Outcomes reported in high-impact surgical journals.

BACKGROUND: With advances in operative technique and perioperative care, traditional endpoints such as morbidity and mortality provide an incomplete description of surgical outcomes. There is increasing emphasis on the need for patient-reported outcomes (PROs) to evaluate fully the effectiveness and quality of surgical interventions. The objective of this study was to identify the outcomes reported in clinical studies published in high-impact surgical journals and the frequency with which PROs are used. METHODS: Electronic versions of material published between 2008 and 2012 in the four highest-impact non-subspecialty surgical journals (Annals of Surgery, British Journal of Surgery (BJS), Journal of the American College of Surgeons (JACS), Journal of the American Medical Association (JAMA) Surgery) were hand-searched. Clinical studies of adult patients undergoing planned abdominal, thoracic or vascular surgery were included. Reported outcomes were classified into five categories using Wilson and Cleary's conceptual model. RESULTS: A total of 893 articles were assessed, of which 770 were included in the analysis. Some 91·6 per cent of studies reported biological and physiological outcomes, 36·0 per cent symptoms, 13·4 per cent direct indicators of functional status, 10·6 per cent general health perception and 14·8 per cent overall quality of life (QoL). The proportion of studies with at least one PRO was 38·7 per cent overall and 73·4 per cent in BJS (P < 0·001). The proportion of studies using a formal measure of health-related QoL ranged from 8·9 per cent (JAMA Surgery) to 33·8 per cent (BJS). CONCLUSION: The predominant reporting of clinical endpoints and the inconsistent use of PROs underscore the need for further research and education to enhance the applicability of these measures in specific surgical settings.

Systematic review of outcomes used to evaluate enhanced recovery after surgery.

BACKGROUND: Enhanced recovery pathways (ERPs) aim to improve patient recovery. However, validated outcome measures to evaluate this complex process are lacking. The objective of this review was to identify how recovery is measured in ERP studies and to provide recommendations for the design of future studies. METHODS: A systematic search of MEDLINE, Embase and Cochrane databases was conducted. Prospective studies evaluating ERPs compared with traditional care in abdominal surgery published between 2000 and 2013 were included. All reported outcomes were classified into categories: biological and physiological variables, symptom status, functional status, general health perceptions and quality of life (QoL). The phase of recovery measured was defined as baseline, intermediate (in hospital) and late (following discharge). RESULTS: A total of 38 studies were included based on the systematic review criteria. Biological or physiological variables other than postoperative complications were reported in 30 studies, and included return of gastrointestinal function (25 studies), pulmonary function (5) and physical strength (3). Patient-reported symptoms, including pain (16 studies) and fatigue (9), were reported less commonly. Reporting of functional status outcomes, including mobilization (16 studies) and ability to perform activities of daily living (4), was similarly uncommon. Health aspects of QoL were reported in only seven studies. Length of follow-up was generally short, with 24 studies reporting outcomes within 30 days or less. All studies documented in-hospital outcomes (intermediate phase), but only 17 reported postdischarge outcomes (late phase) other than complications or readmission. CONCLUSION: Patient-reported outcomes, particularly postdischarge functional status, were not commonly reported. Future studies of the effectiveness of ERPs should include validated, patient-reported outcomes to estimate better their impact on recovery, particularly after discharge from hospital.

Colonoscopy performance correlates with scores on the FES™ manual skills test.

INTRODUCTION: Achieving proficiency in flexible endoscopy is a major priority for general surgery training programs. The Fundamentals of Endoscopic Surgery (FES™) is a high-stakes examination of the knowledge and skills required to perform flexible endoscopy. The objective of this study was to establish additional evidence for the validity of the FES™ hands-on test as a measure of flexible endoscopy skills by correlating clinical colonoscopy performance with FES™ score. METHODS: Participants included FES™-naïve general surgery residents, gastroenterology fellows at all levels of training and attending physicians who regularly perform colonoscopy. Each participant completed a live colonoscopy and the FES™ hands-on test within 2 weeks. Performance on live colonoscopy was measured using the Global Assessment of Gastrointestinal Endoscopic Skills-Colonoscopy (GAGES-C, maximum score 20), and performance on the FES™ hands-on test was assessed by the simulator's computerized scoring system. The clinical assessor was blinded to simulator performance. Scores were compared using Pearson's correlation coefficient. RESULTS: A total of 24 participants were enrolled (mean age 30; 54 % male) with a broad range of endoscopy experience; 17 % reported no experience, 54 % had <25 previous colonoscopies; and 21 % had >100. The FES™ and GAGES scores reflected the broad range of endoscopy experience of the study group (FES™ score range 32-105; GAGES score range 5-20). Pearson's correlation coefficient between GAGES-C scores and FES™ hands-on test scores was 0.78 (0.54-0.90, p < 0.0001). All eight participants with GAGES-C score >15/20 achieved a passing score on the FES™ hands-on test. CONCLUSION: There is a strong correlation between clinical colonoscopy performance and scores achieved on the FES™ hands-on test. These data support the validity of FES™ as a measure of colonoscopy skills.

Impact of a bladder scan protocol on discharge efficiency within a care pathway for ambulatory inguinal herniorraphy.

BACKGROUND: Postoperative urinary retention (POUR) is a common complication of ambulatory inguinal herniorraphy, with an incidence reaching 38%, and many surgeons require patients to void before discharge. This study aimed to assess whether the implementation of a bladder scan-based voiding protocol reduces the time until discharge after ambulatory inguinal herniorraphy without increasing the rate of POUR. METHODS: As part of a perioperative care pathway, a protocol was implemented to standardize decision making after elective inguinal hernia repair (February 2012). Patients were assessed with a bladder scan, and those with <600 mL of urine were discharged home, whereas those with more than 600 mL of urine had an in-and-out catheterization before discharge. The patients received written information about urinary symptoms and instructions to present to the emergency department if they were unable to void at home. An audit of scheduled outpatient inguinal hernia repairs between October 2011 and July 2012 was performed. Comparisons were made using the t test, Fisher's exact test, and Wilcoxon rank sum test where appropriate. Statistical significance was defined a priori as a p value lower than 0.05. RESULTS: During the study period, 124 patients underwent hernia repair: 60 before and 64 after implementation of the protocol. The findings showed no significant differences in patient characteristics, laparoscopic approach (35 vs. 33%; p = 0.80), proportion receiving general anesthesia (70 vs. 73%; p = 0.67), or amount of intravenous fluids given (793 vs. 663 mL; p = 0.07). The proportion of patients voiding before discharge was higher after protocol implementation (73 vs. 89%; p = 0.02). The protocol had no impact on median time to discharge (190 vs. 205 min; p = 0.60). Only one patient in each group presented to the emergency department with POUR (2%). CONCLUSION: After ambulatory inguinal herniorraphy, implementation of a bladder scan-based voiding protocol did not result in earlier discharge. The incidence of POUR was lower than reported in the literature.

Standard diagnostic laparoscopy is superior to NOTES approaches: results of a blinded, randomized controlled porcine study.

BACKGROUND AND STUDY AIM: The use of transluminal endoscopic access via the stomach or colon for flexible diagnostic peritoneoscopy has been proposed, although the diagnostic value of the technique has not yet been fully clarified. In this animal trial, the two main natural orifice transluminal endoscopic surgery (NOTES) approaches - transgastric (TG) and transcolonic (TC) - were compared with standard transabdominal access using both rigid (TAR) and flexible instruments (TAF) for diagnostic laparoscopy. METHODS: A total of 48 peritoneoscopies were performed using two randomly assigned approaches in 24 anesthetized pigs. The ability of the examinations to detect 576 electrocautery markings simulating intraperitoneal metastases, to achieve complete organ visualization, and to simulate organ biopsies was analyzed. RESULTS: Sensitivities for the detection of lesions were 78.5 %, 59.7 %, 48.6 %, and 38.9 % for TAR, TAF, TC, and TG, respectively; standard laparoscopy was superior to all other approaches (P < 0.01). Among the NOTES approaches, TC was superior for examining the upper abdomen (P = 0.03). Complete organ visualization was better with the transabdominal approach (visual analogue scale TAR 7.15, TAF 6.71) than with the NOTES access routes (TC 5.07, TG 4.35); standard rigid laparoscopy was superior to both NOTES approaches (P < 0.01). Organ biopsy simulation was possible in 87 %, 85 %, 72 %, and 65 % of cases with TAR, TAF, TC, and TG, respectively. Standard rigid laparoscopy was again superior to both NOTES approaches (TAR vs. TC, P = 0.03; TAR vs. TG, P < 0.01). CONCLUSIONS: In this experimental trial, rigid standard laparoscopy provided better organ visualization, better lesion detection, and better biopsy capability than the transgastric and transcolonic NOTES approaches. In its current form, NOTES appears to be unsuitable for diagnostic laparoscopy.

Endoscopic vs. laparoscopic gastrojejunostomy for duodenal obstruction: a randomized study in a porcine model.

BACKGROUND AND STUDY AIMS: Open or laparoscopic gastrojejunostomy is an established treatment for malignant duodenal obstruction but may be associated with significant morbidity and mortality. The purpose of this study was to develop a model for an entirely endoscopic gastrojejunostomy to treat duodenal obstruction, and to compare this with the laparoscopic technique. METHODS: During the first part of the study the endoscopic technique was developed and tested in porcine nonsurvival and survival experiments (n=12). During the second part of the study (n=10), endoscopic gastrojejunostomy for duodenal occlusion was compared with laparoscopic gastrojejunostomy in a survival randomized controlled trial (RCT). For both groups duodenal occlusion was achieved by the laparoscopic approach. RESULTS: In the RCT, the median times for laparoscopic vs. endoscopic gastrojejunostomy were 70 minutes (interquartile range [IQR] 65-75) vs. 210 minutes (IQR 197-220; P=0.01). There was a trend toward increased anastomotic diameter at necropsy in the laparoscopic group (2 cm, IQR 2-3) compared to the endoscopic group (1.8 cm, IQR 1.6-1.8; P=0.06). One animal in the endoscopic group died secondarily to bowel ischemia from volvulus of the jejunal loop. One animal in the laparoscopic group was prematurely sacrificed due to extensive pulmonary congestion and edema. All anastomoses were intact and patent. CONCLUSIONS: Purely endoscopic gastrojejunostomy using the developed technique and devices is feasible and can result in adequate relief of duodenal obstruction. Endoscopic anastomoses tend to be smaller than laparoscopic anastomoses, with the procedures being more time-consuming and associated with higher complication rates.

Endoscopic mucosal resection using a grasp-and-snare technique.

BACKGROUND AND STUDY AIM: Endoscopic mucosal resection (EMR) is a minimally invasive method for en bloc removal of superficial gastrointestinal lesions. The aim of this study was to evaluate the feasibility of a novel grasp-and-snare EMR technique. METHODS: In 10 domestic pigs, gastric lesions of approximately 3 cm were marked using electrocautery. EMR was performed using a double-channel endoscope. A novel tissue anchor was used through one channel, and a monofilament snare through the other. After submucosal injection, a circumferential mucosal incision was created. The tissue-anchoring device was then advanced through the open snare and anchored into the submucosal layer. The tissue-anchoring device was partly retracted into the endoscope and the snare was positioned into the circular incision. The snare was subsequently closed and the specimen resected by applying high-frequency electrocautery. RESULTS: Mean time to perform EMR was 32.4 minutes (range 22-41 minutes, SD 6.3). EMR yielded specimens that ranged in area from 2.7 cm (minor axis) by 2.8 cm (major axis) to 4.0 cm by 4.2 cm (mean area 9.36 cm(2); range 5.94-13.19 cm(2); SD +/- 2.50). Complete en bloc resection including all electrocautery markings was achieved in 9/10 cases. In one case, resection was achieved in two steps. One gastric wall perforation occurred. No other adverse events were observed. CONCLUSIONS: Grasp-and-snare EMR is feasible in an animal model. The technique can be performed efficiently compared with standard methods. To avoid perforation, caution is needed to ensure that tissue anchor needles are placed within and not deeper than the submucosal layer prior to tissue retraction.

The oaxaca, Mexico, earthquake of 29 november 1978: a preliminary report on aftershocks.

Aftershocks of the 29 November 1978 Oaxaca, Mexico, earthquake (surface-wave magnitude Ms = 7.8) define a rupture area of about 6000 square kilometers along the boundary of the Cocos sea-plate subduction. This area had not ruptured in a large (Ms >/= 7), shallow earthquake since the years 1928 and 1931 and had been designated a seismic "gap." The region has also been seismically quiet for small to moderate (M >/= 4), shallow (depth

Randomized controlled trial comparing endoscopic clips and over-the-scope clips for closure of natural orifice transluminal endoscopic surgery gastrotomies.

BACKGROUND AND STUDY AIM: Secure and reliable endoscopic closure is of paramount importance before clinical introduction of transgastric natural orifice transluminal endoscopic surgery (NOTES). Gastrotomy closure in humans using standard endoclips has been reported. The aim of this study was to assess the safety of standard endoclip closure and to compare it to a new over-the-scope clip (OTSC) specifically designed for gastrotomy closure. MATERIAL AND METHODS: Gastric wall puncture and balloon dilation followed by peritoneoscopy was carried out in 20 female swine. After randomization, closure of the gastric incision was performed using a tissue approximation grasper and either endoclips or OTSCs. RESULTS: Mean (+/- SD) time for gastrotomy closure using endoclips was 31.5 +/- 24.2 minutes (range 8 - 88 minutes) compared with 8.5 +/- 9.1 minutes (range 2 - 31 minutes) using OTSC (P = 0.002). No intraoperative complications occurred. Laparoscopic leak tests with insufflation and saline immersion demonstrated three minor leaks and one major leak in the endoclip closures. No leaks were observed in the OTSC group. At necropsy, complete sealing of the gastrotomy sites was found in all OTSC closures. Small, localized perigastric abscesses were observed in two animals in the OTSC group and in three animals in the endoclip group. One animal in the endoclip group was sacrificed prematurely due to signs of sepsis and was found to have gross peritonitis secondary to a leak. At necropsy, evidence of peritonitis was identified in two other animals in the endoclip group. CONCLUSION: NOTES gastrotomy closure using standard endoclips, even with a tissue approximation grasper, is associated with an increased risk of leakage and intra-abdominal infection compared with OTSC. The significance of perigastric abscesses, which were seen in both groups, warrants further investigation.

Endoscopic closure of large colonic perforations using an over-the-scope clip: a randomized controlled porcine study.

BACKGROUND AND STUDY AIMS: Perforation of the colon is considered to be one of the most serious complications of flexible endoscopy. The over-the-scope clip system (OTSC) has previously been shown to close small colonic perforations effectively. The aim of this randomized controlled porcine study was to compare acute closure strengths between the novel 11-mm over-the-scope-clip and surgical closure for large colonic perforations. MATERIAL AND METHODS: In 24 anesthetized domestic pigs, an 18-mm sigmoid perforation was created endoscopically using a needle knife and dilating balloon. The animals were randomly assigned to undergo either open surgical repair (n = 12) or endoscopic closure using the OTSC system (n = 12). Pressurized leak tests were performed during necropsy. RESULTS: Mean time to perform the incision in the sigmoid colon and obtain peritoneal access was 5.5 minutes (range 3-12; SD +/- 2.5). Mean time for endoscopic closure was 6.8 minutes (range 3-14; SD +/- 3). At necropsy, all OTSC and surgical closures demonstrated complete sealing of colotomy sites. In one case peritoneum and in a second case adjacent small intestine were found incorporated into the OTSC closure. No other complications occurred. Pressurized leak tests revealed a mean burst pressure of 62.8 mmHg (range 18-112; SD 35.7) for OTSC closures and 67.4 mmHg (range 30-90; SD 19) for sutured closure. No significant differences in burst pressures were noted between the OTSC closures and surgical repair. CONCLUSION: Closure of acute perforations using the OTSC system is comparable to surgical closure in a nonsurvival porcine model.

An evaluation of techniques used in superficial radiotherapy for non-melanoma skin cancer to replicate the planned treatment area: A prospective study.

INTRODUCTION: Accuracy of superficial radiotherapy for non-melanoma skin cancer is dependent on replicating the original clinical mark-up. Responses from 18 UK Radiotherapy centres identified the four most common replication techniques; the accuracy and time-efficiency of each was evaluated, as well as participant preference and confidence. METHODS: A 2.0  cm × 2.5  cm ellipse field was drawn around the nasal ala of a surrogate patient. Templates for each replication method (1-4) were created, and skin marks removed. Twenty-five therapeutic radiographers used each method to replicate the mark-up. Measurements were recorded for lateral and longitudinal displacement, ellipse diameter and time taken. A post-study questionnaire recorded participant preference and perceived confidence. RESULTS: Comparison of the mean ellipse areas for methods 1-4 identified no statistically significant differences (ANOVA test; p = 0.579 to p = 0.999). Lateral and longitudinal displacements for method 1-4 showed a statistically significant difference between method 3 and each of methods 1, 2, 4 for lateral and longitudinal respectively (ANOVA; lateral: p = 0.008, p = 0.002, p = 0.05; longitudinal: p = 0.036, p = 0.000, and p = 0.000). Mean time taken was longest for method 3, and was compared using a Friedman test (p = 0.000) identifying a statistically significant difference. Twenty-two participants completed the questionnaire. 48% favoured method 2, 41% method 4. Method 3 was least favourite. A Likert scale (1-10) measured confidence. Participants had most confidence in methods 2 and 4. CONCLUSION: In this study, method 3 was least accurate, most time consuming, and was least favoured by users. The clinical significance of these results will depend on the margins used in local practise.

FLS simulator performance predicts intraoperative laparoscopic skill.

INTRODUCTION: Simulators are being used more and more for teaching and testing laparoscopic skills. However, it has yet to be firmly established that simulator performance reflects operative laparoscopic skill. The study reported here was designed to test the hypothesis that laparoscopic simulator performance predicts intraoperative laparoscopic skill. METHODS: A review of our prospectively maintained database identified 40 subjects who underwent Fundamentals of Lapraoscopic Surgery (FLS) skills testing and objective intraoperative assessments within the same 6-month period. Subjects consisted of 22 novice (postgraduate year [PGY] 1-2), 10 intermediate (PGY 3-4), and 8 experienced (PGY 5, fellows, and attendings) laparoscopic surgeons. Laparoscopic performance was objectively assessed in the operating room using the previously validated Global Operative Assessment of Laparoscopic Skill (GOALS). Analysis of variance (ANOVA) was used to compare mean FLS scores and mean GOALS scores across experience levels. The relationship between individual FLS scores and GOALS scores was assessed with linear regression analysis. A multivariate analysis evaluated FLS score and surgeon experience as predictors of intraoperative GOALS score. A receiver-operator curve (ROC) was constructed in order to define an FLS cutoff score that predicts intraoperative performance at or above the level of experienced surgeons. Significance was defined as p < 0.05. RESULTS: Mean FLS scores and mean GOALS scores increased with increasing experience. Individual FLS scores correlated significantly with intraoperative GOALS scores (0.77, p < 0.001). Multivariate analysis confirmed that FLS score is an independent predictor of intraoperative GOALS scores. The ROC identified an FLS cutoff score of 70 with optimal sensitivity (91%) and specificity (86%) for predicting a GOALS score at or above the level of experienced surgeons. CONCLUSIONS: In this study sample, FLS simulator scores were independently predictive of intraoperative laparoscopic performance as measured by GOALS. More precisely, an FLS cutoff score of 70 optimized sensitivity and specificity for expert intraoperative performance. A larger prospective study is justified to validate these findings.

The MISTELS program to measure technical skill in laparoscopic surgery : evidence for reliability.

BACKGROUND: The McGill Inanimate System for Training and Evaluation of Laparoscopic Skills (MISTELS) is a series of five tasks with an objective scoring system. The purpose of this study was to estimate the interrater and test-retest reliability of the MISTELS metrics and to assess their internal consistency. METHODS: To determine interrater reliability, two trained observers scored 10 subjects, either live or on tape. Test-retest reliability was assessed by having 12 subjects perform two tests, the second immediately following the first. Interrater and test-retest reliability were assessed using intraclass correlation coefficients. Internal consistency between tasks was estimated using Cronbach's alpha. RESULTS: The interrater and test-retest reliabilities for the total scores were both excellent at 0.998 [95% confidence interval (CI), 0.985-1.00] and 0.892 (95% CI, 0.665-0.968), respectively. Cronbach's alpha for the first assessment of the test-retest was 0.86. CONCLUSIONS: The MISTELS metrics have excellent reliability, which exceeds the threshold level of 0.8 required for high-stakes evaluations. These findings support the use of MISTELS for evaluation in many different settings, including residency training programs.

A survey of general surgeons regarding laparoscopic inguinal hernia repair: practice patterns, barriers, and educational needs.

PURPOSE: Practice patterns for inguinal hernia repair vary significantly among surgeons. The purpose of this study was to identify perceived indications for laparoscopic inguinal hernia repair (LIHR), and to identify barriers to its adoption and educational needs for surgeons. METHODS: A web-based survey was sent to general surgery members of several North American surgical societies, and to surgical residents through program directors. The 33-item survey was divided in 4 sections: demographics, utilization of techniques, management based on 11 clinical scenarios, reasons for not performing LIHR and educational needs for those who want to learn. RESULTS: Six hundred and ninety-seven general surgeons and 206 general surgery residents responded to the survey. Surgeons with MIS fellowships, and surgeons at the beginning of their careers are more likely to perform LIHR. Out of the 11 clinical scenarios, surgeons preferred a laparoscopic approach (totally extraperitoneal or transabdominal preperitoneal) for bilateral (48 %) and recurrent (44 %) hernias. However, 46 % of respondents never perform LIHR. Of these, 70 % consider the benefits of laparoscopy to be minimal, 59 % said they lack the requisite training, and 26 % are interested in learning. Surgeons (70 %) and residents (73 %) agreed that the best educational method would be a course followed by expert proctoring. CONCLUSION: Surgeons remain divided on the utility of laparoscopic surgery for inguinal hernia repair. Nearly half of responding surgeons never perform LIHR, and the other half offer it selectively. One quarter of surgeons who do not perform LIHR are interested in learning. This reveals a knowledge gap that could be addressed with educational programs.

Current practices of laparoscopic inguinal hernia repair: a population-based analysis.

PURPOSE: The selection of a laparoscopic approach for inguinal hernias varies among surgeons. It is unclear what is being done in actual practice. The purpose of this study was to report practice patterns for treatment of inguinal hernias among Quebec surgeons, and to identify factors that may be associated with the choice of operative approach. METHODS: We studied a population-based cohort of patients who underwent an inguinal hernia repair between 2007 and 2011 in Quebec, Canada. A generalized linear model was used to identify predictors associated with the selection of a laparoscopic approach. RESULTS: 49,657 inguinal hernias were repaired by 478 surgeons. Laparoscopic inguinal hernia repair (LIHR) was used in 8 % of all cases. LIHR was used to repair 28 % of bilateral hernias, 10 % of recurrent hernias, 6 % of unilateral hernias, and 4 % of incarcerated hernias. 268 (56 %) surgeons did not perform any laparoscopic repairs, and 11 (2 %) surgeons performed more than 100 repairs. These 11 surgeons performed 61 % of all laparoscopic cases. Patient factors significantly associated with having LIHR included younger age, fewer comorbidities, bilateral hernias, and recurrent hernias. CONCLUSION: An open approach is favored for all clinical scenarios, even for situations where published guidelines recommend a laparoscopic approach. Surgeons remain divided on the best technique for inguinal hernia repair: while more than half never perform LIHR, the small proportion who perform many use the technique for a large proportion of their cases. There appears to be a gap between the best practices put forth in guidelines and what surgeons are doing in actual practice. Identification of barriers to the broader uptake of LIHR may help inform the design of educational programs to train those who have the desire to offer this technique for certain cases, and have the volume to overcome the learning curve.

Volume dependence of respiratory system resistance during artificial ventilation in rabbits.

The volume dependence of respiratory resistance (Rrs), usually observed during normal breathing, is expected to be accentuated during expiratory flow limitation (EFL). In order to quantify this dependence we studied the pressure, flow, and volume data obtained from eight New Zealand rabbits, artificially ventilated at different levels of applied expiratory pressure (0-10 hPa), before and during histamine i. v. infusion. EFL was provoked by lowering the expiratory pressure and was detected by the application of an additional negative expiratory pressure and by forced oscillations. The analysis of respiratory system mechanics was performed by multiple regression, using the classical linear first-order model and also a nonlinear model, accounting for volume dependence of Rrs. Both models satisfactorily fitted the data in the absence of EFL. The nonlinear model proved to be more appropriate in the presence of EFL. The coefficient expressing the volume dependence of Rrs (Rvd) was significantly more negative during EFL. Rvd values were highly correlated with the fraction of the tidal volume left to be expired at the onset of EFL. A threshold Rvd value of -1,000 (hPa x s x l(-2)) detected EFL with high sensitivity and specificity. We conclude that a strongly negative volume dependence of Rrs is a reliable and noninvasive index of EFL during artificial ventilation.

Linear and nonlinear analysis of pressure and flow during mechanical ventilation.

OBJECTIVE: Linear modeling as a method of exploring respiratory mechanics during mechanical ventilation, was compared to nonlinear modeling for flow dependence of resistance in three distinct groups of patients, those with: (a) normal respiratory function (NRF), (b) chronic obstructive pulmonary disease (COPD), or (c) adult respiratory distress syndrome (ARDS). DESIGN AND PATIENTS: Airways opening pressure (Pao), flow (V'), and volume (V) signals were recorded in 32 ICU mechanically ventilated patients, under sedation and muscle relaxation (10 NRF, 11 COPD, 11 ARDS). All patients were ventilated with controlled mandatory ventilation mode at three levels of end-expiratory pressure (PEEPe): 0, 5, and 10 hPa. Data were analyzed according to: (a) Pao = PE + Ers V + Rrs V' and (b) Pao = PE + Ers V + k1V' + k2¿V'¿V'; where Ers and Rrs represent the intubated respiratory system (RS) elastance and resistance, k1 and k2 the linear and the nonlinear RS resistive coefficients, and PE the end-expiratory pressure. The model's goodness of fit to the data was evaluated by the root mean square difference of predicted minus measured Pao values. RESULTS: NRF data fit both models well at all PEEPe levels. ARDS and particularly COPD data fit the nonlinear model better. Values of k2 were often negative in COPD and ARDS groups, and they increased in parallel with PEEPe. A gradual increase in PEEPe resulted in better fit of ARDS and COPD data to both models. CONCLUSIONS: The model of V' dependence of resistance is more suitable for the ARDS and particularly the COPD groups. PEEP tends to diminish the V' dependence of respiratory resistance during the respiratory cycle, particularly in the COPD group, probably through an indirect effect of the increased lung volume.

Thermal artifacts in plethysmographic airway resistance measurements.

Measurements of airway resistance (Raw) by body plethysmography during unconditioned air breathing are implicitly based on the assumption that the warming and humidification of air in the airways are instantaneous. Simulation with a simple model suggests that Raw may be frequency dependent and substantially underestimated at most breathing frequencies if the time constant of gas conditioning (theta) is between 0.01 and 0.3 s. We measured the frequency dependence of the real (Re) and imaginary parts of the relationship between the plethysmographic signal and airway flow from 0.5 to 3 Hz in six healthy subjects in several situations. During breathing of unconditioned air through a heated pneumotachograph, Re increased by 47 +/- 70% between 2 and 3 Hz; the data were consistent with a theta of 0.087 +/- 0.023 s. Additional dead spaces moderately increased theta: 0.105 +/- 0.031 and 0.120 +/- 0.027 s with 50-cm-long polyvinyl chloride and copper tubes, respectively. During breathing of saturated air conditioned at 34, 36, 38, and 40 degrees C, Re exhibited a much smaller positive frequency dependence, most of which was probably due to a flow dependence of Raw. We conclude that unless the inspired gas is conditioned, plethysmographic Raw is likely to be substantially underestimated, particularly when measured during spontaneous breathing.