Static pressure–volume curves of the respiratory system: were they just a passing fad?

Setting positive end-expiratory pressure: using the pressure-volume curve

PURPOSE OF REVIEW

To discuss the role of pressure-volume curve (PV curve) in exploring elastic properties of the respiratory system and setting mechanical ventilator to reduce ventilator-induced lung injury.

RECENT FINDINGS

Nowadays, quasi-static PV curves and loops can be easily obtained and analyzed at the bedside without disconnection of the patient from the ventilator. It is shown that this tool can provide useful information to optimize ventilator setting. For example, PV curves can assess for patient's individual potential for lung recruitability and also evaluate the risk for lung injury of the ongoing mechanical ventilation setting.

SUMMARY

In conclusion, PV curve is an easily available bedside tool: its correct interpretation can be extremely valuable to enlighten potential for lung recruitability and select a high or low positive end-expiratory pressure (PEEP) strategy. Furthermore, recent studies have shown that PV curve can play a significant role in PEEP and driving pressure fine tuning: clinical studies are needed to prove whether this technique will improve outcome.

Study of Tidal Volume and Positive End-Expiratory Pressure on Alveolar Recruitment Using Spiro Dynamics in Mechanically Ventilated Patients

Background and Aims

Ventilator setting in the intensive care unit patients is a topic of debate and setting of tidal volume (TV) should be patient-specific based on lung mechanics. In this study, we have evaluated to develop optimal ventilator strategies through continuous and thorough monitoring of respiratory mechanics during ongoing ventilator support to prevent alveolar collapse and alveolar injury in mechanically ventilated patients.

Methods

In our monocentric, randomized, observational study, we had recruited 60 patients and divided them into two groups of 30 each. In Group 1 patients, TV and positive end-expiratory pressure (PEEP) were set according to pressure-volume (P/V) curve obtained by the mechanical ventilator in a conventional manner (control group), and in Group 2, TV and PEEP were set according to P/V curve obtained by the mechanical ventilator using intratracheal catheter. PEEP and TV were set accordingly. TV, PEEP, and PaO₂/FiO₂ (P/F) ratio at days 1, 3, and 7, mortality within 7 days and mortality within 28 days were measured in each group and compared.

Results

We found a significant difference between PEEP and P/F ratio in both groups while intragroup comparison at days 1, 3, and 7. After the intergroup comparison of Group 1 and 2, we observed a significant difference of PEEP and P/F ratio between the groups at day 7 and not on day 1 or 3.

Conclusion

This study concludes that optimal PEEP is more accurate using an intratracheal catheter than the conventional method of deciding ventilator setting. Hence, it is recommended to use intratracheal catheter to obtain more accurate ventilator settings.

[Lung recruitment maneuvers and positive end-expiratory pressure titration in children with acute respiratory distress syndrome]

The application of small tidal volume and the limitation of airway pressure during mechanical ventilation in acute respiratory distress syndrome (ARDS) are well accepted. Lung recruitment and positive end-expiratory pressure (PEEP) titration can improve oxygenation and protect the lungs. However, the approaches of lung recruitment and PEEP titration remain controversial. This article reviews the lung recruitment maneuvers and PEEP titration in children with ARDS.

Assessing Respiratory System Mechanical Function

The main goals of assessing respiratory system mechanical function are to evaluate the lung function through a variety of methods and to detect early signs of abnormalities that could affect the patient's outcomes. In ventilated patients, it has become increasingly important to recognize whether respiratory function has improved or deteriorated, whether the ventilator settings match the patient's demand, and whether the selection of ventilator parameters follows a lung-protective strategy. Ventilator graphics, esophageal pressure, intra-abdominal pressure, and electric impedance tomography are some of the best-known monitoring tools to obtain measurements and adequately evaluate the respiratory system mechanical function.

Impact of Recruitment on Static and Dynamic Lung Strain in Acute Respiratory Distress Syndrome

BACKGROUND

Lung strain, defined as the ratio between end-inspiratory volume and functional residual capacity, is a marker of the mechanical load during ventilation. However, changes in lung volumes in response to pressures may occur in injured lungs and modify strain values. The objective of this study was to clarify the role of recruitment in strain measurements.

METHODS

Six oleic acid-injured pigs were ventilated at positive end-expiratory pressure (PEEP) 0 and 10 cm H2O before and after a recruitment maneuver (PEEP = 20 cm H2O). Lung volumes were measured by helium dilution and inductance plethysmography. In addition, six patients with moderate-to-severe acute respiratory distress syndrome were ventilated with three strategies (peak inspiratory pressure/PEEP: 20/8, 32/8, and 32/20 cm H2O). Lung volumes were measured in computed tomography slices acquired at end-expiration and end-inspiration. From both series, recruited volume and lung strain (total, dynamic, and static) were computed.

RESULTS

In the animal model, recruitment caused a significant decrease in dynamic strain (from [mean ± SD] 0.4 ± 0.12 to 0.25 ± 0.07, P < 0.01), while increasing the static component. In patients, total strain remained constant for the three ventilatory settings (0.35 ± 0.1, 0.37 ± 0.11, and 0.32 ± 0.1, respectively). Increases in tidal volume had no significant effects. Increasing PEEP constantly decreased dynamic strain (0.35 ± 0.1, 0.32 ± 0.1, and 0.04+0.03, P < 0.05) and increased static strain (0, 0.06 ± 0.06, and 0.28 ± 0.11, P < 0.05). The changes in dynamic and total strain among patients were correlated to the amount of recruited volume. An analysis restricted to the changes in normally aerated lung yielded similar results.

CONCLUSION

Recruitment causes a shift from dynamic to static strain in early acute respiratory distress syndrome.

[Hypoxic lung failure]

Hypoxic lung failure is among the major indications for patients' referral to intensive care units either for surveillance or if necessary therapy. There are a vast number of pathophysiological causes of lung failure and the optimal treatment highly depends on the underlying pathology; therefore, no standard algorithm exists. So-called acute respiratory distress syndrome (ARDS) represents a very severe manifestation of hypoxemic lung failure that is of particular relevance for intensivists and is therefore the focus of this review. In addition to fundamental pathophysiology of lung injury, the article also focuses on established and modern treatment strategies. Moreover, we will briefly highlight innovative concepts of ARDS treatment that might become relevant in the future.

[Monitorization of respiratory mechanics in the ventilated patient]

Monitoring during mechanical ventilation allows the measurement of different parameters of respiratory mechanics. Accurate interpretation of these data can be useful for characterizing the situation of the different components of the respiratory system, and for guiding ventilator settings. In this review, we describe the basic concepts of respiratory mechanics, their interpretation, and their potential use in fine-tuning mechanical ventilation.

[Ventilation in acute respiratory distress. Lung-protective strategies]

Ventilation of patients suffering from acute respiratory distress syndrome (ARDS) with protective ventilator settings is the standard in patient care. Besides the reduction of tidal volumes, the adjustment of a case-related positive end-expiratory pressure and preservation of spontaneous breathing activity at least 48 h after onset is part of this strategy. Bedside techniques have been developed to adapt ventilatory settings to the individual patient and the different stages of ARDS. This article reviews the pathophysiology of ARDS and ventilator-induced lung injury and presents current evidence-based strategies for ventilator settings in ARDS.

[Positive end-expiratory pressure : adjustment in acute lung injury]

Treatment of patients suffering from acute lung injury is a challenge for the treating physician. In recent years ventilation of patients with acute hypoxic lung injury has changed fundamentally. Besides the use of low tidal volumes, the most beneficial setting of positive end-expiratory pressure (PEEP) has been in the focus of researchers. The findings allow adaption of treatment to milder forms of acute lung injury and severe forms. Additionally computed tomography techniques to assess the pulmonary situation and recruitment potential as well as bed-side techniques to adjust PEEP on the ward have been modified and improved. This review gives an outline of recent developments in PEEP adjustment for patients suffering from acute hypoxic and hypercapnic lung injury and explains the fundamental pathophysiology necessary as a basis for correct treatment.

[Interpretation of ventilator curves in patients with acute respiratory failure]

Mechanical ventilation is a therapeutic intervention involving the temporary replacement of ventilatory function with the purpose of improving symptoms in patients with acute respiratory failure. Technological advances have facilitated the development of sophisticated ventilators for viewing and recording the respiratory waveforms, which are a valuable source of information for the clinician. The correct interpretation of these curves is crucial for the correct diagnosis and early detection of anomalies, and for understanding physiological aspects related to mechanical ventilation and patient-ventilator interaction. The present study offers a guide for the interpretation of the airway pressure and flow and volume curves of the ventilator, through the analysis of different clinical scenarios.

Novel technologies to detect atelectotrauma in the injured lung

Cyclical recruitment and derecruitment of lung parenchyma (R/D) remains a serious problem in ALI/ARDS patients, defined as atelectotrauma. Detection of cyclical R/D to titrate the optimal respiratory settings is of high clinical importance. Image-based technologies that are capable of detecting changes of lung ventilation within a respiratory cycle include dynamic computed tomography (dCT), synchrotron radiation computed tomography (SRCT), and electrical impedance tomography (EIT). Time-dependent intra-arterial oxygen tension monitoring represents an alternative approach to detect cyclical R/D, as cyclical R/D can result in oscillations of PaO₂ within a respiratory cycle. Continuous, ultrafast, on-line in vivo measurement of PaO₂ can be provided by an indwelling PaO₂ probe. In addition, monitoring of fast changes in SaO₂ by pulse oximetry technology at the bedside could also be used to detect those fast changes in oxygenation.

Automated measurement of the lower inflection point in a pediatric lung model

OBJECTIVE

To determine which flow setting most accurately detects the lower inflection point (Pflex) using an automated constant flow method and varying endotracheal tube (ETT) sizes with and without an airleak in a pediatric lung model.

DESIGN

Interventional laboratory study.

SETTING

Children's hospital research center.

INTERVENTIONS

A pediatric lung model was created with Pflexs of the inspiratory pressure-volume (P-V) curve set at 5 and 10 cm H2O using the ASL 5000 Test Lung (IngMar Medical, Pittsburgh, PA). Three ETT sizes (3.0, 4.0, 5.0 mm) were tested with and without a 25% airleak. P-V curves were obtained using an automated constant flow method at ten different flow rates.

MEASUREMENTS AND MAIN RESULTS

Without an ETT airleak, the lowest flow of 0.5 L/min led to the most accurate determination of Pflex regardless of ETT size or set Pflex (p < 0.001). When a 25% leak was introduced, accuracy of measured Pflex depended on both ETT size (p < 0.001) and flow rate (p < 0.001). Optimum flow rates for Pflex determination were 0.5, 1.0, and 1.5 L/min at Pflex of 5 cm H2O, and 2.0, 3.5, and 4.5 L/min at 10 cm H2O for 3.0, 4.0, and 5.0 mm ETTs, respectively (p < 0.001).

CONCLUSIONS

Estimation of Pflex can be achieved using automated P-V curves with ETTs appropriate for pediatric use, with and without an airleak. ETT size and flow rate affect the accuracy of these measurements when an airleak is present, and use of increased flow rates to create the automated P-V curves can reduce error. These data support the idea that a low-flow technique provides the most accurate determination of Pflex in pediatric patients without a leak around their ETT, whereas increased flows are needed to compensate when an ETT airleak is present.