring FRC, by comparing it with set volumes in a lung simulator.

Methods

We conducted a diagnostic bench study in the Intensive Care Unit and Radiology Department of a tertiary hospital in Switzerland. Using a fully controllable high fidelity lung simulator (TestChest®), we set the functional residual capacity at 1500 ml, 2000 ml and 2500 ml and connected to the GE Carestation respirator, which includes the nitrogen washout/washin technique (INview™ tool). FRC was then set to vary by different levels of PEEP (5, 8, 12 and 15 cmH₂O). The main outcome measures were bias and precision of the TestChest® when compared to the results from the washout/washin technique, according to the results of a Bland Altman Analysis. We verified our findings with volumetric computed tomography.

Results

One hundred and thirty-five nitrogen-wash-in/wash-out measurements were taken at three levels of F_IO₂ (0.4, 0.5, 0.6). The CT volumetry reproduced the set end-expiratory volumes at the Simulator with a bias of 4 ml. The nitrogen-wash-in/wash-out method had a bias of 603 ml with acceptable limits of agreement (95% CI 252 to − 953 ml). Changes were detected with a concordance rate of 97%.

Conclusions

We conclude that the TestChest® simulator is an accurate simulation tool, concerning the simulation of lung volumes. The nitrogen wash-in/wash out method correlated positively with FRC changes, despite a relatively large bias in absolute measurements. The reference volumes in the lung simulator verified with CT volumetry were very close to their expected values. The reason for the bias could not be determined.

Validating the inspired sinewave technique to measure the volume of the 'baby lung' in a porcine lung-injury model

BACKGROUND

Bedside lung volume measurement could personalise ventilation and reduce driving pressure in patients with acute respiratory distress syndrome (ARDS). We investigated a modified gas-dilution method, the inspired sinewave technique (IST), to measure the effective lung volume (ELV) in pigs with uninjured lungs and in an ARDS model.

METHODS

Anaesthetised mechanically ventilated pigs were studied before and after surfactant depletion by saline lavage. Changes in PEEP were used to change ELV. Paired measurements of absolute ELV were taken with IST (ELV_IST) and compared with gold-standard measures (sulphur hexafluoride wash in/washout [ELV_SF6] and computed tomography (CT) [ELV_CT]). Measured volumes were used to calculate changes in ELV (ΔELV) between PEEP levels for each method (ΔELV_IST, ΔELV_SF6, and ΔELV_CT).

RESULTS

The coefficient of variation was <5% for repeated ELV_IST measurements (n=13 pigs). There was a strong linear relationship between ELV_IST and ELV_SF6 in uninjured lungs (r²=0.97), and with both ELV_SF6 and ELV_CT in the ARDS model (r²=0.87 and 0.92, respectively). ELV_IST had a mean bias of -12 to 13% (95% limits=±17 - 25%) compared with ELV_SF6 and ELV_CT. ΔELV_IST was concordant with ΔELV_SF6 and ΔELV_CT in 98-100% of measurements, and had a mean bias of -73 to -77 ml (95% limits=±128 - 186 ml) compared with ΔELV_SF6 and -1 ml (95% limits ±333 ml) compared with ΔELV_CT.

CONCLUSIONS

IST provides a repeatable measure of absolute ELV and shows minimal bias when tracking PEEP-induced changes in lung volume compared with CT in a saline-lavage model of ARDS.

Respiratory Mechanics and Diaphragmatic Dysfunction in COPD Patients Who Failed Non-Invasive Mechanical Ventilation

Background

Although non-invasive mechanical ventilation (NIV) is the gold standard treatment for patients with acute exacerbation of COPD (AECOPD) developing respiratory acidosis, failure rates still range from 5% to 40%. Recent studies have shown that the onset of severe diaphragmatic dysfunction (DD) during AECOPD increases risk of NIV failure and mortality in this subset of patients. Although the imbalance between the load and the contractile capacity of inspiratory muscles seems the main cause of AECOPD-induced hypercapnic respiratory failure, data regarding the influence of mechanical derangement on DD in this acute phase are lacking. With this study, we investigate the impact of respiratory mechanics on diaphragm function in AECOPD patients experiencing NIV failure.

Methods

Twelve AECOPD patients with respiratory acidosis admitted to the Respiratory ICU of the University Hospital of Modena from 2017 to 2018 undergoing mechanical ventilation (MV) due to NIV failure were enrolled. Static respiratory mechanics and end-expiratory lung volume (EELV) were measured after 30 mins of volume control mode MV. Subsequently, transdiaphragmatic pressure (Pdi) was calculated by means of a sniff maneuver (Pdisniff) after 30 mins of spontaneous breathing trial. Linear regression analysis and Pearson's correlation coefficient served to assess associations.

Results

Average Pdisniff was 23.3 cmH2O (standard deviation 29 cmH2O) with 3 patients presenting bilateral diaphragm palsy. Pdisniff was directly correlated with static lung elastance (r=0.69, p=0.001) while inverse correlation was found with dynamic intrinsic PEEP (r=-0.73, p=0.007). No significant correlation was found with static intrinsic PEEP (r=-0.55, p=0.06), EELV (r=-0.4, p=0.3), airway resistance (r=-0.2, p=0.54), chest wall, and total elastance (r=-0-01, p=0.96 and r=0.3, p=0.36, respectively). Significant linear inverse correlation was found between Pdisniff and the ratio between Pdi assessed at tidal volume and Pdi sniff (r=-0.82, p=0.02).

Conclusion

The causes of extreme DD in AECOPD patients who experienced NIV failure might be predominantly mechanical, driven by a severe dynamic hyperinflation that overlaps on an elastic lung substrate favoring volume overload.

Respiratory mechanics to understand ARDS and guide mechanical ventilation

OBJECTIVE

As precision medicine is becoming a standard of care in selecting tailored rather than average treatments, physiological measurements might represent the first step in applying personalized therapy in the intensive care unit (ICU). A systematic assessment of respiratory mechanics in patients with the acute respiratory distress syndrome (ARDS) could represent a step in this direction, for two main reasons. Approach and Main results: On the one hand, respiratory mechanics are a powerful physiological method to understand the severity of this syndrome in each single patient. Decreased respiratory system compliance, for example, is associated with low end expiratory lung volume and more severe lung injury. On the other hand, respiratory mechanics might guide protective mechanical ventilation settings. Improved gravitationally dependent regional lung compliance could support the selection of positive end-expiratory pressure and maximize alveolar recruitment. Moreover, the association between driving airway pressure and mortality in ARDS patients potentially underlines the importance of sizing tidal volume on respiratory system compliance rather than on predicted body weight.

SIGNIFICANCE

The present review article aims to describe the main alterations of respiratory mechanics in ARDS as a potent bedside tool to understand severity and guide mechanical ventilation settings, thus representing a readily available clinical resource for ICU physicians.

Controlled invasive mechanical ventilation strategies in obese patients undergoing surgery

INTRODUCTION

The obesity prevalence is increasing in surgical population. As the number of obese surgical patients increases, so does the demand for mechanical ventilation. Nevertheless, ventilatory strategies in this population are challenging, since obesity results in pathophysiological changes in respiratory function. Areas covered: We reviewed the impact of obesity on respiratory system and the effects of controlled invasive mechanical ventilation strategies in obese patients undergoing surgery. To date, there is no consensus regarding the optimal invasive mechanical ventilation strategy for obese surgical patients, and no evidence that possible intraoperative beneficial effects on oxygenation and mechanics translate into better postoperative pulmonary function or improved outcomes. Expert commentary: Before determining the ideal intraoperative ventilation strategy, it is important to analyze the pathophysiology and comorbidities of each obese patient. Protective ventilation with low tidal volume, driving pressure, energy, and mechanical power should be employed during surgery; however, further studies are required to clarify the most effective ventilation strategies, such as the optimal positive end-expiratory pressure and whether recruitment maneuvers minimize lung injury. In this context, an ongoing trial of intraoperative ventilation in obese patients (PROBESE) should help determine the mechanical ventilation strategy that best improves clinical outcome in patients with body mass index≥35kg/m².

Monitoring respiration: what the clinician needs to know

A recent large prospective cohort study showed an unexpectedly high in-hospital mortality after major non-cardiac surgery in Europe, as well as a high incidence of postoperative pulmonary complications. The direct effect of postoperative respiratory complications on mortality is still under investigation, for intensive care unit (ICU) and in the perioperative period. Although respiratory monitoring has not been actually proven to affect in-hospital mortality, it plays an important role in patient care, leading to appropriate setting of ventilatory support as well as risk stratification. The aim of this article is to provide an overview of various respiratory monitoring techniques including the role of conventional and most recent methods in the perioperative period and in critically ill patients. The most recent techniques proposed for bedside respiratory monitoring, including lung imaging, are presented and discussed, comparing them to those actually considered as gold standards.

Drainage of pleural effusion in mechanically ventilated patients: time to measure chest wall compliance?

PURPOSE

Pleural effusion (PE) is commonly encountered in mechanically ventilated, critically ill patients and is generally addressed with evacuation or by fluid displacement using increased airway pressure (P(AW)). However, except when massive or infected, clear evidence is lacking to guide its management. The aim of this study was to investigate the effect of recruitment maneuvers and drainage of unilateral PE on respiratory mechanics, gas exchange, and lung volume.

MATERIALS AND METHODS

Fifteen critically ill and mechanically ventilated patients with unilateral PE were enrolled. A 3-step protocol (baseline, recruitment, and effusion drainage) was applied to patients with more than 400 mL of PE, as estimated by chest ultrasound. Predefined subgroup analysis compared patients with normal vs reduced chest wall compliance (C(CW)). Esophageal and P(AW)s, respiratory system, lung and C(CW)s, arterial blood gases, and end-expiratory lung volumes were recorded.

RESULTS

In the whole case mix, neither recruitment nor drainage improved gas exchange, lung volume, or tidal mechanics. When C(CW) was normal, recruitment improved lung compliance (81.9 [64.8-104.1] vs 103.7 [91.5-111.7] mL/cm H2O, P < .05), whereas drainage had no significant effect on total respiratory system mechanics or gas exchange, although it measurably increased lung volume (1717 vs 2150 mL, P < .05). In the setting of reduced C(CW), however, recruitment had no significant effect on total respiratory system mechanics or gas exchange, whereas pleural drainage improved respiratory system and C(CW)s as well as lung volume (42.7 [38.9-50.0] vs 47.0 [43.8-63.3], P < .05 and 97.4 [89.3-97.9] vs 126.7 [92.3-153.8] mL/cm H2O, P < .05 and 1580 vs 1750 mL, P < .05, respectively).

CONCLUSIONS

Drainage of a moderate-sized effusion should not be routinely performed in unselected population of critically ill patients. We suggest that measurement of C(CW) may help in the decision-making process.

Assessment of ventilation inhomogeneity during mechanical ventilation using a rapid-response oxygen sensor-based oxygen washout method

Purpose

Ventilatory inhomogeneity indexes in critically ill mechanically ventilated patients could be of importance to optimize ventilator settings in order to reduce additional lung injury. The present study compared six inhomogeneity indexes calculated from the oxygen washout curves provided by the rapid oxygen sensor of the LUFU end-expiratory lung volume measurement system.

Methods

Inhomogeneity was tested in a porcine model before and after induction of acute lung injury (ALI) at four different levels of positive end-expiratory pressure (PEEP; 15, 10, 5 and 0 cm H₂O). The following indexes were assessed: lung clearance index (LCI), mixing ratio, Becklake index, multiple breath alveolar mixing inefficiency, moment ratio and pulmonary clearance delay.

Results

LCI, mixing ratio, Becklake index and moment ratio were comparable with previous reported values and showed acceptable variation coefficients at baseline with and without ALI. Moment ratio had the highest precision, as calculated by the variation coefficients. LCI, Becklake index and moment ratio showed comparable increases in inhomogeneity during decremental PEEP steps before and after ALI.

Conclusions

The advantage of the method we introduce is the combined measurement of end-expiratory lung volume (EELV) and inhomogeneity of lung ventilation with the LUFU fast-response medical-grade oxygen sensor, without the need for external tracer gases. This can be combined with conventional breathing systems. The moment ratio and LCI index appeared to be the most favourable for integration with oxygen washout curves as judged by high precision and agreement with previous reported findings. Studies are under way to evaluate the indexes in critically ill patients.

Clinical review: Respiratory monitoring in the ICU - a consensus of 16

Monitoring plays an important role in the current management of patients with acute respiratory failure but sometimes lacks definition regarding which 'signals' and 'derived variables' should be prioritized as well as specifics related to timing (continuous versus intermittent) and modality (static versus dynamic). Many new techniques of respiratory monitoring have been made available for clinical use recently, but their place is not always well defined. Appropriate use of available monitoring techniques and correct interpretation of the data provided can help improve our understanding of the disease processes involved and the effects of clinical interventions. In this consensus paper, we provide an overview of the important parameters that can and should be monitored in the critically ill patient with respiratory failure and discuss how the data provided can impact on clinical management.

Accuracy and precision of end-expiratory lung-volume measurements by automated nitrogen washout/washin technique in patients with acute respiratory distress syndrome

Introduction

End-expiratory lung volume (EELV) is decreased in acute respiratory distress syndrome (ARDS), and bedside EELV measurement may help to set positive end-expiratory pressure (PEEP). Nitrogen washout/washin for EELV measurement is available at the bedside, but assessments of accuracy and precision in real-life conditions are scant. Our purpose was to (a) assess EELV measurement precision in ARDS patients at two PEEP levels (three pairs of measurements), and (b) compare the changes (Δ) induced by PEEP for total EELV with the PEEP-induced changes in lung volume above functional residual capacity measured with passive spirometry (ΔPEEP-volume). The minimal predicted increase in lung volume was calculated from compliance at low PEEP and ΔPEEP to ensure the validity of lung-volume changes.

Methods

Thirty-four patients with ARDS were prospectively included in five university-hospital intensive care units. ΔEELV and ΔPEEP volumes were compared between 6 and 15 cm H₂O of PEEP.

Results

After exclusion of three patients, variability of the nitrogen technique was less than 4%, and the largest difference between measurements was 81 ± 64 ml. ΔEELV and ΔPEEP-volume were only weakly correlated (r² = 0.47); 95% confidence interval limits, -414 to 608 ml). In four patients with the highest PEEP (≥ 16 cm H₂O), ΔEELV was lower than the minimal predicted increase in lung volume, suggesting flawed measurements, possibly due to leaks. Excluding those from the analysis markedly strengthened the correlation between ΔEELV and ΔPEEP volume (r² = 0.80).

Conclusions

In most patients, the EELV technique has good reproducibility and accuracy, even at high PEEP. At high pressures, its accuracy may be limited in case of leaks. The minimal predicted increase in lung volume may help to check for accuracy.

Evaluation of a CO2 partial rebreathing functional residual capacity measurement method for use during mechanical ventilation

OBJECTIVE

There is a need for an automated bedside functional residual capacity (FRC) measurement method that does not require a step change in inspired oxygen fraction. Such a method can be used for patients who require a high inspired oxygen fraction to maintain arterial oxygenation and for patients ventilated using a circle breathing system commonly found in operating rooms, which is not capable of step changes in oxygen. We developed a CO(2) rebreathing method for FRC measurement that is based on the change in partial pressure of end-tidal carbon dioxide and volume of CO(2) eliminated at the end of a partial rebreathing period. This study was designed to assess the accuracy and precision of the proposed FRC measurement system compared to body plethysmography and nitrogen washout FRC.

METHODS

Accuracy and precision of measurements were assessed by comparing the CO(2) rebreathing FRC values to the gold standard, body plethysmography FRC, in twenty spontaneously breathing volunteers. The CO(2) rebreathing FRC measurements were then compared to nitrogen washout FRC in twenty intensive care patients whose lungs were mechanically ventilated. For each subject, an average value of CO(2) rebreathing FRC was compared to the average gold standard method. Measurements were accepted for statistical analysis if they had been recorded from periods of stable tidal ventilation, defined as a coefficient of variation of tidal volume of <0.13.

RESULTS

Compared to body plethysmography, the accuracy (average error) for the CO(2) rebreathing method during stable ventilation (n = 8) was 0.03 L and precision (1 standard deviation of the error) was 0.29 L (0.8 ± 7.6% of body plethysmography). During stable mechanical ventilation (n = 9), the accuracy was -0.02 L and precision was 0.26 L (-1.1 ± 12.6% of nitrogen washout).

CONCLUSIONS

The CO(2) rebreathing method for FRC measurement provides acceptable accuracy and precision during stable ventilation compared to the gold standards of body plethysmography and nitrogen washout. The results based on periods of stable ventilation best approximate the performance of the system in the likely areas of application during controlled mechanical ventilation. Further study of the CO(2) rebreathing method is needed to evaluate accuracy in a larger group of controlled mechanical ventilation patients, including patients with respiratory insufficiency and significant lung injury.

PEEP-induced changes in lung volume in acute respiratory distress syndrome. Two methods to estimate alveolar recruitment

PURPOSE

Lung volumes, especially functional residual capacity (FRC), are decreased in acute respiratory distress syndrome (ARDS). Positive end-expiratory pressure (PEEP) contributes to increased end-expiratory lung volume (EELV) and to improved oxygenation, but differentiating recruitment of previously nonaerated lung units from distension of previously open lung units remains difficult. This study evaluated simple methods derived from bedside EELV measurements to assess PEEP-induced lung recruitment while monitoring strain.

METHODS

Prospective multicenter study in 30 mechanically ventilated patients with ARDS in five university hospital ICUs. Two PEEP levels were studied, each for 45 min, and EELV (nitrogen washout/washin technique) was measured at both levels, with the difference (Δ) reflecting PEEP-induced lung volume changes. Alveolar recruitment was measured using pressure-volume (PV) curves. High and low recruiters were separated based on median recruitment at high PEEP. Minimum predicted increase in lung volume computed as the product of ΔPEEP by static compliance was subtracted from ΔEELV as an independent estimate of recruitment. Estimated and measured recruitments were compared. Strain induced by PEEP was also calculated from the same measurements.

RESULTS

FRC was 31 ± 11% of predicted. Median [25th-75th percentiles] PEEP-induced recruitment was 272 [187-355] mL. Estimated recruitment correlated with recruited volume measured on PV curves (ρ = 0.68), with a slope close to identity. The ΔEELV/FRC ratio differentiated high from low recruiters (110 [76-135] vs. 55 [23-70]%, p = 0.001). Strain increase due to PEEP was larger in high recruiters (p = 0.002).

CONCLUSION

PEEP-induced recruitment and strain can be assessed at the bedside using EELV measurement. We describe two bedside methods for predicting low or high alveolar recruitment during ARDS.

Measurement of functional residual capacity by modified multiple breath nitrogen washout for spontaneously breathing and mechanically ventilated patients

BACKGROUND

There is a need for a bedside functional residual capacity (FRC) measurement method that performs well in intensive care patients during many modes of ventilation including controlled, assisted, spontaneous, and mixed. We developed a modified multiple breath nitrogen washout method for FRC measurement that relies on end-tidal gas fractions and alveolar tidal volume measurements as inputs but does not require the traditional measurements of volume of nitrogen or oxygen. Using end-tidal measurements, not volume, reduces errors from signal synchronization. This study was designed to assess the accuracy, precision, and repeatability of the proposed FRC system in subjects with variable ventilation patterns including some spontaneous effort.

METHODS

The accuracy and precision of measurements were assessed by comparing the novel N₂ washout FRC values to the gold standard, body plethysmography, in 20 spontaneously breathing volunteers. Repeatability was assessed by comparing subsequent measurements in 20 intensive care patients whose lungs were under controlled and assisted mechanical ventilation.

RESULTS

Compared with body plethysmography, the accuracy (mean bias) of the novel method was -0.004 litre and precision [1 standard deviation (sd)] was 0.209 litre [mean (sd)] [-0.1 (5.9)% of body plethysmography]. The difference between repeated measurements was 0.009 (0.15) litre [mean (sd)] [0.4 (6.4)%]. The coefficient of repeatability was 0.31 litre (12.7%).

CONCLUSIONS

The modified multiple breath nitrogen washout method for FRC measurement provides improved precision and equivalent accuracy and repeatability compared with existing methods during ventilation with variable ventilation patterns. Further study of the novel N₂ washout method is needed.

Respiratory impact of paracentesis in cirrhotic patients with acute lung injury

INTRODUCTION

Ascites may contribute to the loss of lung volume and alter the gas exchange in cirrhotic patients with acute lung injury (ALI).

AIM

The aim of the study was to evaluate the effects of paracentesis on respiratory parameters in ventilated cirrhotic patients with ALI.

STUDY DESIGN

This was a prospective trial in an intensive care unit of a university hospital.

PATIENTS AND METHODS

Thirty-one cirrhotic patients on mechanical ventilation (with ALI) requiring paracentesis were included in this study. Arterial blood gases, intraabdominal pressures, ventilator parameters, and lung volumes were measured before and after the ascitic drainage.

RESULTS

Following paracentesis, the intraabdominal pressure decreased (24.1 ± 7.0 vs 12.3 ± 8.9 mm Hg, P < .0001) and the Pao(2)/Fio(2) improved significantly (190.0 ± 65.2 vs 284.9 ± 76.1 mm Hg, P < .0001), without hemodynamic disturbances. End-expiratory lung volume, markedly reduced before drainage, increased significantly following paracentesis (Δ end-expiratory lung volume: +463 ± 249 mL, P = .0009). No adverse effects related to the paracentesis were encountered.

CONCLUSION

In contrast to ventilatory recruitment maneuvers, paracentesis is a simple and well-tolerated technique able to improve oxygenation and alveolar recruitment without the risk of the lung overdistension in severely hypoxemic cirrhotic patients.

Acute severe asthma: new approaches to assessment and treatment

The precise definition of a severe asthmatic exacerbation is an issue that presents difficulties. The term 'status asthmaticus' relates severity to outcome and has been used to define a severe asthmatic exacerbation that does not respond to and/or perilously delays the repetitive or continuous administration of short-acting inhaled beta(2)-adrenergic receptor agonists (SABA) in the emergency setting. However, a number of limitations exist concerning the quantification of unresponsiveness. Therefore, the term 'acute severe asthma' is widely used, relating severity mostly to a combination of the presenting signs and symptoms and the severity of the cardiorespiratory abnormalities observed, although it is well known that presentation does not foretell outcome. In an acute severe asthma episode, close observation plus aggressive administration of bronchodilators (SABAs plus ipratropium bromide via a nebulizer driven by oxygen) and oral or intravenous corticosteroids are necessary to arrest the progression to severe hypercapnic respiratory failure leading to a decrease in consciousness that requires intensive care unit (ICU) admission and, eventually, ventilatory support. Adjunctive therapies (intravenous magnesium sulfate and/or others) should be considered in order to avoid intubation. Management after admission to the hospital ward because of an incomplete response is similar. The decision to intubate is essentially based on clinical judgement. Although cardiac or respiratory arrest represents an absolute indication for intubation, the usual picture is that of a conscious patient struggling to breathe. Factors associated with the increased likelihood of intubation include exhaustion and fatigue despite maximal therapy, deteriorating mental status, refractory hypoxaemia, increasing hypercapnia, haemodynamic instability and impending coma or apnoea. To intubate, sedation is indicated in order to improve comfort, safety and patient-ventilator synchrony, while at the same time decrease oxygen consumption and carbon dioxide production. Benzodiazepines can be safely used for sedation of the asthmatic patient, but time to awakening after discontinuation is prolonged and difficult to predict. The most common alternative is propofol, which is attractive in patients with sudden-onset (near-fatal) asthma who may be eligible for extubation within a few hours, because it can be titrated rapidly to a deep sedation level and has rapid reversal after discontinuation; in addition, it possesses bronchodilatory properties. The addition of an opioid (fentanyl or remifentanil) administered by continuous infusion to benzodiazepines or propofol is often desirable in order to provide amnesia, sedation, analgesia and respiratory drive suppression. Acute severe asthma is characterized by severe pulmonary hyperinflation due to marked limitation of the expiratory flow. Therefore, the main objective of the initial ventilator management is 2-fold: to ensure adequate gas exchange and to prevent further hyperinflation and ventilator-associated lung injury. This may require hypoventilation of the patient and higher arterial carbon dioxide (PaCO(2)) levels and a more acidic pH. This does not apply to asthmatic patients intubated for cardiac or respiratory arrest. In this setting the post-anoxic brain oedema might demand more careful management of PaCO(2) levels to prevent further elevation of intracranial pressure and subsequent complications. Monitoring lung mechanics is of paramount importance for the safe ventilation of patients with status asthmaticus. The first line of specific pharmacological therapy in ventilated asthmatic patients remains bronchodilation with a SABA, typically salbutamol (albuterol). Administration techniques include nebulizers or metered-dose inhalers with spacers. Systemic corticosteroids are critical components of therapy and should be administered to all ventilated patients, although the dose of systemic corticosteroids in mechanically ventilated asthmatic patients remains controversial. Anticholinergics, inhaled corticosteroids, leukotriene receptor antagonists and methylxanthines offer little benefit, and clinical data favouring their use are lacking. In conclusion, expertise, perseverance, judicious decisions and practice of evidence-based medicine are of paramount importance for successful outcomes for patients with acute severe asthma.

Measurements of functional residual capacity during intensive care treatment: the technical aspects and its possible clinical applications

Direct measurement of lung volume, i.e. functional residual capacity (FRC) has been recommended for monitoring during mechanical ventilation. Mostly due to technical reasons, FRC measurements have not become a routine monitoring tool, but promising techniques have been presented. We performed a literature search of studies with the key words 'functional residual capacity' or 'end expiratory lung volume' and summarize the physiology and patho-physiology of FRC measurements in ventilated patients, describe the existing techniques for bedside measurement, and provide an overview of the clinical questions that can be addressed using an FRC assessment. The wash-in or wash-out of a tracer gas in a multiple breath maneuver seems to be best applicable at bedside, and promising techniques for nitrogen or oxygen wash-in/wash-out with reasonable accuracy and repeatability have been presented. Studies in ventilated patients demonstrate that FRC can easily be measured at bedside during various clinical settings, including positive end-expiratory pressure optimization, endotracheal suctioning, prone position, and the weaning from mechanical ventilation. Alveolar derecruitment can easily be monitored and improvements of FRC without changes of the ventilatory setting could indicate alveolar recruitment. FRC seems to be insensitive to over-inflation of already inflated alveoli. Growing evidence suggests that FRC measurements, in combination with other parameters such as arterial oxygenation and respiratory compliance, could provide important information on the pulmonary situation in critically ill patients. Further studies are needed to define the exact role of FRC in monitoring and perhaps guiding mechanical ventilation.

Nitrogen washout/washin, helium dilution and computed tomography in the assessment of end expiratory lung volume

INTRODUCTION

End expiratory lung volume (EELV) measurement in the clinical setting is routinely performed using the helium dilution technique. A ventilator that implements a simplified version of the nitrogen washout/washin technique is now available. We compared the EELV measured by spiral computed tomography (CT) taken as gold standard with the lung volume measured with the modified nitrogen washout/washin and with the helium dilution technique.

METHODS

Patients admitted to the general intensive care unit of Ospedale Maggiore Policlinico Mangiagalli Regina Elena requiring ventilatory support and, for clinical reasons, thoracic CT scanning were enrolled in this study. We performed two EELV measurements with the modified nitrogen washout/washin technique (increasing and decreasing inspired oxygen fraction (FiO2) by 10%), one EELV measurement with the helium dilution technique and a CT scan. All measurements were taken at 5 cmH2O airway pressure. Each CT scan slice was manually delineated and gas volume was computed with custom-made software.

RESULTS

Thirty patients were enrolled (age = 66 +/- 10 years, body mass index = 26 +/- 18 Kg/m2, male/female ratio = 21/9, partial arterial pressure of carbon dioxide (PaO2)/FiO2 = 190 +/- 71). The EELV measured with the modified nitrogen washout/washin technique showed a very good correlation (r2 = 0.89) with the data computed from the CT with a bias of 94 +/- 143 ml (15 +/- 18%, p = 0.001), within the limits of accuracy declared by the manufacturer (20%). The bias was shown to be highly reproducible, either decreasing or increasing the FiO2 being 117+/-170 and 70+/-160 ml (p = 0.27), respectively. The EELV measured with the helium dilution method showed a good correlation with the CT scan data (r2 = 0.91) with a negative bias of 136 +/- 133 ml, and appeared to be more correct at low lung volumes.

CONCLUSIONS

The EELV measurement with the helium dilution technique (at low volumes) and modified nitrogen washout/washin technique (at all lung volumes) correlates well with CT scanning and may be easily used in clinical practice.

TRIAL REGISTRATION

Current Controlled Trials NCT00405002.