Anesth Analg 2003;97:145-150
© 2003 International Anesthesia Research Society
TECHNOLOGY, COMPUTING, AND SIMULATION
Guidelines for Inspiratory Flow Setting When Measuring the Pressure-Volume Relationship
Fábio E. Bensenor, MD PhD,
Joaquim E. Vieira, MD PhD, and
Jose Otávio C. Auler, Jr., MD PhD
Anesthesia Department, Hospital das Clinicas, Faculdade de Medicina da Universidade de Sao Paulo, Sao Paulo, Brazil
Address correspondence to Fábio Ely Bensenor, MD, PhD, Rua Mauá, 934/936, Sao Paulo, SP 01028–000, Brazil. Address e-mail to bensenor{at}aol.com
 |
Abstract
|
|---|
Acquisition of pressure-volume (PV) curves to improve ventilation strategy is time consuming when using static methods. Low-flow techniques use less time, but compliance values can be decreased by the resistance to flow in airways and tracheal tube (P-t). In this study, we determined the impact of three flows on the resistive component of airway pressure during anesthesia. We studied 10 ASA status P1/P2 patients with normal respiratory function. Airway and esophageal pressures were measured while volume-control ventilated with 6, 12, and 30 L/min continuous flows. PV curves, lower inflection point, respiratory system, and chest wall compliances at 250, 500, 750, and 1000 mL tidal volume were established before and after removing P-t. Data were submitted to analysis of variance. The inflection point was lower for the lower flow when comparing 6 and 12 with 30 L/min (P < 0.001). No difference was found between 6 and 12 L/min. Removal of P-t showed a difference only for 30 L/min (P = 0.004). Higher flows generated lower compliances. P-t subtraction reduced compliances only for 30 L/min. Chest wall compliances showed no difference between flows. We concluded that flows ≤12 L/min minimize P-t during intraoperative PV curves acquisition. Compliances suggest 6 L/min as the most adequate flow.
IMPLICATIONS: We suggest guidelines for inspiratory flow setting when measuring the pressure-volume relationship during anesthesia based on the comparison among three different continuous flow values, aiming at better intraoperative respiratory settings in patients with normal respiratory function.
|
Introduction
|
|---|
An analysis of respiratory system mechanics has drawn attention because of its potential ability to guide ventilator settings and to avoid eventual pulmonary injuries secondary to mechanical ventilation (1,2). Nevertheless, this attention has been limited to intensive care units (ICUs). Monitoring of respiratory mechanics during anesthesia has been very unusual, even when caring for patients with known respiratory disease.
Pressure-volume (PV) curves are a useful tool for optimizing the ventilation strategy during positive pressure ventilation by helping to assess the interaction between the ventilator and the patient’s lungs. The relationship between these two variables in the respiratory system has been applied as a diagnostic, prognostic, or therapeutic tool (3–5). PV curves are classically obtained by a static process (no inspiratory flow) whereby the lungs are inflated with a series of known volumes and the resulting pressures measured. Once several measurements have been made, a PV curve can be created.
The static method for measuring the PV relationship is inconvenient in the operating room (OR). One of its main drawbacks in anesthesia is that acquisition of static PV curves is time consuming (1). Alternatively, clinical instruments for measuring PV relationships generate a dynamic PV curve by measuring pressure and flow continuously for each breath. The dynamic method is convenient, but tends to indicate a lung compliance that is inferior to the actual one. This is because the pressure measured dynamically includes both the pressure caused by lung compliance and the pressure generated by the flow of gas through the resistance of the breathing circuit and upper airways. This resistive component has to be eliminated to determine true lung compliance.
The use of a quasi-static approach, the "low flow inflation technique," has been proposed as a reliable and quick method to obtain PV curves (1,3,4,6). This technique uses a very low inspiratory flow to minimize the pressure caused by the gas flow through the resistance of the breathing circuit and lungs (Fig. 1). The goal is to reduce the inspiratory flow to the point in which the pressure generated from resistance does not contribute significantly to the total pressure. This is a practical approach for use in the OR, once it can be performed using the existing anesthesia ventilator.
The aim of this study was to evaluate the influence of inspiratory flow on PV curve measurement in the OR and to determine the appropriate flow value to use when making these measurements.
|
Methods
|
|---|
Patients
Patients requiring pulmonary segmental resection, pulmonary lobectomy, pulmonary biopsy, and mediastinal nodular resection were prospectively studied. Written informed consent was obtained preoperatively from each patient. Exclusion criteria were a diagnosis of moderate or severe obstructive or any degree of restrictive respiratory disease by spirometry and/or clinical signs. To accomplish the exclusion criteria, patients performed a spirometry test a day before surgery. Patients classified ASA status P3 or higher were also excluded. All aspects of the study were approved by the Ethics Committee of the Hospital das Clinicas da Faculdade de Medicina da Universidade de São Paulo.
Anesthesia
All patients were administered epidural and general anesthesia. They were premedicated with 10 mg of diazepam per os the night before surgery and 0.1 mg/kg midazolam IM 45 min before surgery. Once in the OR, electrocardiogram (CB-5), arterial pressure, pulse oximetry, and temperature were monitored.
Epidural anesthesia was performed in the T7–8 interspace. Two milligrams of morphine chlorhydrate in a 15-mL 0.9% saline solution was injected and an epidural catheter positioned to allow postoperative analgesia.
Patients received 100% oxygen via face mask for 5 min. Anesthesia was induced with propofol 2 mg/kg, sufentanil citrate 0.5 µg/kg, and oral tracheal intubation was performed using a 37 left endobronchial tube (Smith Industries Medical Systems Inc./Portex, Keene, NH) facilitated by 0.1 mg/kg vecuronium bromide. Correct tube positioning was checked by fiberoptic bronchoscopy. Anesthesia was maintained using continuous infusions pumps (ANNETM Anesthesia Infuser; Abbott Laboratories, Chicago, IL) for propofol, vecuronium, and sufentanil and, when necessary, small boluses of these drugs. After induction, invasive radial and right atrial pressures were monitored.
Patients were ventilated in volume-controlled mode using a circle system with a CO2 absorber connected to the anesthesia machine (Linea Anesthesia Apparatus; Intermed, Sao Paulo, SP, Brazil). Except for the moments when PV curves were being recorded, patients were ventilated with the following settings: control mode, square-wave constant flow of 30 L/min, tidal volume equal to 8 mL/kg, respiratory frequency of 10 breaths/min, 33% I/E ratio, positive end-expiratory pressure level of 5 cm H2O. Fresh gas flow composition was a mixture of air and oxygen in equal parts. Inspired and expired gas analyses were performed with a Capnomac Ultima respiratory monitor (Datex Instrumentarium, Helsinki, Finland).
Respiratory Mechanics Measurements
Immediately before each measurement, airways were cleaned to remove accumulated mucus. After being ventilated as described above, ventilation was stopped, fresh gas manifolds were closed, and the ventilator was adjusted as follows: volume-controlled mode, tidal volume of 1000 mL, respiratory rate of 3 breaths/min, and zero positive end-expiratory pressure. Three different square-waveform constant flow values were used, in this sequence: 6, 12, and 30 L/min.
Inspiratory and expiratory flows and airway pressure (Paw) were measured using a variable area pneumotachograph (Bicore CP-100 respiratory monitor, Irvine, CA), with its sensor (Var-Flex® Flow Transducer; Allied Healthcare, Los Angeles, CA) inserted between the Y-piece and the tracheal tube. Tidal volumes were obtained by integration of the flow curve. Before each case, the anesthesia apparatus flow controls were calibrated by means of a Timeter RT-200 (Allied Healthcare) to ensure that the set flows were absolutely precise during measurements.
Esophageal pressure (Pes) was measured using an air-filled catheter (SmartCath® Esophageal Catheter; BEAR Medical Systems, Palm Springs, CA) inserted orally and connected to the pneumotachograph. Catheter positioning at the lower third of the esophagus was confirmed according to the occlusion test (7,8): airway and Pes signals were recorded for 1 min, and changes in the esophagic pressure-to-airway pressure relationship measured. Values between 0.9 and 1.1 were considered acceptable.
Resistance relative to the tracheal cannula was measured by connecting the proximal end of the cannula to the Y-piece and the distal end left open. The pneumotachograph’s sensor was inserted between the Y-piece and the tracheal tube. Application of the three tested inspiratory flows allowed determination of the resistive pressure related to the tracheal cannula by dividing the measured pressure by the correspondent inspiratory flow.
Data Formatting and Analysis
Bicore’s analog signals were recorded in ASCII format on an IBM PC (IBM Computers, SP, Brazil) by using an analog-to-digital converter (CAD 12 bit/32 channels; Lynx, SP, Brazil) for 1 min at 200 Hz. Files were converted to Excel for Windows 2000 format (Microsoft, Sao Paulo, SP, Brazil) before analysis. A flow curve was then built to determine the exact transition between expiratory and inspiratory phases (inspiratory flow beginning), the moment at which the inspiratory flow becomes constant, and that when the flow returned to zero before expiratory valve opening (end of inspiratory pause). Flow values were double-checked by observing the inspiratory time at the pressure curve (the 1000-mL tidal volume had to be reached in exactly 10, 5, and 2 s to ensure a flow respectively equal to 6, 12, and 30 L/min).
PV curves for the respiratory system were constructed by plotting lung volumes against Paw. Quasi-static respiratory system compliances were calculated by dividing tidal volumes by end-inspiratory pressures. Intrinsic positive end-expiratory pressure, considered as any pressure measured at zero flow during an expiratory pause, was subtracted when detected. Conversely, PV curves for chest wall plotted tidal volumes against the Pes. Quasi-static chest wall compliances were calculated by dividing tidal volumes by Pes. Tidal volumes used for statistical analysis were 250, 500, 750, and 1000 mL, in order to evaluate the curve in its entire length.
After the PV curves were built, a polynomial tendency line was obtained for each curve to remove artifacts from cardiac rhythm. These tendency lines and the equations originated from them were used for data analysis. Figure 2, A and B show the curves obtained in one case.
View larger version (19K):
[in this window]
[in a new window]
|
Figure 2. Pressure-volume curves obtained from one patient, showing the tendency line and derived equations used for compliance values acquisition. A, Respiratory system. B, Chest wall. C, The intersection between the starting compliance (Cstart) and the inflation compliance (Cinf) allows acquisition of the lower inflection point (Pflex). Paw = airway pressure, PEEP = positive end-expiratory pressure, Pes = esophageal pressure.
|
|
Lower inflection point (Pflex) was obtained intersecting the starting compliance (the ratio between the first 100-mL inflation and the corresponding pressure) with the inflation compliance (the slope of the PV curve in its most linear segment), according to the method proposed by Gattinoni et al. (5). A Pflex determination curve is shown in Figure 2C.
Results were expressed as mean ± SD in the text and in the tables. Statistical analysis was performed using SAS software version 8.0 (SAS, Sao Paulo, SP, Brazil) and Excel for Windows 2000 (Microsoft). One-way analysis of variance was used to compare pressure and compliance values obtained with different flows. Application of the Bonferroni t-test allowed pairwise multiple comparisons (comparison of flows used two-by-two), except for the Pflex analysis, compared by application of the Tukey test. The level of significance was 5%.
|
Results
|
|---|
A total of 14 patients were initially recruited. Of these, two were excluded because of uncorrected flow settings noticed during analysis, another because of the impossibility of performing epidural punction, and one more because of the impossibility of esophageal catheter location according to the established reference method (7,8). Consequently, compliances from 10 patients, 5 men and 5 women, were analyzed, and their characteristics were: age 46.2 ± 15.1 yr, weight 61.7 ± 8.4 kg, height 1.66 ± 0.1 m, body mass index 22.6 ± 3.5 kilo · m-2. Patients’ lungs were ventilated with an average inspired oxygen percentage of 71.2% ± 10%.
Preoperative pulmonary function tests showed an average forced vital capacity of 98.3% ± 16.6% of the predicted, a forced expiratory volume in 1 s of 88.4% ± 18.3% of the predicted, and an average forced expiratory flow rate measured over the middle portion of the forced vital capacity of 86.6% ± 22.7% of the predicted, according to the values established by Crapo et al. (9). Such values were also within normal ranges when compared with a Brazilian reference (10). Although a smoking habit was not an exclusion criterion, only 2 patients had smoked in the last 2 yr before study (no active smokers at the moment of evaluation).
Pflex were statistically less for the lowest flow when comparing 6 and 12 L/min with 30 L/min. There was no significant difference when comparing the 6 and 12 L/min flows. Conversely, compliances for 250-, 500-, and 750-mL tidal volumes were statistically more for the lowest flow when comparing 6 with 12, 6 with 30, and 12 with 30 L/min. Nevertheless, they were statistically larger for 1000-mL tidal volume only when comparing 6 with 30 L/min. Results for pressures and calculated compliances are showed in Table 1.
View this table:
[in this window]
[in a new window]
|
Table 1. Lower Inflection Points (Pflex) and Calculated Compliances (C) for 250-, 500-, 750-, and 1000-mL Tidal Volumes for Respiratory System (rs) and Chest Wall (w) Before and After Removal of the Resistive Component Related to the Tracheal Cannula (P-t)
|
|
After removal of the pressure component related to the tracheal cannula (P-t), there was no difference in Pflex when comparing the evaluated flows two by two. Concerning compliances, there was a statistical difference only at 250-mL tidal volume when comparing 6 and 12 L/min, but no difference for the other 3 tidal volumes evaluated and for all tidal volumes when comparing 12 and 30 L/min flows. There were significantly higher compliances with the lower flow for the 4 tidal volumes when comparing the 6 and 30 L/min flows. Table 1 shows Paw and compliance values after removal of the pressure component related to the P-t.
When comparing measured and calculated values before and after removal of the pressure component related to the P-t, only those obtained under 30 L/min flow were statistically different. No difference was found when the 6 and 12 L/min flows were used. Table 2 shows the comparison between the values with and without the P-t component.
View this table:
[in this window]
[in a new window]
|
Table 2. Lower Inflection Points (Pflex) and Calculated Compliances (C) for 250-, 500-, 750-, and 1000-mL Tidal Volumes for Respiratory System (rs) With or Without the Resistive Component Related to the Tracheal Cannula (P-t)
|
|
Figures 3 and 4 show PV curves respectively for respiratory system and chest wall, with and without the P-t component.
View larger version (24K):
[in this window]
[in a new window]
|
Figure 3. Respiratory system pressure-volume curves for the three flows studied, before and after subtraction of the pressure component related to the tracheal cannula (P-t). Dots show infused volumes of 250, 500, 750, and 1000 mL. Paw = airway pressure.
|
|
View larger version (21K):
[in this window]
[in a new window]
|
Figure 4. Chest wall pressure-volume curves for the three flows studied. Dots show infused volumes of 250, 500, 750, and 1000 mL. Pes = esophageal pressure.
|
|
|
Discussion
|
|---|
PV curves can be obtained during anesthesia by the application of the low-flow technique with little effort, once the necessary equipment is available. Although the 6 and 12 L/min flows created a resistive component related to the P-t without clinical relevance, larger compliance values in the 250–750-mL range with the lower flow favor the 6 L/min flow for PV curves acquisition during anesthesia.
Despite previous studies showing the increased use of PV curves in the ICU to guide ventilator management (11), its use in anesthesia has been infrequent. One of the reasons is the time necessary for PV curves acquisition. Rodriguez et al. (4) related 38 minutes with the conventional static methods (super syringe, multiple occlusion technique), compared with only 3 minutes when the low flow technique was used to obtain PV curves. We did not spend more than five minutes in any case to record curves under the three evaluated flows. It seems a wise use of time to obtain adequate ventilatory settings. The reason why we did not compare different techniques for PV curves acquisition is that time spent in the static approach makes this method pointless during anesthesia.
Time spent was not the only reason why quasi-static conditions seem to be the most suitable to acquire intraoperative PV curves. A myriad of components is involved when studying the mechanical properties of the respiratory system (12). Nevertheless, the dynamics of breathing can be adequately evaluated, at least for clinical purposes, if the elastic, resistive, and viscoelastic forces are represented. The low flow technique measures the elastic properties of lung parenchyma and chest wall in a dynamic approach. This allows not only the study of elastic and viscoelastic forces, but also the interaction between them, and, theoretically, could offer more clinically relevant information than static evaluation (6,12,13). The price to pay for this benefit seems small, because the only drawback is to determine a flow value small enough to reduce the resistive forces to a nonsignificant level, which was the main purpose of this study. Figure 1 shows graphically the importance of using an adequate flow value. It is important to bear in mind, however, that the use of an excessively low flow would affect the PV curve by continuing gas exchange as when the super syringe method is used (4). From the 3 tested flows, the 6 and 12 L/min have shown to be low enough to make the resistive component negligible, but the differences found in compliances in the 250–750 tidal volume range, with which most patients are ventilated during anesthesia, favor the lower one.
To assure precise results, we decided that Pes should be measured to evaluate eventual participation of lung parenchyma into pressure curves. Although it is not often evaluated during anesthesia practice, Pes allows the partitioning of respiratory system compliance into its chest wall and lung parenchyma components. As expected, we did not get different results for Pes with the evaluated flows. Also, the option for total IV anesthesia was intended to avoid the bronchodilating effects of volatile anesthetics.
Technical requirements for PV curves acquisition during anesthesia demand, preferably, a microprocessor-equipped anesthesia ventilator that allows direct flow settings, as the one used in this study. Despite the fact that flow value can be indirectly determined by setting inspiratory time and tidal volume variables, a microprocessor ventilator might guarantee a constant flow during the entire inspiration. The other technical requirement would be a screen where the PV curve could be displayed and analyzed.
Acquisition and analysis of PV curves in anesthesia may not be worth the equipment cost when dealing with healthy patients. However, respiratory settings based on a tidal volume adjusted to the ideal body weight and sequential arterial gases analysis might be inadequate under extreme conditions. For instance, respiratory mechanics information could allow more precise ventilator settings adjustment in thoracic trauma victims. Conversely, patients with pulmonary emphysema or chronic obstructive pulmonary disease could be ventilated safely during elective or emergency surgery and directed to a postoperative ICU with preset adequate ventilation.
Anesthesia ventilators have improved in the last years by incorporating features previously exclusive to intensive care machines. Aiming at adequate respiratory settings based on all information collected and recorded during the intraoperative period might guarantee less respiratory complications after surgery. Our study suggests the use of 6 L/min as a reliable and safe flow to acquire respiratory system dynamic compliance values on time.
|
Acknowledgments
|
|---|
Supported by Grant 00/10847–0 from FAPESP (Fundação de Amparo à Pesquisa do Estado de São Paulo).
|
References
|
|---|
- Harris RS, Hess DR, Venegas JG. An objective analysis of the pressure-volume curve in the acute respiratory distress syndrome. Am J Respir Crit Care Med 2000; 161: 432–39.[Abstract/Free Full Text]
- Amato MB, Barbas CS, Medeiros DM, et al. Effect of a protective-ventilation strategy on mortality in the acute respiratory distress syndrome. N Engl J Med 1998; 338: 347–54.[Abstract/Free Full Text]
- Lu Q, Vieira SR, Richecoeur J, et al. A simple automated method for measuring pressure-volume curves during mechanical ventilation. Am J Respir Crit Care Med 1999; 159: 275–82.[Abstract/Free Full Text]
- Rodriguez L, Marquer B, Mardrus P, et al. A new simple method to perform pressure-volume curves obtained under quasi-static conditions during mechanical ventilation. Intensive Care Med 1999; 25: 173–9.[ISI][Medline]
- Gattinoni L, Pesenti A, Rossi F, Bombino M. Pressure-volume curve of total respiratory system in acute respiratory failure. Am Rev Respir Dis 1987; 136: 730–6.[ISI][Medline]
- Maggiore SM, Brochard L. Pressure-volume curve: methods and meaning. Minerva Anestesiol 2001; 67: 228–37.[Medline]
- Baydur A, Behrakis PK, Zin WA, et al. A simple method for assessing the validity of the esophageal balloon technique. Am Rev Respir Dis 1982; 126: 788–91.[ISI][Medline]
- Zin WA, Milic-Emili J. Esophageal pressure measurement. In: Tobin MJ, ed. Principles and practice of intensive care monitoring. 1st ed. New York: McGraw-Hill Health Professions Division, 1998: 545–52.
- Crapo RO, Morris AH, Gardner RM. Reference spirometric values using techniques and equipment that meets ATS recommendations. Am Rev Respir Dis 1981; 123: 659–64.[ISI][Medline]
- Neder JA, Andreoni S, Castelo-Filho A, Nery LE. Reference values for lung function tests. I. Static volumes. Braz J Med Biol Res 1999; 32: 703–17.[ISI][Medline]
- Hess DR, Medoff BD, Fessler MB. Pulmonary mechanics and graphics during positive pressure ventilation. Int Anesthesiol Clin 1999; 37: 15–34.
- Rossi A, Polese G, Milic-Emili J. Monitoring respiratory mechanics in ventilator-dependent patients. In: Tobin MJ, ed. Principles and practice of intensive care monitoring. 1st ed. New York: McGraw-Hill Health Professions Division, 1998: 553–96.
- Beydon L, Svantesson C, Brauer K, et al. Respiratory mechanics in patients ventilated for critical lung disease. Eur Respir J 1996; 9: 262–73.[Abstract]
Accepted for publication February 28, 2003.
Top
Abstract
Introduction
P1 = Pmax - Pi) increases with higher flows, whereas the component related to the viscoelastic properties of lung parenchyma (
