Norbert Weiler

Respiratory mechanics, gastric insufflation pressure, and air leakage of the laryngeal mask airway.

A potential risk of the laryngeal mask airway (LMA) is incomplete mask seal, which causes air leakage or insufflation of air into the stomach.The objective of the present study was to assess respiratory mechanics, quantify air leakage, and measure gastric air insufflation in patients ventilated via the LMA. Thirty patients were studied after induction of anesthesia but prior to any surgical manipulations. After the insertion of the LMA, patients were ventilated with increasing tidal volumes until one of the three following end points were reached: 1) gastric air insufflation, 2) airway pressure >40 cm H2 O, or 3) limitation of further increase in tidal volume by air leakage. The following variables were determined: inspired volume (V (I)), expired volume (VE), maximum inspiratory pressure (Pmax), airway pressure at gastric inflation (Pinfl), respiratory time constant (RC), compliance (C), resistance (R), and leakage fraction (FL). Respiratory mechanics were in the physiological range. Gastric insufflation occurred in 27% of the patients at inspiratory pressures between 19 and 33 cm H2 O. Air leakage of more than 10% was evident at inspiratory pressures between 25 and 34 cm H2 O. The end point of 40 cm H2 O airway pressure was reached in only three patients. We conclude that the LMA is not better in preventing airway pressure transmission to the esophagus than a conventional face mask. However, a high FL is associated with reduced gastric air insufflation. (Anesth Analg 1997;84:1025-8)

Assessment of pulmonary mechanics and gastric inflation pressure during mask ventilation.

INTRODUCTIONnMask ventilation is a procedure routinely used in emergency medicine. Potential hazards are inadequate alveolar ventilation and inflation of the stomach with air, leading to subsequent regurgitation and aspiration. The aim of this study was to measure lung function and gastric inflation pressures during mask ventilation.nnnMETHODSnFor this purpose, 31 patients scheduled for routine urological procedures were studied during induction of anesthesia. Lung function was assessed by recording respiratory flow and pressure directly at the face mask. Gastric inflation was observed with a microphone taped to the epigastric area.nnnRESULTSnGastric inflation occurred in 22 of the 31 patients. Mean gastric inflation pressure was 27.5 +/- 6.55 cm H2O, mean compliance was 67 +/- 24.1 ml/cm H2O, mean resistance was 17.4 +/- 6.41 cm H2O/L/sec, and the mean respiratory time constant was 1.1 +/- 0.26 seconds.nnnCONCLUSIONSnThese data suggest that inspiratory pressure be limited to 20 cm H2O, and that an inspiratory time of at least four times the respiratory time constant be allowed. Monitoring airway pressure and gastric inflation is a simple technique that may improve the safe-ty of patients during mask ventilation.

3He-MRI-based measurements of intrapulmonary pO2 and its time course during apnea in healthy volunteers: first results, reproducibility, and technical limitations

We applied a recently developed method of following the time course of the intrapulmonary oxygen partial pressure pO2(t) during apnea by 3He MRI to healthy volunteers. Using two imaging series with different interscan times during two breathholds (double acquisition technique), relaxation of 3He due to paramagnetic oxygen and depolarization by RF pulses were discriminated. In all four subjects, the temporal evolution of pO2 was found to be linear, and was described by an initial partial pressure p0 and a decrease rate R. Also, regional differences of both p0 and R were observed. A correlation between p0 and R was apparent. Finally, we discuss limitations of the double acquisition approach. Copyright

3He MRI in healthy volunteers: preliminary correlation with smoking history and lung volumes

MRI with hyperpolarized helium‐3 (3He) provides high‐resolution imaging of ventilated airspaces. The first aim of this 3He‐study was to compare observations of localized signal defects in healthy smokers and non‐smokers. A second aim was to describe relationships between parameters of lung function, volume of inspired 3He and signal‐to‐noise ratio. With Ethics Committee approval and informed consent, 12 healthy volunteers (seven smokers and five non‐smokers) were studied. Imaging was performed in a 1.5u2005T scanner using a two‐dimensional FLASH sequence at 30V transmitter amplitude (TR/TE/αu2005=u200511 ms/4.2 ms/<10°). Known amounts of 3He were inhaled from a microprocessor‐controlled delivery device and imaged during single breath‐holds. Images were evaluated visually, and scored using a prospectively defined ‘defect‐index’. Signal‐to‐noise ratio of the images were correlated with localization, 3He volumes and static lung volumes. Due to poor image quality studies of two smokers were not eligible for the evaluation. Smokers differed from non‐smokers in total number and size of defects: the ‘defect‐index’ of smokers ranged between 0.8 and 6.0 (medianu2005=u20051.1), that of non‐smokers between 0.1 and 0.8 (medianu2005=u20050.4). Intraindividually, an anteroposterior gradient of signal‐to‐noise ratio was apparent. Signal‐to‐noise ratio correlated with the estimated amount of hyperpolarization administered (ru2005=u20050.77), but not with static lung volumes. We conclude that 3He MRI is a sensitive measure to detect regional abnormalities in the distribution of ventilation in clinically healthy persons with normal pulmonary function tests. Copyright

Dynamic 19F-MRI of pulmonary ventilation using sulfur hexafluoride (SF6) gas

A new method for dynamic imaging of pulmonary wash‐in and wash‐out kinetics of inhaled sulfur hexafluoride (SF6) gas was developed. Measurements at the fluorine‐19 Larmor frequency were performed in pigs using a gradient echo pulse sequence with 0.5 ms echo time and a measurement time of 9.1 s per image. Dynamic MRI was performed during wash‐in and wash‐out of SF6 gas in mechanically ventilated porcine lungs. A postprocessing strategy was developed for quantitative determination of wash‐out time constants in the presence of noise. Mean wash‐out constants were 4.78 ± 0.48 breaths vs. 4.33 ± 0.76 breaths for left and right lung when ventilation was performed with low tidal volume, and 1.73 ± 0.16 breaths vs. 1.85 ± 0.11 breaths with high tidal volume ventilation. In conclusion, breath‐hold MRI of SF6 gas is feasible in large animals. Moreover, regional wash‐in and wash‐out kinetics of SF6 can be determined noninvasively with this new method. Potential human applications are discussed. Magn Reson Med 45:605–613, 2001.

19F-MRI of perflubron for measurement of oxygen partial pressure in porcine lungs during partial liquid ventilation.

A method for in vivo measurement of oxygen partial pressure (pO2) in porcine lungs during partial liquid ventilation (PLV) with perflubron (PFOB) was developed. A pulse sequence for high‐resolution MRI of the distribution of PFOB in the lung after intratracheal administration was developed as well. Moreover, quantitative measurements of longitudinal relaxation time T1 of 19F resonances for assessment of regional pO2 are described. Due to the need to acquire data during a single expiratory breathhold, only low SNRs were achieved in vivo. Therefore, simulations were performed to investigate the influence of background noise on T1 values calculated from data with low SNR. Based on these simulations, a postprocessing strategy was developed to correct for systematic errors by background noise prior to quantitative analysis. Results of a pilot study in pigs under conditions of PLV are presented. Magn Reson Med 47:82–89, 2002.

Computed tomography-based tracheobronchial image reconstruction allows selection of the individually appropriate double-lumen tube size

OBJECTIVESnTo determine whether individualized selection of double-lumen tubes or alternatives based on three-dimensional reconstruction of the tracheobronchial image from routine preoperative computed tomography (CT) scans leads to clinically appropriate choices.nnnDESIGNnProspective observational study; comparison to historic controls.nnnSETTINGnAnesthesia and radiology facilities of a university medical center.nnnPARTICIPANTSnForty-nine patients undergoing thoracic surgery requiring one-lung ventilation.nnnINTERVENTIONSnThree-dimensional image reconstruction of individual tracheobronchial anatomy was performed from routine preoperative spiral CT scans as well as from scans of five left-sided and four right-sided double-lumen tubes. Results of image-based tube size selection were compared with literature recommendations. Prospectively, individualized tube selection was performed by superimposition of printed transparencies of tubes over the tracheobronchial system and was validated using bronchoscopic and clinical criteria (n = 24).nnnMEASUREMENTS AND MAIN RESULTSnThree-dimensional reconstruction visualized individual anatomy with good accuracy and resolution. Correlations between patient morphology and tracheobronchial dimensions were weak (height versus mainstem bronchial diameters: r < 0.50). In 11 of 48 patients (23%). CT-fitted double-lumen tube sizes would have differed from a conventional height-based and gender-based selection. Individual, prospective, CT-based double-lumen tube selection was associated with (1) good fit and positioning confirmed by fiberoptic bronchoscopy, (2) adequate bronchial cuff seal volumes, (3) complete lung separation, and (4) oxygenation and ventilation parameters during one-lung ventilation similar to those with conventional size selection. In one patient, three-dimensional CT study allowed noninvasive evaluation of a tracheal stenosis precluding double-lumen tube placement.nnnCONCLUSIONnIndividualized selection of double-lumen tube size using CT-based reconstructions of tracheobronchial anatomy leads to clinically appropriate choices. Risks resulting from variations in tracheobronchial morphology are recognized in advance.

Laryngeal mask airway position and the risk of gastric insufflation

A potential risk of the laryngeal mask airway (LMA) is an incomplete mask seal causing gastric insufflation or oropharyngeal air leakage.The objective of the present study was to assess the incidence of LMA malpositions by fiberoptic laryngoscopy, and to determine their influence on gastric insufflation and oropharyngeal air leakage. One hundred eight patients were studied after the induction of anesthesia, before any surgical manipulations. After clinically satisfactory LMA placement, tidal volumes were increased stepwise until air entered the stomach, airway pressure exceeded 40 cm H2 O, or air leakage from the mask seal prevented further increases in tidal volume. LMA position in relation to the laryngeal entrance was verified using a flexible bronchoscope. The overall incidence of LMA malpositions was 40% (43 of 108). Gastric air insufflation occurred in 19% (21 of 108), and in 90% (19 of 21) of these patients, the LMA was malpositioned. Oropharyngeal air leakage occurred in 42%, and was independent of LMA position. We conclude that clinically unrecognized LMA malposition is a significant risk factor for gastric air insufflation. Implications: Routine placement of laryngeal mask airways does not require laryngoscopy. In our study, fiberoptic verification of mask position revealed suboptimal placement in 40% of cases. Such malpositioning considerably increased the risk of gastric air insufflation. (Anesth Analg 1998;86:867-71)

Dynamic ventilation 3He-magnetic resonance imaging with lung motion correction: Gas flow distribution analysis

Gast KK, Puderbach MU, Rodriguez I, et al. Dynamic ventilation 3He-MRI with lung motion correction: gas flow distribution analysis. Invest Radiol 2002;37:126–134. rationale and objectives. Software was developed to correct for lung motion to improve the description of hyperpolarized 3He gas distribution in the lung. methods. Five volunteers were studied by dynamic ventilation 3He-MRI using an ultrafast FLASH 2D sequence with a temporal resolution of 128 milliseconds. Signal kinetics were evaluated in the trachea and seven parenchymal Regions of Interest. Reference ranges for healthy subjects were defined for motion-corrected and uncorrected images. results. Motion correction was successfully performed. Reference ranges were 0.11–1.21 seconds for tracheal transit time, 0–0.02 seconds for trachea-alveolar interval, 0.22–0.62 seconds for alveolar rise time and 0–76.6 arbitrary units for alveolar amplitude for motion corrected images, and 0–1.09 seconds, 0–0.11 seconds, 0.26–0.85 seconds, 46.4–99.8 arbitrary units for uncorrected images. conclusions. Evaluation of 3He-distribution in the lung using motion correction of dynamic 3He-ventilation imaging is feasible and gives more narrow reference ranges.

Distribution of ventilation in lung transplant recipients: evaluation by dynamic 3He-MRI with lung motion correction.

Rationale and objectives.The ability of motion corrected dynamic 3He-magnetic resonance imaging (MRI) to discriminate distributional patterns of inhaled hyperpolarized 3He between different groups of lung transplant recipients was evaluated. MethodsAn ultrafast low-angle shot 2D sequence (temporal resolution 128 ms) was used for ventilation 3He-MRI of 11 single and 6 double lung transplant recipients. After digital motion correction, signal kinetics were evaluated in a tracheal and 7 pulmonary regions of interest. Results from grafts and native lungs as well as from normal and rejected grafts were compared with each other and to reference values from healthy subjects. ResultsIn emphysema patients, median alveolar rise time, a parameter for increase of alveolar signal, was 0.28 seconds for the graft and 0.48 seconds for the native lung, in fibrosis patients its median was 0.46 seconds for the graft and 0.21 seconds for the native lung. In double lung recipients, alveolar rise time was 0.29 seconds in normal and clinically rejected grafts. ConclusionsDynamic ventilation 3He-MRI discriminated normal lung grafts from diseased native lungs in single lung recipients. Graft rejection in double lung recipients could not be discriminated.