Respiratory mechanics refers to the expression of lung function through measures of pressure and flow. From these measurements, a variety of derived indices can be determined, such as volume, compliance, resistance, and work of breathing. Plateau pressure is a measure of end-inspiratory distending pressure. It has become increasingly appreciated that end-inspiratory transpulmonary pressure (stress) might be a better indicator of the potential for lung injury than plateau pressure alone. This has resulted in a resurgence of interest in the use of esophageal manometry in mechanically ventilated patients. End-expiratory transpulmonary pressure might also be useful to guide the setting of PEEP to counterbalance the collapsing effects of the chest wall. The shape of the pressure-time curve might also be useful to guide the setting of PEEP (stress index). This has focused interest in the roles of stress and strain to assess the potential for lung injury during mechanical ventilation. This paper covers both basic and advanced respiratory mechanics during mechanical ventilation.

http://rc.rcjournal.com/content/respcare/59/11/1773.full.pdf

Respiratory Mechanics in Mechanically Ventilated Patients

Objective: To investigate the breathing pattern and the inspiratory work of breathing (WOBI) in patients with chronic obstructive pulmonary disease (COPD) assisted with proportional assist ventilation (PAV) and conventional pressure support ventilation (PSV). Design: Prospective controlled study. Setting: Intensive care unit of a university hospital. Patients: Thirteen COPD patients being weaned from mechanical ventilation. Interventions: All patients were breathing PSV and two different levels of PAV. Measurements and main results: During PAV (EVITA 2 prototype, Drager, Germany), the resistance of the endotracheal tube (Ret) was completely compensated while the patients' resistive and elastic loads were compensated for by approximately 80 % and 50 % (PAV80 and PAV50), respectively. PSV was adjusted to match the same mean inspiratory pressure (Pinspmean) as during PAV80. Airway pressure, esophageal pressure and gas flow were measured over a period of 5 min during each mode. Neuromuscular drive (P0.1) was determined by inspiratory occlusions. Mean tidal volume (VT) was not significantly different between the modes. However, the coefficient of variation of VT was 10 ± 4.%, 20 ± 13 % and 15 ± 8 % during PSV, PAV80 and PAV50, respectively. Respiratory rate (RR) and minute ventilation (VE) were significantly lower during PAV80 as compared with both other modes, but the differences did not exceed 10 %. PAV80 and PSV had comparable effects on WOBI and P0.1, whereas WOBI and P0.1 increased during PAV50 compared with both other modes. Conclusion: Mean values of breathing pattern did not differ by a large amount between the investigated modes. However, the higher variability of VT during PAV indicates an increased ability of the patients to control VT in response to alterations in respiratory demand. A reduction in assist during PAV50 resulted in an increase in WOB and indices of patient effort.

/pdf/proportional-assist-versus-pressure-support-ventilation-54tm8drtvu.pdf

Proportional assist versus pressure support ventilation: effects on breathing pattern and respiratory work of patients with chronic obstructive pulmonary disease.

The aim of this article is to identify and interpret the data provided by modern ventilators that provide the greatest clinical help in evaluating respiratory mechanics during mechanical ventilation. In intensive care, respiratory mechanics can be assessed in dynamic conditions (no flow-interruption) or static conditions (occlusion techniques) to record compliance and resistance and to monitor pressure, flow, and volume. Real-time visualization of the pressure curve is crucial for monitoring during volume-controlled ventilation, in which pressure is the dependent variable. Analysis of the pressure curve has little clinical utility during pressure-controlled ventilation, in which the dependent variable is the flow waveform, which varies according to changes in the mechanics of the respiratory system. Pressure-volume loops and flow-volume loops provide useful information on the dynamic trends of the respiratory system compliance and resistance, respectively. Modern ventilators provide complete monitoring of respiratory system mechanics, which is our guideline for optimizing ventilatory support and avoiding complications associated with mechanical ventilation.

http://rc.rcjournal.com/content/respcare/50/1/55.full.pdf

Respiratory Mechanics Derived From Signals in the Ventilator Circuit

An analysis of the efficacy of bag-valve-mask ventilation and chest compression during different compression–ventilation ratios in manikin-simulated paediatric resuscitation

Background: The formulation of sulfite-containing propofol (SCP) has not been thoroughly investigated in patients with the extensive smoking history for the effects on the total respiratory system resistance after tracheal intubation. However adverse effects, including acute asthma and bronchospasm, have been reported with several other parenteral formulations of drugs containing sulfite as preservative. Therefore, the aim of this prospective randomized and double blind study was to investigate the effects of EDTA-containing propofol (ECP) and SCP on total respiratory system resistance (Rrs) in patients with the prolonged smoking history and undergoing propofol-based total intravenous anesthesia with tracheal intubation. Methods: 40 patients scheduled for general anesthesia were enrolled into the study. Anesthesia was induced with either 2 mg/kg ECP, or 2 mg/kg SCP followed by vecuronium (0.1 mg/kg) to ensure complete neuromuscular relaxation for the time of the study. Maintenance anesthesia was continued with propofol infusion at 0.15 mg/kg/min for the first 15 min after intubation. Total respiratory system resistance (Rrs), was measured continuously for 10 min postintubation. Results: The analysis of repeated Rrs measurements taken every minute for 10 min postintubation revealed trend consisting of higher Rrs in the SCP group when compared to the ECP group. The statistical analysis of the data performed using repeated measures analysis of covariance demonstrated statistically significant effect (P < 0.05) of the treatment group factor (SCP vs. ECP) and the time factor (time after intubation) on the postintubation Rrs. Conclusion: The total respiratory system resistance measured repeatedly for 10 min after tracheal intubation in patients with smoking history is significantly elevated after induction with SCP than after induction with ECP. The preservative used for propofol formulation may alter the effects of propofol on the total respiratory system resistance in smokers.

Effects of EDTA- and sulfite-containing formulations of propofol on respiratory system resistance after tracheal intubation in smokers.

Objective. A variety of methods are used to calculate indices of lung mechanics. We conducted this study to compare 6 methods of calculating airway resistance.Methods. Data were recorded from 20 adult mechanically ventilated patients. All were relaxed and breathing in synchrony with the ventilator, and an end-inspiratory pause sufficient to produce a pressure plateau (0.5–1.5 s) was used. Pressure and flow rate were measured at the proximal airway using a calibrated lung mechanics analyzer (VenTrak, Med Science, St Louis, MO). Flow rate, pressure, and volume were printed simultaneously. Airway resistance was calculated using 6 methods: Suter, Krieger, Neergard, Bergman, Comroe, and Jonson.Results. Mean calculated resistances (± SD) (cm H2O/L/s) were 11.7 ± 4.8 (Suter method), 13.3 ± 5.0 (Krieger method), 14.9 ± 5.3 (Neergard method), 25.0 ± 6.6 (Bergman method), 24.7 ± 6.4 (Comroe method), and 26.9 ± 4.8 (Jonson method). By repeated measures analysis of variance, these differences were significant (p < 0.001). Using Scheffe analysis, no difference was found between the calculations using the Bergman, Comroe, and Jonson methods; these were significantly greater than the other 3 methods (p < 0.05).Conclusions. Methods that evaluate expiratory resistance (Comroe, Bergman, and Jonson) produce higher values than methods that evaluate inspiratory resistance (Suter and Neergard) or a combination of inspiratory and expiratory resistance (Krieger). Because of these differences, investigators should clearly describe their calculations when reporting airway resistance values.

Comparison of six methods to calculate airway resistance during mechanical ventilation in adults.

Comparison of six methods to calculate airway resistance in adult mechanically ventilated patients

Pulmonary compliance of cardiac arrest victims. a reassessment

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Comparison of six methods to calculate airway resistance during mechanical ventilation in adults.

Comparison of six methods to calculate airway resistance in adult mechanically ventilated patients

Pulmonary compliance of cardiac arrest victims. a reassessment