Is pulmonary resistance constant, within the range of tidal volume ventilation, in patients with ARDS?

When managing patients with acute respiratory distress syndrome (ARDS), respiratory system compliance is usually considered first and changes in resistance, although recognized, are neglected. Resistance can change considerably between minimum and maximum lung volume, but is generally assumed to be constant in the tidal volume range (V(T)). We measured resistance during tidal ventilation in 16 patients with ARDS or acute lung injury by the slice method and multiple linear regression analysis. Resistance was constant within V(T) in only six of 16 patients. In the remaining patients, resistance decreased, increased or showed complex changes. We conclude that resistance within V(T) varies considerably from patient to patient and that constant resistance within V(T) is not always likely.

[Anaplastic astrocytomas of the medulla oblongata. A rare differential diagnosis in orthostatic hypotension]

HISTORY AND PHYSICAL EXAMINATION

A 39-year-old women was admitted for evaluation of dizziness and hypotension. During standing the blood pressure dropped from 130/80 to 80/40 mmHg. Ten weeks before admission she had recurrent cerebral convulsions. Neurological evaluation showed a slight left hemiparesis. Computed tomography of the brain revealed a 1.5 +/- 1 cm cyst in the left tempral region. It was thought that the cause of the recurrent convulsions was alcohol abuse. During the next few weeks orthostatic hypotension increased and she was not able to work.

INVESTIGATIONS

At admission abnormal findings included hypotension, horizontal nystagmus, and deviation of the soft palate to the right. After the patient was brought to an upright position during standardized passive tilt testing she showed a defect in the sympathetic limb of the baroreceptor reflex arc. Head magnetic resonance tomography showed a signal-enhancing tumour in the cervicomedullary region.

TREATMENT AND CLINICAL COURSE

Before a planned biopsy could be performed the patient died of respiratory arrest. Postmortem examination revealed an anaplastic grade III astrocytoma extending form the pons to the medulla oblongata.

CONCLUSION

Upright tilting leads to pooling of blood in the legs. One of the normal compensatory responses is a reflex tachycardia which our patient did not show as a sign of an afferent defect. Patients with orthostatic hypotension as a prominent symptom should be investigated with a standardized tilt test. In special patients, additional neurological investigations are necessary.

Volume-dependent compliance and ventilation-perfusion mismatch in surfactant-depleted isolated rabbit lungs

OBJECTIVE

Volume-dependent alterations of lung compliance are usually studied over a very large volume range. However, the course of compliance within the comparably small tidal volume (intratidal compliance-volume curve) may also provide relevant information about the impact of mechanical ventilation on pulmonary gas exchange. Consequently, we determined the association of the distribution of ventilation and perfusion with the intratidal compliance-volume curve after modification of positive end-expiratory pressure (PEEP).

DESIGN

Repeated measurements in randomized order.

SETTING

An animal laboratory.

SUBJECTS

Isolated perfused rabbit lungs (n = 14).

INTERVENTIONS

Surfactant was removed by bronchoalveolar lavage. The lungs were ventilated thereafter with a constant tidal volume (10 mL/kg body weight). Five levels of PEEP (0-4 cm H2O) were applied in random order for 20 mins each.

MEASUREMENTS AND MAIN RESULTS

The intratidal compliance-volume curve was determined with the slice method for each PEEP level. Concurrently, pulmonary gas exchange was assessed by the multiple inert gas elimination technique. At a PEEP of 0-1 cm H2O, the intratidal compliance-volume curve was formed a bow with downward concavity. At a PEEP of 2 cm H2O, concavity was minimal or compliance was almost constant, whereas higher PEEP levels (3-4 cm H2O) resulted in a decrease of compliance within tidal inflation. Pulmonary gas exchange did not differ between PEEP levels of of 0, 1, and 2 cm H2O. Pulmonary shunt was lowest and perfusion of alveoli with a normal ventilation-perfusion was highest at a PEEP of 3-4 cm H2O. Deadspace ventilation did not change significantly but tended to increase with PEEP.

CONCLUSIONS

An increase of compliance at the very beginning of tidal inflation was associated with impaired pulmonary gas exchange, indicating insufficient alveolar recruitment by the PEEP level. Consequently, the lowest PEEP level preventing alveolar atelectasis could be detected by analyzing the course of compliance within tidal volume without the need for total lung inflation.

Extracorporeal membrane oxygenation: a ten-year experience

BACKGROUND

Extracorporeal membrane oxygenation (ECMO) is a supportive therapy used for severe acute respiratory distress syndrome (ARDS). We present outcome, clinical parameters, and complications in a cohort of 245 ARDS patients of whom 62 were treated with ECMO.

METHODS

Data of all ARDS patients were prospectively collected between 1991 and 1999. Outcome and clinical parameters of patients treated with and without ECMO were evaluated.

RESULTS

One hundred thirty-eight patients were referred from other hospitals, 107 were primarily located in our hospital. About one fourth of these patients were treated with ECMO. The survival rate was 55% in ECMO patients and 61% in non-ECMO patients.

CONCLUSIONS

ECMO is a therapeutic option for patients with severe ARDS, likely to increase survival. However, a randomized controlled study proving its benefit is still awaited. Until the development of a causal or otherwise superior therapy ECMO should be used in selected patients.

Respiratory comfort and breathing pattern during volume proportional assist ventilation and pressure support ventilation: a study on volunteers with artificially reduced compliance

OBJECTIVE

To assess respiratory comfort and associated breathing pattern during volume assist (VA) as a component of proportional assist ventilation and during pressure support ventilation (PSV).

DESIGN

Prospective, double-blind, interventional study.

SETTING

Laboratory.

SUBJECTS

A total of 15 healthy volunteers (11 females, 4 males) aged 21-31 yrs.

INTERVENTIONS

Decreased respiratory system compliance was simulated by banding of the thorax and abdomen. Volunteers breathed via a mouthpiece with VA and PSV each applied at two levels (VA, 8 cm H2O/L and 12 cm H2O/L; PSV, 10 cm H2O and 15 cm H2O) using a positive end-expiratory pressure of 5 cm H2O throughout. The study was subdivided into two parts. In Part 1, volunteers breathed three times with each of the four settings for 2 mins in random order. In Part 2, the first breath effects of multiple, randomly applied mode, and level shifts were studied.

MEASUREMENTS AND MAIN RESULTS

In Part 1, the volunteers were asked to estimate respiratory comfort in comparison with normal breathing using a visual analog scale. In Part 2, they were asked to estimate the change of respiratory comfort as increased, decreased, or unchanged immediately after a mode shift. Concomitantly, the respiratory pattern (change) was characterized with continuously measured tidal volume, respiratory rate, pressure, and gas flow. Respiratory comfort during VA was higher than during PSV. The higher support level was less important during VA but had a major negative influence on comfort during PSV. Both modes differed with respect to the associated breathing pattern. Variability of breathing was higher during VA than during PSV (Part 1). Changes in respiratory variables were associated with changes in respiratory comfort (Part 2).

CONCLUSIONS

For volunteers breathing with artificially reduced respiratory system compliance, respiratory comfort is higher with VA than with PSV. This is probably caused by a better adaptation of the ventilatory support to the volunteer's need with VA.

Continuous calculation of intratracheal pressure in the presence of pediatric endotracheal tubes

OBJECTIVE

To measure the pressure-flow relationship of pediatric endotracheal tubes (ETTs) in trachea models, to mathematically describe this relationship, and to evaluate in trachea/lung models a method for calculation of pressure at the distal end of the ETT (Ptrach) by subtracting the flow-dependent pressure drop across the ETT from the airway pressure measured at the proximal end of the ETT.

DESIGN

Trachea models and trachea/lung models.

SETTING

Research laboratory in a university medical center.

INTERVENTIONS

The pressure-flow relationship of pediatric ETTs (inner diameter, 2.5-6.5 mm) was determined using a physical model consisting of a tube connector, an anatomically curved ETT, and an artificial trachea. The model was ventilated with sinusoidal gas flow (12-60 cycles/min). The coefficients of an approximation equation considering ETT resistance and inertance were fitted separately to the measured pressure-flow curves for inspiration and expiration. Calculated Ptrach was compared with directly measured Ptrach in mechanically ventilated physical trachea/lung models.

MEASUREMENTS AND MAIN RESULTS

The pressure-flow relationship was considerably nonlinear and showed hysteresis around the origin caused by the inertia of accelerated gas. ETT inertance ranged from 0.1 to 0.4 cm H2O/L x sec2 (inner diameter, 6-2.5 mm). The abrupt change in cross-sectional area at the tube connector caused an inspiration-to-expiration asymmetry. Calculated and measured Ptrach were within +/- 1 cm H2O. Correspondence between measured and calculated Ptrach is improved even further when the ETT inertance is taken into account.

CONCLUSIONS

Ptrach can continuously be monitored in the presence of pediatric ETT by combining ETT coefficients and the flow and airway pressure continuously measured at the proximal end of the ETT.

Breathing pattern associated with respiratory comfort during automatic tube compensation and pressure support ventilation in normal subjects

BACKGROUND

Automatic tube compensation (ATC) is a new option to support spontaneously breathing tracheally intubated patients. We have previously demonstrated an increased respiratory comfort compared to pressure support ventilation (PSV) in volunteers. Here we characterized the breathing pattern during ATC associated with respiratory comfort in comparison to PSV. Furthermore, we studied whether ATC can be substituted by a simple modification of PSV.

METHODS

We exposed 10 volunteers breathing through a 7.5 mm endotracheal tube via mouthpiece to PSV with 1) immediate and 2) delayed pressure rise and to 3) ATC. Immediate changes of the respiratory pattern after mode shifts were analyzed in detail. Furthermore, the volunteers were instructed to indicate changes in comfort after transitions between these modes as increased, unchanged, or decreased.

RESULTS

Decreased comfort was associated with a substantial increase of tidal volume, minute ventilation, gas flow, and pressure. No differences in respiratory comfort were perceived between immediate and delayed pressure rise during PSV.

CONCLUSION

PSV resulted in excessive tidal volumes and airflow, which was perceived as discomfort. This cannot be avoided by a delayed pressure rise but can be by the more comfortable ATC. ATC seems to adapt better to the ventilatory demand than PSV.

Reduction of the bacterial load by the silver-coated endotracheal tube (SCET), a laboratory investigation

Microaspiration enabled by high-volume-low-pressure cuffed endotracheal tubes is the most likely explanation for ventilator-associated pneumonia. To decontaminate the secretion at the proximal end of the cuff we developed a silver-coated endotracheal tube (SCET). In an in vitro model we investigated the efficacy of SCET to lower the bacterial load of secretion and aspirate. We developed a continuously contaminated and mechanically ventilated oropharynx-larynx-lung model to investigate the reduction of the bacterial count by SCET compared to controls. The model was continuously contaminated via the oropharynx-larynx with Pseudomonas aeruginosa ATCC 27853. During the investigation period of 50 hours the bacterial count of oropharynx-larynx and lung was measured as colony-forming-units/ml. In addition, the characteristic curve of silver ion release of SCET was determined. SCET significantly reduced the bacterial count in oropharynx-larynx at all timepoints (p < 0.05). In lung the bacterial count was significantly lower beginning with the 36th hour of recording (p < 0.05). A reduction of greater than 2 log was found from 28 hours on in oropharynx-larynx and from 50 hours on in lung. The release of silver ions was very rapid and was described by a mono-exponential function with a time-constant tau of about 60 minutes and a saturation concentration of 200 +/- 80 microg/l. SCET showed a significant inhibition of growth of P. aeruginosa in the continuously contaminated and mechanically ventilated oropharynx-larynx-lung model. SCET by thus might be helpful in reducing ventilator-associated pneumonia.

Ventilation-perfusion mismatch after lung ischemia-reperfusion. Protective effect of nitric oxide

Lung ischemia-reperfusion provokes pulmonary hypertension and increased microvascular permeability with subsequent edema formation and hypoxemia. We exposed buffer-perfused rabbit lungs to 120 and 180 min of warm ischemia. After reperfusion, gas exchange disturbances were analyzed by the multiple inert gas elimination technique (MIGET). Additionally, ischemic lungs were treated with different doses of inhaled nitric oxide (NO) throughout reperfusion. Reperfusion provoked a transient pulmonary artery pressure elevation, followed by progressive pulmonary edema formation. After 120 min of ischemia, severe ventilation-perfusion (V A/Q) mismatch developed within 15 min of reperfusion, with the appearance of low V A/Q areas and marked broadening of both perfusion and ventilation distribution in the midrange V A/Q regions. In parallel, shunt flow increased from less than 2% to approximately 17%. Inhalation of NO suppressed the pressor response, edema formation, as well as V A/Q mismatch and shunt flow. Concentrations of 10 and 50 ppm NO were equipotent, surpassing the efficacy of 1 or 250 ppm NO. Inhalation of NO, however, did not protect from the overwhelming gas exchange and fluid balance disturbances provoked by 180 min ischemia. In conclusion, severe abnormalities in gas exchange occurred rapidly upon reperfusion of ischemic lungs. Prophylactic NO inhalation may be considered for maintenance of gas exchange in settings of ischemia-reperfusion including lung transplantation.

Volume-dependent compliance in ARDS: proposal of a new diagnostic concept

OBJECTIVE

Adaptation of ventilator settings to the individual's respiratory system mechanics requires information about the pressure-volume relationship and the change of compliance which is dependent on inflated volume. Unfortunately, established methods of obtaining this information are invasive and time-consuming, and, therefore, not well suited for clinical routine. We propose a new standardized diagnostic concept based on the recently developed slice method. This multiple linear regression method (MLR) determines volume-dependent respiratory system compliance (C(SLICE)) within the tidal volume (V(T)) during ongoing mechanical ventilation. The impact of a ventilator strategy, recommended by a consensus conference, on the course of compliance within V(T) was investigated in patients with the acute respiratory distress syndrome (ARDS) or acute lung injury (ALI).

DESIGN

Prospective observational study.

SETTING

Intensive care unit of a university hospital.

PATIENTS

14 ARDS patients, 2 patients with ALI.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

After measurement of flow and airway pressure and calculation of tracheal pressure, C(SLICE) was determined. The resulting course of C(SLICE) within V(T) was estimated using a mathematical algorithm. C(SLICE) data were compared to those obtained by standard MLR. We found decreasing C(SLICE) mainly in the upper part of V(T) in all patients. In 7 patients, we found an additional increasing C(SLICE) mainly in the lower part of V(T).

CONCLUSIONS

C(SLICE) was not constant in patients with ARDS/ALI whose lungs were ventilated according to consensus conference recommendations. The proposed diagnostic concept may serve as a new tool to obtain a standardized estimation of respiratory system compliance within V(T) non-invasively without interfering with ongoing mechanical ventilation.

Unintended inhalation of nitric oxide by contamination of compressed air: physiologic effects and interference with intended nitric oxide inhalation in acute lung injury

BACKGROUND

Compressed air from a hospital's central gas supply may contain nitric oxide as a result of air pollution. Inhaled nitric oxide may increase arterial oxygen tension and decrease pulmonary vascular resistance in patients with acute lung injury and acute respiratory distress syndrome. Therefore, the authors wanted to determine whether unintentional nitric oxide inhalation by contamination of compressed air influences arterial oxygen tension and pulmonary vascular resistance and interferes with the therapeutic use of nitric oxide.

METHODS

Nitric oxide concentrations in the compressed air of a university hospital were measured continuously by chemiluminescence during two periods (4 and 2 weeks). The effects of unintended nitric oxide inhalation on arterial oxygen tension (n = 15) and on pulmonary vascular resistance (n = 9) were measured in patients with acute lung injury and acute respiratory distress syndrome by changing the source of compressed air of the ventilator from the hospital's central gas supply to a nitric oxide-free gas tank containing compressed air. In five of these patients, the effects of an additional inhalation of 5 ppm nitric oxide were evaluated.

RESULTS

During working days, compressed air of the hospital's central gas supply contained clinically effective nitric oxide concentrations (> 80 parts per billion) during 40% of the time. Change to gas tank-supplied nitric oxide-free compressed air decreased the arterial oxygen tension by 10% and increased pulmonary vascular resistance by 13%. The addition of 5 ppm nitric oxide had a minimal effect on arterial oxygen tension and pulmonary vascular resistance when added to hospital-supplied compressed air but improved both when added to tank-supplied compressed air.

CONCLUSIONS

Unintended inhalation of nitric oxide increases arterial oxygen tension and decreases pulmonary vascular resistance in patients with acute lung injury and acute respiratory distress syndrome. The unintended nitric oxide inhalation interferes with the therapeutic use of nitric oxide.

The Traveling Shutter Wave analyses non-linear compliance during mechanical ventilation

Mechanical ventilation is an important, often life-saving component of modern intensive care medicine. However, it may further aggravate pulmonary pathology by endinspiratory overdistension of the alveoli or by their endexpiratory collapse. To prevent both the ventilator may be adjusted based on the slope of the pressure-volume curve, named as compliance, which is often determined by a stepwise inflation of the lungs. This maneuver gained no widespread clinical acceptance because of being cumbersome and invasive. Therefore, we developed a modification of the well known interrupter technique - the Traveling Shutter Wave. A wave of short-term (300 ms) occlusions "travels" over the tidal volume range. Differential compliance is calculated by division of volume and pressure differences between two adjacent occlusion maneuvers. The technique is well suited for the clinical setting because the ventilatory pattern does not need to be changed. This manuscript describes the realization of the Traveling Shutter Wave as well as its application in two patients.

[Importance and perspectives of breath analysis]

Quantitative chemical analysis of volatile constituents in exhaled gas can provide useful insights into biochemical processes in the body. Relations between the chemical composition of human exhaled air and inflammatory processes, states of high oxidative activity, inhalation or ingestion of various noxious substances as well as diseases like ARDS, pneumonia or sleep apnea have been described. Clinical interpretation of these findings, however, remains difficult because definite marker substances for certain diseases could not yet be identified, substance concentrations in the exhaled air change under various conditions, and results vary in a wide range. Because of very low substance concentrations in the exhaled air sophisticated analytical techniques are necessary. Analyses are hampered by high water content and numerous contaminants in the samples. A clear distinction has to be made between substances originating from within the patient and those coming from outside of the body. Despite all these limitations there is no doubt that relations exist between the chemical composition of human exhaled air and clinical parameters. Upcoming new analytical techniques will allow more efficient extraction and preconcentration of substances in minute concentrations. Fast track bedside analyses will shortly become possible with the introduction of miniature gas chromatographic and mass spectrometric equipment. This will open a new area for clinical and basic research.

In vivo evaluation of a new method for chemical analysis of volatile components in the respiratory gas of mechanically ventilated patients

Using the volatile anaesthetic isoflurane as a marker substance a gas chromatographic method for analysis of exhaled gas in mechanically ventilated patients was evaluated. Twelve patients with and 10 patients without preceding isoflurane exposure and seven blank respiratory circuits were investigated. Respiratory gas was sampled at four different sites in the respiratory tubing system. Analysis of volatile components was based upon adsorption onto activated charcoal, desorption by means of microwave energy, gas chromatographic separation and flame ionisation or mass spectrometric detection. Isoflurane concentrations in the blank respiratory circuits, in the control group, and in the inspiratory limb of patients with preceding isoflurane inhalation were at the limit of detection (<10(-12) mol/l). In the isoflurane group, isoflurane concentrations in the inspiratory and expiratory limb differed considerably. Using the method described, substances at very low concentrations could be identified as originating in the patient or as coming from the gas delivery system.

[In Process Citation]

Soaring health care costs are also forcing surgeons and anaesthesiologists to introduce practice patterns that allow more efficient use of expensive hospital resources. Because perioperative ward expenditures account for approximately one-third of the hospital costs incurred for surgical inpatients, with personnel costs being a major component, interventions that decrease the length of stay can result in considerable savings without affecting the quality of care. Interventious that can reduce the length of stay are outpatient preoperative evaluation and same-day admission, improvement of OR efficiency and sufficient cost-effective facilities for qualified postoperative care.

Evaluation of forced oscillation technique for early detection of airway obstruction in sleep apnea: a model study

The forced oscillation technique (FOT) is a non-invasive method which may be suitable for assessing upper airway obstruction in obstructive sleep apnea/hypopnea syndrome (OSAS) patients. The aim of this study was to determine in vitro if FOT can provide an early detection index of total or partial upper airway occlusion. A respiratory system analog was developed which includes an upper airway analog that allows simulation of upper airway collapse (thus mimicking the situation in patients with OSAS). We simulated different degrees of upper airway obstructions ranging from 0 (unobstructed airways) to 1 (total occlusion). Furthermore, we imitated the collapsible upper airway wall by means of elastic membranes with ten different wall compliances ranging from 3.3 x 10(-4) to 1 1/mbar. For the two stiffest rubber membranes (Cwall = 0.01 and 3.3 x 10(-4) l/mbar) the absolute value of the impedance (¿Z¿) showed a marked increase for obstructions greater than 0.6. For the two membranes with the highest wall compliances (Cwall = 0.03 and 1 1/mbar) obstructions with an increase in ¿Z¿ could not be detected before the obstruction reached 0.8. For degrees of obstruction less than 0.6 the phase angle of collapsible membranes with low compliance (stiff airway wall) were about 1.5pi which significantly differed from phase angles of 1.77pi measured in membranes with high compliance (elastic airway wall); p < 0.01. We hypothesized that stiffness of upper airway walls corresponds with their muscle tone, i.e., stiff airway walls are related with high muscle tone and vice versa. Thus, a decrease in upper airway muscle activity would cause an increase of upper airway wall elasticity that enables upper airway collapse. As a consequence the phase angle phi could be expected to change from values characterizing stiff membranes to values characterizing more elastic membranes which could be used as early indicator for obstructive respiratory events. We have frequently observed such changes in morphology of phi(t) data obtained from patients with OSAS.

Detection of endotracheal tube obstruction by analysis of the expiratory flow signal

OBJECTIVE

Acute obstruction of endotracheal tubes (ETT) increases airway pressure, decreases tidal volume, increases the risk of dynamic hyperinflation by prolonging the duration of passive expiration, and prevents reliable calculation of tracheal pressure. We propose a computer-assisted method for detecting ETT obstruction during controlled mechanical ventilation. The method only requires measurement of the expiratory flow.

DESIGN

Computer simulation; prospective study in two cases; retrospective study in one case and in seven patients with the adult respiratory distress syndrome (ARDS).

SETTING

Laboratory of the Section of Experimental Anaesthesiology (University of Freiburg); surgical adult intensive care units in a university hospital (University of Basel) and in a university affiliated hospital (Zentralklinikum Augsburg).

PATIENTS

3 patients with partial ETT or bronchial obstructions and 7 ARDS patients.

MEASUREMENTS AND RESULTS

Expiratory flow was measured using a pneumotachograph and integrated to obtain expiratory volume. The time-constant of passive expiration (tauE) as a function of expired volume [tauE(V(E)) function] was calculated from the expiratory volume/flow curve. We investigated the tauE(V(E)) function of data obtained from: (1) computer simulation of mechanically ventilated homogeneous and inhomogeneous lungs intubated with ETTs of different sizes; (2) one patient with an artificial ETT obstruction of 7.5 and 25% of the cross-sectional area of the ETT (case 1); (3) one patient with ETT obstruction due to secretions (case 2); (4) one patient with acute bronchial constriction (case 3); (5) seven ARDS patients who showed an increase in airway resistance of more than 2 cm H2O x s/l. It was found that an ETT obstruction caused an increase in tauE in early expiration (at high flow), whereas tauE in late expiration was virtually unchanged. The reason for this is the flow dependency of the increase in ETT resistance produced by ETT obstruction. Unlike ETT obstruction, an increase in pure airway resistance produced an increase in tauE throughout expiration.

CONCLUSIONS

An ETT obstruction can be reliably distinguished from an increase in pure airway resistance by a characteristic pattern change in the tauE(V(E)) function, which can be detected easily even by an automated pattern recognition system.

Method for analysis of exhaled air by microwave energy desorption coupled with gas chromatography-flame ionization detection-mass spectrometry

A method for chemical analysis of volatile constituents in exhaled air of mechanically ventilated patients is described. Exhaled substances are adsorbed and concentrated onto activated charcoal, desorbed by microwave energy and transferred into a gas chromatograph for separation without prior cryofocusing. Substances are identified by flame ionisation detection and mass spectrometry. This method yields reproducible results and is well suited for clinical studies.

Application of a new method for analysis of exhaled gas in critically ill patients

OBJECTIVE

Application of a new method for analysis of exhaled gas in critically ill patients.

DESIGN

Open study.

SETTING

Surgical intensive care unit of an university hospital.

PATIENTS

Thirty-seven consecutive, critically ill, mechanically ventilated patients.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Chemical analysis of the patient's exhaled gas was based upon substance adsorption and concentration onto activated charcoal, microwave desorption and gas chromatographic separation. Patients with acute respiratory distress syndrome (ARDS) exhaled less isoprene than those without ARDS [9.8 (8.2-21.6) vs 21.8 (13.9-41.4) nmol/m2 per min [median (95% confidence interval)], p = 0.04]. In patients who developed pulmonary infection, pentane elimination increased from 0.4 (0.0-5.4) to 2.7 (0.6-6.1, p = 0.05) nmol/m2 per min and isoprene elimination decreased from 5.2 (0-33) to 5.0 (0-17, p = 0.05) nmol/m2 per min, resulting in a significant increase in pentane/isoprene ratio from 0.1 (0-0.3) to 0.4 (0-15, p = 0.007) when compared to patients without pulmonary infection.

CONCLUSIONS

The new method allows quantitative analysis of human gas samples with low substance concentrations and is well suited for clinical studies which involve the investigation of metabolic processes in the lung and the body.

Effect of different doses of inhaled nitric oxide on pulmonary capillary pressure and on longitudinal distribution of pulmonary vascular resistance in ARDS

Inhaled nitric oxide lowers pulmonary capillary pressure (PCP) in animals and in patients with acute respiratory distress syndrome (ARDS). A dose-response relationship in patients with ARDS has not yet been established. Therefore, we studied the effects of four concentrations of nitric oxide (1, 10, 20 and 40 volumes per million (vpm)) in random order, on PCP in 19 patients with ARDS. PCP was estimated by visual analysis of the pressure decay curve after balloon inflation of the pulmonary artery catheter. Haemodynamic and gas exchange variables were measured at each nitric oxide concentration. Patients were classified as responders when PCP decreased by at least 2 mm Hg after nitric oxide 20 vpm. In responders (n = 8), nitric oxide decreased PCP and post-capillary vascular resistance dose-dependently and changed longitudinal distribution of pulmonary vascular resistance with a maximum effect at 20 vpm. In non-responders (n = 11), PCP did not change. In both groups, the nitric oxide-induced decrease in pre-capillary vascular resistance was small with a maximum effect at 1 vpm. In ARDS, vasodilatation of pre-capillary vessels is achieved at low concentrations of nitric oxide, whereas the effect of nitric oxide on postcapillary vessels is variable. Higher concentrations may be required for optimal post-capillary vasodilatation in a subgroup of ARDS patients.

Bees travel novel homeward routes by integrating separately acquired vector memories

The question of whether bees can take novel short cuts between familiar sites has been central to the discussion about the existence of cognitive maps in these insects. The failure of bees to show this capacity in the majority of previous studies may be a result of the training procedure, because extensive training to one feeding site may have eliminated or weakened memories for other sites that were previously trained. Here we present a novel approach to this problem, by rewarding honey bees, Apis mellifera carnica, at two feeding sites, one (Sm, 630 m southeast from the hive) at which they could feed in the morning, and the other (Sa, 790 m northeast) at which they could feed in the afternoon. We then displaced bees to Sa in the morning and to Sm in the afternoon either from the other feeding site or from the hive. Bees were also displaced to two novel sites, one at a completely unfamiliar location (S4) and another that was located halfway between the two feeding sites (S3). Bees displaced from either of the feeding sites never took novel short cuts; instead, they used the homeward directions that would have been correct had they not been displaced. Bees caught at the hive entrance, however, chose the correct homeward direction not only when displaced to both feeding sites, but also when displaced to S3, although not from S4. Control bees that had been trained to only one of the feeding sites were not able to travel directly home from S3 excluding the possibility that bees used landmarks close to the hive. This is the first evidence that bees take a novel short cut by activating two vector memories simultaneously. The potential mechanisms of integrating the two memories are discussed. Since bees took novel short cuts in only one direction (to the hive) and only when displaced from the hive (not the feeders), we conclude that inference of a cognitive map in bees would be premature. Copyright 1998 The Association for the Study of Animal Behaviour.

Respiratory comfort of automatic tube compensation and inspiratory pressure support in conscious humans

OBJECTIVE

To compare the new mode of ventilatory support, which we call automatic tube compensation (ATC), with inspiratory pressure support (IPS) with respect to perception of respiratory comfort. ATC unloads the resistance of the endotracheal tube (ETT) in inspiration by increasing the airway pressure, and in expiration by decreasing the airway pressure according to the non-linear pressure-flow relationship of the ETT.

DESIGN

Prospective randomized single blind cross-over study.

SETTING

Laboratory of the Section of Experimental Anaesthesiology (Clinic of Anaesthesiology; University of Freiburg).

SUBJECTS

Ten healthy volunteers.

INTERVENTIONS

The subjects breathed spontaneously through an ETT of 7.5 mm i.d. Three different ventilatory modes, each with a PEEP of 5 cmH2O, were presented in random order using the Dräger Evita 2 ventilator with prototype software: (1) IPS (10 cmH2O, 1 s ramp), (2) inspiratory ATC (ATC-in), (3) inspiratory and expiratory ATC (ATC-in-ex).

MEASUREMENTS AND MAIN RESULTS

Immediately following a mode transition, the volunteers answered with a hand sign to show how they perceived the new mode compared with the preceding mode in terms of gain or loss in subjective respiratory comfort: "better", "unchanged" or "worse". Inspiration and expiration were investigated separately analyzing 60 mode transitions each. Flow rates were continuously measured. The transition from IPS to either type of ATC was perceived positively, i.e. as increased comfort, whereas the opposite transition from ATC to IPS was perceived negatively, i.e. as decreased comfort. The transition from ATC-in to ATC-in-ex was perceived positively whereas the opposite mode transition was perceived negatively in expiration only. Tidal volume was 1220 +/- 404 ml during IPS and 1017 +/- 362 ml during ATC. The inspiratory peak flow rate was 959 +/- 78 ml/s during IPS and 1048 +/- 197 ml/s during ATC.

CONCLUSIONS

ATC provides an increase in respiratory comfort compared with IPS. The predominant cause for respiratory discomfort in the IPS mode seems to be lung over-inflation.

[Double isotope albumin flux measurement: diagnosis and therapeutic monitoring of acute lung injury]

PURPOSE

Acute Lung Injury (ALI) is a clinical condition which is associated with a high lethality. It is characterized by an increased pulmonary capillary permeability and non-cardiogenic pulmonary edema. This study was designed to answer the question whether double isotope albumin-flux measurement is a useful tool both for diagnosis of increased pulmonary capillary permeability and for monitoring therapeutic interventions (nitric oxide (NO) inhalation).

METHOD

In 12 patients with clinical signs of ALI, transvascular albumin-flux was measured by a double radioisotope technique before, during and after NO inhalation. 99mTc labeled albumin and 51Cr labeled autologous erythrocytes were used as tracer. The radioactivity of both radiopharmaceuticals was measured externally over the right lung by a radiation probe and simultaneously in arterial blood. For quantification of transvascular albumin-flux Normalized Index (NI) and Normalized Slope Index (NSI) were calculated. Furthermore, pulmonal vascular pressures and other physiological parameters were recorded.

RESULTS

All 12 patients showed markedly increased NSI before inhalation of NO. NSI decreased from 0.0074 +/- 0.0046 min-1 without nitric oxide to -0.0051 +/- 0.0041 min-1 during nitric oxide and increased to 0.0046 +/- 0.0111 min-1 after nitric oxide. The decrease of the NSI correlated well with decrease of venous pulmonary resistance during inhalation of NO.

CONCLUSION

Inhalation of NO reduces transvascular albumin-flux in patients with ALI. Double isotope albumin-flux measurement enables diagnosis of increased capillary permeability as well as monitoring therapeutic interventions.

Hypoxic pulmonary vasoconstriction in nonventilated lung areas contributes to differences in hemodynamic and gas exchange responses to inhalation of nitric oxide

BACKGROUND

Enhancement of hypoxic pulmonary vasoconstriction (HPV) in nonventilated lung areas by almitrine increases the respiratory response to inhaled nitric oxide (NO) in patients with acute respiratory distress syndrome (ARDS). Therefore the authors hypothesized that inhibition of HPV in nonventilated lung areas decreases the respiratory effects of NO.

METHODS

Eleven patients with severe ARDS treated by venovenous extracorporeal lung assist were studied. Patients' lungs were ventilated at a fraction of inspired oxygen (F[I(O2)]) of 1.0. By varying extracorporeal blood flow, mixed venous oxygen tension (P[O2]; partial oxygen pressure in mixed venous blood [PV(O2)]) was adjusted randomly to four levels (means, 47, 54, 64 and 84 mmHg). Extracorporeal gas flow was adjusted to prevent changes in mixed venous carbon dioxide tension [PV(CO2)]). Hemodynamic and gas exchange variables were measured at each level before, during, and after 15 ppm NO.

RESULTS

Increasing PV(O2) from 47 to 84 mmHg resulted in a progressive decrease in lung perfusion pressure (PAP-PAWP; P < 0.05) and pulmnonary vascular resistance index (PVRI; P < 0.05) and in an increase in intrapulmonary shunt (Q[S]/Q[T]; P < 0.05). PV(CO2) and cardiac index did not change. Whereas the NO-induced reduction in PAP-PAWP was smaller at high PV(O2), NO-induced decrease in Q(S)/Q(T) was independent of baseline PV(O2). In response to NO, arterial P(O2) increased more and arterial oxygen saturation increased less at high compared with low PV(O2).

CONCLUSION

In patients with ARDS, HPV in nonventilated lung areas modifies the hemodynamic and respiratory response to NO. The stronger the HPV in nonventilated lung areas the more pronounced is the NO-induced decrease in PAP-PAWP. In contrast, the NO-induced decrease in Q(S)/Q(T) is independent of PV(O2) over a wide range of PV(O2) levels. The effect of NO on the arterial oxygen tension varies with the level of PV(O2) by virtue of its location on the oxygen dissociation curve.

Large increase in cardiac output in a patient with ARDS and acute right heart failure during inhalation of nitric oxide

BACKGROUND

Inhaled nitric oxide (NO), a selective pulmonary vasodilator, reduces pulmonary artery pressure in patients with acute respiratory distress syndrome (ARDS). In spite of the reduction of right ventricular afterload, the effect of NO on cardiac output remains unclear.

METHODS

A patient with ARDS and echocardiographically determined severe acute right heart failure was treated with increasing concentrations of inhaled nitric oxide (NO). Haemodynamic and gas exchange variables were determined for each concentration of NO. NO treatment was continued for 3 days.

RESULTS

During initial right heart failure, administration of NO resulted in a large increase (32%) in cardiac output in a dose-dependent manner. When right ventricular function had improved, inhalation of NO did not increase cardiac output.

CONCLUSION

Our observations suggest that inhalation of NO is likely to increase cardiac output in ARDS when severe acute right heart failure is present.

The effects of desflurane on splanchnic hemodynamics and oxygenation in the anesthetized pig

This study was designed to investigate the effects of desflurane on systemic and splanchnic hemodynamics, O2 delivery and O2 uptake, tissue oxygenation (as monitored by surface PO2 electrodes), and hepatic oxygen-dependent intermediary metabolism (hepatic lactate uptake, intestinal lactate production, ketone-body ratio) in the pig. We studied 11 anesthetized (i.e., ketamine, flunitrazepam, vecuronium) and ventilated domestic pigs (17-23 kg). After instrumentation, desflurane was administered randomly at 0.5 minimum alveolar anesthetic concentration (MAC) (4.2 vol %) and 1.0 MAC (8.3 vol %). Desflurane caused dose-dependent decreases in heart rate, mean arterial blood pressure, and cardiac output. Hepatic arterial blood flow was not affected at 0.5 MAC but decreased at 1.0 MAC. In contrast, portal and superior mesenteric arterial blood flow decreased at 0.5 MAC but did not show any further significant decrease at 1.0 MAC. Total hepatic blood flow decreased dose-dependently. Although O2 deliveries of whole body, liver, and small intestine were markedly reduced at both concentrations, respective O2 uptakes did not change significantly. The decreases in O2 deliveries were reflected by moderate disturbances in hepatic and small intestinal surface PO2. No evidence for severe tissue hypoxia could be detected. Desflurane had no adverse effects on hepatic and small intestinal metabolic function. These data indicate that hepatic and small intestinal O2 reserve capacity is impaired by desflurane.

Doppelisotopen-Albuminfluxmessung: Diagnose und Therapiemonitoring des Acute Lung Injury

Ziel: Acute Lung Injury (ALI) ist ein Krankheitsbild mit hoher Letalität, das durch eine erhöhte pulmonale Kapillarpermeabilität mit einem nichtkardialen Lungenödem gekennzeichnet ist. In der vorliegenden Studie sollte überprüft werden, ob die Doppelisotopen-Albuminfluxmessung sich neben der Diagnostik einer erhöhten pulmonalen Kapillarpermeabilität auch zum Monitoring therapeutischer Interventionen (Stickstoffmonoxid (NO)-lnhalation) eignet. Methoden: Bei 12 Patienten mit ALI wurde der transvaskuläre Albuminflux vor, während und nach NO-Inhalation mittels Doppelisotopenmethode bestimmt. Als Tracer dienten 99mTc markiertes Albumin und 51Cr markierte autologe Erythrozyten. Die Aktivität beider Radiopharmazeutika wurde simultan über der Lunge mit einem Sondenmeßgerät und im arteriellen Blut ermittelt. Zur Quantifizierung des transvaskulären Albuminfluxes wurden der Normalized Index (Nl) und Normalized Slope Index (NSI) errechnet. Darüber hinaus wurden die pulmonalen Gefäßdrücke und weitere physiologische Parameter bestimmt. Ergebnisse: Alle 12 Patienten wiesen, als Ausdruck einer schweren Permeabilitätsstörung vor der NO-Inhalation deutlich erhöhte NSI auf. Während der NO-Inhalation fiel der NSI von durchschnittlich 0,0074 ± 0,0046 min-1 auf -0,0051 ± 0,0041 min-1. Nach Beendigung der NO Gabe stieg der NSI erneut auf durchschnittlich 0,0046 ± 0,0111 min-1 an. Der NSI Abfall zeigte eine gute Korrelation zum NO-induzierten Abfall des pulmonal-venösen Gefäßwiderstandes. Schlußfolgerung: Unter NO Inhalation kommt es zu einer Reduzierung des transvaskulären Albuminfluxes. Die Doppelisotopen-Albuminflux-messung eignet sich sowohl zur Diagnostik als auch zum Therapiemonitoring bei ALI-Patienten.

Scoring system for nosocomial pneumonia in ICUs

OBJECTIVE

To develop a scoring system for stratifying patients in intensive care units (ICUs) by risk of developing nosocomial pneumonia (NP), based on variables generally available in an ICU, and to determine the probability of a patient developing NO in the ICU.

DESIGN AND SETTING

a 2-year prospective cohort study conducted in a medical and surgical ICU.

PATIENTS

756 patients admitted to the ICU for 48 h or more were followed up until the development of NP or death or discharge from the ICU.

MEASUREMENTS AND RESULTS

129 (17.1%) patients developed NP, 106 (14%) in the first 2 weeks. The following independent risk factors were identified by multivariate analysis: no infection on admission [relative risk (RR) = 3.1, 95% confidence intervals (CI) = 2.0 to 4.81; thorax drainage (RR = 2.1, 95% CI = 1.2 to 3.5); administration of antacids (RR = 2.1, 95% CI = 1.4 to 3.1); partial pressure of oxygen (PO2) > 110 mmHg (RR = 1.6, 95% CI = 1.0 to 2.6); administration of coagulation factors (RR = 1.8 95% CI = 1.0 to 3.2); male gender (RR = 2.7, 95% CI = 1.2 to 6.3); urgent surgery (RR = 2.4, 95% CI = 0.9 to 6.4); and neurological diseases (RR = 4.2, 95% CI = 1.9 to 9.4). To obtain a predictive risk index for NP, a scoring system was developed using a multivariate model. The probability of developing NP varied between 11.0% in the lowest risk group and 42.3% in the highest risk group. The patients' risk of acquiring NP was seven times higher in the highest score category (i.v.) than in the lowest one (I).

CONCLUSIONS

ICU patients can be stratified into high- and low-risk groups for NP. No infection on admission, thorax drainage, administration of antacids, and PO2 > 110 mmHg were associated with a higher risk of NP during the entire 2-week period.

Different effects of early endotoxaemia on hepatic and small intestinal oxygenation in pigs

OBJECTIVE

Study on simultaneous O2 supply/uptake relationships in liver and gut during endotoxaemia, to determine whether signs of dysoxia develop uniformly in the splanchnic region.

DESIGN

Animal study to assess the early effects of endotoxaemia on oxygenation of both liver and small intestine.

INTERVENTIONS

Eight anaesthetized pigs received a continuous portal venous infusion of lipopolysaccharide (0.5 microgram.kg-1.h-1) for 6 h. Systemic, pulmonary and splanchnic haemodynamics as well as systemic and splanchnic O2 supply/uptake relationships were determined.

RESULTS

There was a multiphasic haemodynamic response pattern characterized by an early (within the 1st h) and a subsequent more prolonged phase (between the 2nd and 6th h) of decreases and recovery of hepatic arterial, portal venous and superior mesenteric arterial blood flows (electromagnetic flow probes) and splanchnic O2 deliveries. Unrelated to perfusion pressure and O2 delivery, there were early and sustained decreases in ileal mucosal surface partial pressure of oxygen (PO2) (multiwire PO2 electrode) and pH (tonometry). This was not reflected by ileal serosal surface PO2, O2 uptake and arteriomesenteric venous pH and partial pressure of carbon dioxide (PCO2) gradients. There was little evidence of concomitant hepatic dysoxia as evaluated by surface PO2.

CONCLUSIONS

The study demonstrates early and sustained regional (mucosa) intestinal hypoxia with little evidence of simultaneous hepatic dysoxia during initial endotoxaemia.

Effect of inhaled nitric oxide on venous admixture depends on cardiac output in patients with acute lung injury and acute respiratory distress syndrome

BACKGROUND

It has been shown that inhaled nitric oxide (NO) reduces intrapulmonary venous admixture (QVA/QT) and improves oxygenation in patients suffering from acute respiratory distress syndrome (ARDS). The change in QVA/QT during NO inhalation varies individually. Factors known to influence the respiratory response to NO are the NO concentration and the level of shunt before NO administration. Other factors that may modify the effect on gas-exchange during NO breathing are unknown.

METHODS

We studied the effect of 40 ppm inhaled NO on pulmonary gas-exchange and haemodynamics in 37 patients with acute lung injury (ALI) and ARDS, respectively, and factors that may influence the respiratory response to NO.

RESULTS

Inhalation of 40 ppm NO produced a decrease in mean pulmonary artery pressure (MPAP) from 33.1 +/- 7.2 to 30.2 +/- 6.8 (mean +/- SD) mmHg (P < 0.0001) while pulmonary artery wedge pressure (PAWP), cardiac output and mean arterial pressure remained constant. Change in QVA/QT during NO inhalation depended on the preinhalation cardiac output and had no association with mixed venous oxygen tension, MPAP-PAWP, and QVA/QT before NO delivery. QVA/QT decreased in 26 patients (group 1) and increased in 11 patients (group 2) during NO inhalation. In group 1, cardiac output was lower than in group 2 (8.6 vs 12.2 l.min-1; P < 0.0005).

CONCLUSION

We conclude that the change in venous admixture during inhalation of 40 ppm NO depends on cardiac output. If preinhalation cardiac output is high, 40 ppm NO can adversely affect gas exchange in patients with ALI and ARDS.

The knowledge base of bee navigation

Navigation in honeybees is discussed against the background of the types of memories employed in the navigational task. Two questions are addressed. Do bees have goal-specific expectations, and when are novel routes travelled? Expectations are deduced from (1) context stimuli as determinants for local cue memories, (2) landmark-dependent path integration, (3) sequential learning of landmarks, and (4) motivation- and context-dependent memory retrieval. Novel routes are travelled under two conditions: (1) goal-cue-based piloting and (2) integration of simultaneously activated vector memories. Our data do not support the conclusion that memory integration in bees is organised by a cognitive map. The assumption of purely separate memories that are only retrieved according to the chain of events during navigational performance also appears to be inadequate. We favour the view that multiple memories are integrated using external and internal sources of information. Such configural memories lead to both specific expectations and novel routes.