Compliance and dead space fraction indicate an optimal level of positive end-expiratory pressure after recruitment in anesthetized patients.

BACKGROUND: "Optimal" positive end-expiratory pressure (PEEP) can be defined as the PEEP that prevents recollapse after a recruitment maneuver, avoids over-distension, and, consequently, leads to optimal lung mechanics at minimal dead space ventilation. In this study, we analyzed the effects of PEEP and recruitment on functional residual capacity (FRC), compliance, arterial oxygen partial pressure (Pao2) and dead space fraction, and we determined the most suitable variables indicating optimal PEEP. METHODS: We studied 20 anesthetized patients with healthy lungs undergoing faciomaxillary surgery. After a stepwise increase of PEEP/inspiratory pressures (0/10, 5/15, 10/20, 15/25 cm H2O, each level lasting for 20 min) using a pressure-controlled ventilation mode, a recruitment maneuver (at 20/45 cm H2O for a maximum of 20 min) was performed, followed by a stepwise pressure reduction (15/25, 10/20, 5/15, 0/10 cm H2O, with 20 min at each level). At each pressure level, FRC, compliance, Pao2, and dead space fraction were measured. RESULTS: When comparing the values before and after recruitment at identical PEEP levels, all variables showed significant changes at 10/20 cm H2O; compliance was also significantly higher at the pressure step 15/25 cm H2O. In addition, FRC values showed significant differences at 5/15 cm H2O and 15/25 cm H2O. CONCLUSIONS: All variables showed the positive effects of PEEP in conjunction with a recruitment maneuver. Optimal PEEP was 10 cm H2O because at this pressure level the highest compliance value in conjunction with the lowest dead space fraction indicated a maximum amount of effectively expanded alveoli. FRC and Pao2 were insensitive to alveolar over-distension.

Determination of functional residual capacity by oxygen washin-washout: a validation study.

OBJECTIVE: To validate a new system for functional residual capacity (FRC) measurements using oxygen washin/washout in spontaneously breathing humans. The system (LUFU, Drägerwerk AG, Lübeck, Germany) consists of an unmodified EVITA 4 ventilator, a side-stream paramagnetic oxygen sensor and a dedicated software. DESIGN: Laboratory study and measurements in spontaneously breathing volunteers. SETTING: Pulmonary function laboratory of a university hospital. PARTICIPANTS: 20 healthy and 15 lung diseased volunteers. INTERVENTIONS: FRC was measured by LUFU (LUFU-FRC) and by helium dilution (He-FRC); intra-thoracic gas volume (ITGV) was determined by body plethysmography. Each measurement cycle consisted of four independent LUFU-FRC determinations (step change of FiO(2) from 0.21 to 0.5 and back and from 0.21 to 1.0 and back), two helium-dilution runs and two body box measurements. Repeatability and agreement between methods were determined by comparing different measurements of one technique and by comparing different techniques among each other. MEASUREMENTS AND RESULTS: Repeatability of LUFU-FRC was estimated by comparing washin to washout and the different FiO(2)steps. The difference of the means was 3.7% at the most. Agreement between methods resulted in the following differences (mean+/-standard deviation of differences) for healthy and lung-diseased volunteers, respectively: LUFU-FRC vs. He-FRC -0.40+/-0.50 L (0.02+/-0.95 L), LUFU-FRC vs. ITGV -0.43+/-0.54 L (-0.18+/-0.61 L) and He-FRC vs. ITGV -0.03+/-0.43 L (-0.20+/-0.98 L). CONCLUSIONS: LUFU is a non-invasive method for the determination of FRC that requires only minor additional equipment and no modification to the ventilator. It can be used in difficult conditions such as breathing patterns with variations from breath to breath. The results of this study show that LUFU is sufficiently reliable and repeatable to warrant its clinical application.

Use of dynamic compliance for open lung positive end-expiratory pressure titration in an experimental study.

OBJECTIVE: We tested whether the continuous monitoring of dynamic compliance could become a useful bedside tool for detecting the beginning of collapse of a fully recruited lung. DESIGN: Prospective laboratory animal investigation. SETTING: Clinical physiology research laboratory, University of Uppsala, Sweden. SUBJECTS: Eight pigs submitted to repeated lung lavages. INTERVENTIONS: Lung recruitment maneuver, the effect of which was confirmed by predefined oxygenation, lung mechanics, and computed tomography scan criteria, was followed by a positive end-expiratory pressure (PEEP) reduction trial in a volume control mode with a tidal volume of 6 mL/kg. Every 10 mins, PEEP was reduced in steps of 2 cm H2O starting from 24 cm H2O. During PEEP reduction, lung collapse was defined by the maximum dynamic compliance value after which a first measurable decrease occurred. Open lung PEEP according to dynamic compliance was then defined as the level of PEEP before the point of collapse. This value was compared with oxygenation (Pao2) and CT scans. MEASUREMENTS AND MAIN RESULTS: Pao2 and dynamic compliance were monitored continuously, whereas computed tomography scans were obtained at the end of each pressure step. Collapse defined by dynamic compliance occurred at a PEEP of 14 cm H2O. This level coincided with the oxygenation-based collapse point when also shunt started to increase and occurred one step before the percentage of nonaerated tissue on the computed tomography exceeded 5%. Open lung PEEP was thus at 16 cm H2O, the level at which oxygenation and computed tomography scan confirmed a fully open, not yet collapsed lung condition. CONCLUSIONS: In this experimental model, the continuous monitoring of dynamic compliance identified the beginning of collapse after lung recruitment. These findings were confirmed by oxygenation and computed tomography scans. This method might become a valuable bedside tool for identifying the level of PEEP that prevents end-expiratory collapse.

Monitoring dead space during recruitment and PEEP titration in an experimental model.

OBJECTIVE: To test the usefulness of dead space for determining open-lung PEEP, the lowest PEEP that prevents lung collapse after a lung recruitment maneuver. DESIGN: Prospective animal study. SETTING: Department of Clinical Physiology, University of Uppsala, Sweden. SUBJECTS: Eight lung-lavaged pigs. INTERVENTIONS: Animals were ventilated using constant flow mode with VT of 6ml/kg, respiratory rate of 30bpm, inspiratory-to-expiratory ratio of 1:2, and FiO(2) of 1. Baseline measurements were performed at 6cmH(2)O of PEEP. PEEP was increased in steps of 6cmH(2)O from 6 to 24cmH(2)O. Recruitment maneuver was achieved within 2min at pressure levels of 60/30cmH(2)O for Peak/PEEP. PEEP was decreased from 24 to 6cmH(2)O in steps of 2cmH(2)O and then to 0cmH(2)O. Each PEEP step was maintained for 10min. MEASUREMENTS AND RESULTS: Alveolar dead space (VD(alv)), the ratio of alveolar dead space to alveolar tidal volume (VD(alv)/VT(alv)), and the arterial to end-tidal PCO(2) difference (Pa-ET: CO(2)) showed a good correlation with PaO(2), normally aerated areas, and non-aerated CT areas in all animals (minimum-maximum r(2)=0.83-0.99; p<0.01). Lung collapse (non-aerated tissue>5%) started at 12[Symbol: see text]cmH(2)O PEEP; hence, open-lung PEEP was established at 14cmH(2)O. The receiver operating characteristics curve demonstrated a high specificity and sensitivity of VD(alv) (0.89 and 0.90), VD(alv)/VT(alv) (0.82 and 1.00), and Pa-ET: CO(2) (0.93 and 0.95) for detecting lung collapse. CONCLUSIONS: Monitoring of dead space was useful for detecting lung collapse and for establishing open-lung PEEP after a recruitment maneuver.

Monitoring of functional residual capacity by an oxygen washin/washout; technical description and evaluation.

OBJECTIVE: It was the goal of this study to develop and test an automated method for measuring functional residual capacity (FRC) by an oxygen washin/washout in intensive care settings. Such a method is required to work with conventional ventilator breathing systems and to use only medical grade sensors. METHODS: The oxygen setting on a standard intensive care ventilator is changed by at least 10%. Ventilatory pressure and flow are measured by the built-in sensors of the intensive care ventilator. Oxygen concentration is measured by a diverting medical oxygen analyzer. In order to overcome the known problem that synchrony between flow and concentration measurement is corrupted by the change of gas viscosity and by the cyclic change of airway pressure, a physical/mathematical model of the pneumatic circuit of the analyzer was developed. With this model, the change of sample flow is calculated continuously. Thus, synchrony between flow and gas concentration measurement is restored. This allows the determination of volumetric gas fluxes as needed for the FRC measurement. The setup was tested in the laboratory with a lung simulator. Simulated lung compliance, breathing frequency and tidal volume were varied. Results. The mean difference between measured and simulated FRC (range 1.7 to 5 L) was less than 1% at tidal volumes greater than 400 mL. This difference ranged from -5% to 8%, depending on simulated lung compliance and ventilator setting. The variability of consecutive measurements was about 2.5%. CONCLUSIONS: A method has been developed for reliable measurement of the FRC with an oxygen washin/washout technique. This method is sufficiently easy to use to suit for application in intensive care units. It does not require any action by the operator except a manual change of inspired oxygen concentration. Accuracy and sensitivity of the method have been proven sufficient to meet clinical and scientific requirements. Future clinical studies will reveal the applicability of the chosen procedure under clinical conditions.

Reduction of anesthesia process times after the introduction of an internal transfer pricing system for anesthesia services.

To improve operating room workflow, an internal transfer pricing system (ITPS) for anesthesia services was introduced in our hospital in 2001. The basic principle of the ITPS is that the department of anesthesia receives reimbursement only for the surgically controlled time, not for anesthesia-controlled time (ACT). A reduction in anesthesia process times is therefore beneficial for the anesthesia department. In this study, we analyzed the ACT (with its parts: preparation before induction, induction, extubation, and recovery room transfer) for 3 yr before and 3 yr after the introduction of the ITPS in 55,776 cases. Furthermore, the anesthesia cases were subsegmented into 10 different anesthesia techniques, and the process times were studied. The average total ACT was reduced from 40.4 +/- 23.5 min in 1998 to 34.3 +/- 21.7 min in 2003. The main effect came from reductions in anesthesia preparation time and recovery room transfer time, whereas induction and extubation time changed little. A significant reduction in average ACT was seen in 7 of 10 analyzed anesthesia techniques, ranging from 4 to 18 min. We conclude that transfer pricing of anesthesia services based on the surgically controlled time can be a successful approach to reduce anesthesia process times.

Suctioning through a double-lumen endotracheal tube helps to prevent alveolar collapse and to preserve ventilation.

OBJECTIVE: Endotracheal suctioning can cause alveolar collapse and impede ventilation. One reason is the gas flow through a single-lumen endotracheal tube (ETT) provoking a gradient between airway opening and tracheal (P(tr)) pressures. Separately extending the patient tubing limbs of a suitable ventilator into the trachea via a double-lumen ETT should maintain P(tr). Can this technique reduce the side effects? DESIGN AND SETTING: Bench and animal studies in a university hospital laboratory. INTERVENTIONS: A lung model was ventilated via single and double-lumen ETTs. Closed-system suctioning was applied with catheters introduced into the single-lumen ETT or the expiratory lumen of the double-lumen ETT via swivel adapter. Seven anesthetized pigs (lungs lavaged) underwent three runs of ventilation and suctioning through (a, b) an 8.0-mm ID single-lumen ETT, (c) a double-lumen ETT (41Ch outer diameter, OD). In (a) the single-lumen ETT was disconnected for suctioning, in (b) and (c) ventilator mode was set to continuous positive airway pressure mode, and the ETTs remained connected. MEASUREMENTS AND RESULTS: Bench: Suction through single-lumen ETTs impaired ventilation and led to strongly negative P(tr) (common: -10 to -20 mbar); the double-lumen ETT technique maintained ventilation and pressures. ANIMALS: Lung gas content (computed tomography, n=4) and arterial oxygen partial pressure, initially 1462+/-65 ml/532+/-76 mmHg, were significantly reduced by suctioning through single-lumen ETT: to 302+/-79 ml/62+/-6 mmHg with disconnection and to 851+/-211 ml/158+/-107 mmHg with closed suction. With double-lumen ETT they remained at 1377+/-95 ml/521+/-56 mmHg. CONCLUSIONS: The double-lumen ETT technique minimizes side effects of suctioning by maintaining P(tr).

Ventilation performance of a mixed group of operators using a new rescue breathing device-the glossopalatinal tube.

INTRODUCTION: We studied how effectively a mixed group of helpers could ventilate a manikin with a new rescue breathing device after a short period of instruction. The device consists of a mouthcap, a "glossopalatinal tube" (GPT) reaching between tongue and palate and a connector for a bag, ventilator or the rescuers mouth. Rather than reaching behind the tongue like an oropharyngeal airway (OP), it is able to scoop the tongue off the posterior pharyngeal wall when tilted by the rescuer. It was compared with a conventional face mask with an OP. METHODS: The study made use of an anaesthesia simulator (MedSim Ltd., Israel) and a manikin. 46 subjects with different professional backgrounds (anaesthesia nurses, medical students, emergency medical technicians (EMTs), physicians training for anaesthesiology) underwent a standard introduction to the GPT and OP (lecture with demonstration on an intubation trainer, illustrated brochure). They ventilated the manikin for 5 min each using the bag plus GPT and the OP plus face mask, respectively, in random order after the simulator had been made apnoeic and the simulated arterial oxygen saturation (S(aO(2))) had dropped to 80%. The actions and the results (tidal volumes (V(t)), S(aO(2))) were recorded on video. The subjects graded difficulty of operation and fatigue on a visual analogue scale (VAS). RESULTS AND CONCLUSIONS: Mean V(t) with the OP plus mask amounted to 463 (230-688 ml), with GPT to 426 (243-610 ml) (median [10-90% percentiles]) (P=0.047). No differences were observed with respect to the time a S(aO(2))> or =90% was maintained (OP plus mask: 255 (139-266 s), GPT: 255 (90-269 s)) or the grades for fatigue (OP plus mask: 58% of VAS, GPT: 48% of VAS, median) and difficulty (OP plus mask: 16% of VAS, GPT: 21% of VAS). Performance and grades were scattered over a wide range. Success with the two devices was correlated, but the subjects judgement tended to diverge. The GPT is an easy to learn alternative to conventional devices and might be helpful in clinical emergencies, including situations of unexpectedly difficult ventilation.

Upper airway patency during ventilation with a new airway device-the glossopalatinal tube.

INTRODUCTION: We studied a new rescue breathing device consisting of a mouthcap and a "glossopalatinal" tube reaching between tongue and palate (the "GPT"), with a connector for a bag, ventilator or rescuers mouth. By tilting the connector in a cranial direction, the tongue can be "scooped" out of the hypopharynx. The study was to test the efficacy and the ease of application of the GPT in anaesthetised patients. It was compared with a conventional face mask with and without an oropharyngeal (OP) airway. METHODS: 19 patients (ethics committee approval, informed consent) anaesthetised for elective surgery were ventilated using an anaesthesia circuit and Ventilog (Draeger) through the GPT and via a face mask (Laerdal) with and without an OP tube. Flow and pressures at the airway opening, in the hypopharynx and the trachea were measured, and the resistance was derived. In addition, the relations of the devices to the anatomical structures were visualised by fibrescope, and ease of operation and fit on the face were scored. RESULTS AND CONCLUSIONS: Inspiratory resistance with the GPT and mask did not differ (1.31+/-0.96 vs. 1.38+/-0.66 kPa s/l at 1 l/s, mean+/-standard deviation (S.D.); reduction of resistance by "scooping" the tongue through angulation of the GPT (to 0.64+/-0.32; P<0.05 vs. GPT without angulation) was equivalent to that by an OP tube used with the mask (to 0.68+/-0.26; P<0.05 vs. mask solo). Pharyngoscopy showed that the effectiveness of the GPT depended on the individual anatomy. The angulating motion caused some fatigue. The GPT is an alternative to established breathing adjuncts; despite not protruding into the pharynx it can enhance airway patency like an OP.