TNF and TNFR polymorphisms in severe sepsis and septic shock: a prospective multicentre study

Tumour necrosis factor (TNF) is an important pro-inflammatory cytokine produced in sepsis. Studies examining the association of individual TNF single nucleotide polymorphisms with sepsis have produced conflicting results. This study investigated whether common polymorphisms of the TNF locus and the two receptor genes, TNFRSF1A and TNFRSF1B, influence circulating levels of encoded proteins, and whether individual polymorphisms or extended haplotypes of these genes are associated with susceptibility, severity of illness or outcome in adult patients with severe sepsis or septic shock. A total of 213 Caucasian patients were recruited from eight intensive care units (ICU) in the UK and Australia. Plasma levels of TNF (P = 0.02), sTNFRSF1A (P = 0.005) and sTNFRSF1B (P = 0.01) were significantly higher in those who died on ICU compared to those who survived. There was a positive correlation between increasing soluble receptor levels and organ dysfunction (increasing SOFA score) (sTNFRSF1A R = 0.51, P < 0.001; sTNFRSF1B R = 0.53, P < 0.001), and in particular with the degree of renal dysfunction. In this study, there were no significant associations between the selected candidate TNF or TNF receptor polymorphisms, or their haplotypes, and susceptibility to sepsis, illness severity or outcome. The influence of polymorphisms of the TNF locus on susceptibility to, and outcome from sepsis remains uncertain.

Local activation of coagulation and inhibition of fibrinolysis in the lung during ventilator associated pneumonia

BACKGROUND

Fibrin deposition is a hallmark of pneumonia. To determine the kinetics of alterations in local coagulation and fibrinolysis in relation to ventilator associated pneumonia (VAP), a single centre prospective study of serial changes in pulmonary and systemic thrombin generation and fibrinolytic activity was conducted in patients at risk for VAP.

METHODS

Non-directed bronchial lavage (NBL) was performed on alternate days in patients expected to require mechanical ventilation for more than 5 days. A total of 28 patients were studied, nine of whom developed VAP.

RESULTS

In patients who developed VAP a significant increase in thrombin generation was observed in the airways, as reflected by a rise in the levels of thrombin-antithrombin complexes in NBL fluid accompanied by increases in soluble tissue factor and factor VIIa concentrations. The diagnosis of VAP was preceded by a decrease in fibrinolytic activity in NBL fluid. Indeed, before VAP was diagnosed clinically, plasminogen activator activity levels in NBL fluid gradually declined, which appeared to be caused by a sharp increase in NBL fluid levels of plasminogen activator inhibitor 1.

CONCLUSION

VAP is characterised by a shift in the local haemostatic balance to the procoagulant side, which precedes the clinical diagnosis of VAP.

Outcome for cardiothoracic surgical patients requiring multidisciplinary intensive care

BACKGROUND AND OBJECTIVE

Patients who require multidisciplinary intensive care after cardiac surgery have a poor prognosis. The aim was to investigate factors in the mortality of this group of patients at 6 months.

METHODS

A retrospective analysis was made of the 6-month mortality rate in 301 adults who required admission to a multidisciplinary intensive care unit following cardiac surgery from 1991 to 1997. Mortality was correlated with clinical and patient characteristic variables.

RESULTS

The intensive care mortality rate was 34% and at 6 months after patients' discharge from intensive care it was 51%. There were positive correlations with death at 6 months for ventricular failure (odds ratio of death 3.4, P = 0.002), sepsis (odds ratio 3.0, P = 0.004) and age over 80 yr (odds ratio of death 9.2, P = 0.034). Patients who had undergone isolated coronary artery graft surgery (odds ratio of death 0.28, P = 0.036) or thoracic surgery (odds ratio of death 0.22, P = 0.042) had better 6-month outcomes. Patients with respiratory or renal failure in the absence of ventricular failure or sepsis had a 6-month mortality rate of 36%; but the lower mortality rate did not achieve statistical significance.

CONCLUSIONS

The 6-month mortality rate of 51% in a group of patients requiring multidisciplinary intensive care after cardiac surgery is consistent with previous studies; mortality was particularly high in extreme old age and in patients referred with sepsis or ventricular failure. Those patients with uncomplicated respiratory or renal failure had a better outcome than the group as a whole.

The pulmonary physician in critical care . 12: Acute severe asthma in the intensive care unit

Most deaths from acute asthma occur outside hospital, but the at-risk patient may be recognised on the basis of prior ICU admission and asthma medication history. Patients who fail to improve significantly in the emergency department should be admitted to an HDU or ICU for observation, monitoring, and treatment. Hypoxia, dehydration, acidosis, and hypokalaemia render the severe acute asthmatic patient vulnerable to cardiac dysrrhythmia and cardiorespiratory arrest. Mechanical ventilation may be required for a small proportion of patients for whom it may be life saving. Aggressive bronchodilator (continuous nebulised beta agonist) and anti-inflammatory therapy must continue throughout the period of mechanical ventilation. Recognised complications of mechanical ventilation include hypotension, barotrauma, and nosocomial pneumonia. Low ventilator respiratory rates, long expiratory times, and small tidal volumes help to prevent hyperinflation. Volatile anaesthetic agents may produce bronchodilation in patients resistant to beta agonists. Fatalities in acute asthmatics admitted to HDU/ICU are rare.

Indices of nitric oxide synthesis and outcome in critically ill patients

We measured the concentrations of serum nitrates/nitrites and plasma cyclic guanosine monophosphate as markers of nitric oxide synthesis in patients with or without septic shock for 5 days following admission to intensive care. We found that nitrate/nitrite concentrations, when corrected for the effect of renal failure, were significantly higher in patients with septic shock, both on admission and in the final samples drawn. In a logistic regression analysis, the rate of change of nitrate/nitrite concentration was associated with survival to day 28 (falling in survivors). The concentration of cyclic guanosine monophosphate when corrected for the confounding effects of renal function and platelet count, was only associated with the septic shock group on admission.

Human-computer interactions: can computers improve the way doctors work?

As medicine becomes more complex and the knowledge base expands, the integration of computer systems into clinical practice would appear to be an inescapable necessity rather than an option. The issues of security and reliability have largely been solved by industrial and business applications of computer technology. The larger challenge lies in designing convenient, efficient and acceptable interfaces between the clinician and computer for data input and presentation. In the future, decision making algorithms are likely to assist the clinician in diagnosis and management to a degree that should significantly improve clinical effectiveness.

Autonomic control of skin microvessels: assessment by power spectrum of photoplethysmographic waves

1. Although it is well known that the microvessels of the skin constantly undergo spontaneous variations in volume, the significance of these rhythmic changes remains uncertain. 2. In 10 healthy males and in 15 patients in intensive care, we assessed the origin of the autonomic influences on spontaneous fluctuations in the microcirculation of the skin, obtained by an infra-red photoplethysmographic device; we used spectral analysis techniques to compare these fluctuations (which were recorded simultaneously in two sites) with those of blood pressure, in order to test the presence of autonomic control of any synchronous fluctuations in these different measurements from the cardiovascular system. In order to minimize mechanical fluctuations caused by occasional slow breaths, rather than nervously mediated fluctuations in skin blood flow, respiration was controlled at 15 breaths/min (0.25 Hz). 3. Spontaneous infra-red photoplethysmographic fluctuations were observed in different body areas (left index finger and left ear lobe, right and left index finger), and all were evident at 0.1 Hz, as well as respiration-related components at 0.25 Hz. Active standing increased the power of the 0.1 Hz fluctuations (sympathetic activity) in both blood pressure (from 62.7 +/- 7.1 to 79.2 +/- 3.7 normalized units, P < 0.05) and IRP (finger: from 68.5 +/- 6.4 to 86.9 +/- 3.4 normalized units, P < 0.05; ear: from 59.0 +/- 5.9 to 88.1 +/- 2.0, P < 0.01). There was a high (> 0.5) coherence between the fluctuations obtained in blood pressure, in IRP signals obtained simultaneously at the finger and at the ear, and in R-R interval. This synchronization between the oscillations in all these signals, which were unrelated to the respiratory frequency or to the pulse rate, suggests a common neural, non-local origin. The phase between IRP and blood pressure was positive in the 0.1 Hz region (+1.65 +/- 0.41 radians, i.e. IRP was leading blood pressure, showing that 0.1 Hz fluctuations were not passively transmitted to the skin microvessels from large arteries) and negative in the 0.25 Hz region (-0.74 +/- 0.19 radians, P < 0.01 compared with phase in the 0.1 Hz region, i.e. IRP was lagging behind blood pressure, suggesting possible passive transmission to the skin microvessels of blood pressure fluctuations caused by respiration). Fluctuations at lower frequency were observed in all IRP recordings, suggesting a local origin for these. Intra-arterial and IRP fluctuations were compared in the 15 intensive care patients and gave similar results. 4. The skin microcirculation is thus not only under local control, but also reflects changes in sympathetic activity; the effect of these changes on the skin microcirculation can be easily evaluated by the spectral analysis of the IRP signal obtained simultaneously in multiple areas, in conjunction with the spectra of R-R interval and blood pressure.

Effect of renal function on serum nitrogen oxide concentrations

Nitric oxide is too short-lived to measure in vivo, but its production can be estimated by measuring its stable oxidation products, nitrites and nitrates, in serum. Renal elimination of these ions has been demonstrated, but the effect of renal function on their concentrations in serum is currently unknown. We evaluated serum and urine nitrates + nitrites as serum nitrogen oxides (sNOx), nitrogen oxide (NOx) clearance, and creatinine clearance in 71 patients on the Intensive Therapy Unit. The correlation between sNOx and plasma creatinine was strong and highly significant (P <0.001). These results suggest that renal function has a significant effect on sNOx concentrations. Studies in which the sNOx concentration is used as an index of nitric oxide production can therefore be interpreted only if renal function has been taken into account.

Effect of renal function on serum nitrogen oxide concentrations

Nitric oxide is too short-lived to measure in vivo, but its production can be estimated by measuring its stable oxidation products, nitrites and nitrates, in serum. Renal elimination of these ions has been demonstrated, but the effect of renal function on their concentrations in serum is currently unknown. We evaluated serum and urine nitrates + nitrites as serum nitrogen oxides (sNOx), nitrogen oxide (NOx) clearance, and creatinine clearance in 71 patients on the Intensive Therapy Unit. The correlation between sNOx and plasma creatinine was strong and highly significant (P &lt;0.001). These results suggest that renal function has a significant effect on sNOx concentrations. Studies in which the sNOx concentration is used as an index of nitric oxide production can therefore be interpreted only if renal function has been taken into account.

Effects of propofol and thiopentone, and benzodiazepine premedication on heart rate variability measured by spectral analysis

We studied the effects of temazepam premedication and induction of anaesthesia with thiopentone or propofol on the heart rate power spectrum in 47 patients undergoing elective minor surgery. Eighteen patients received temazepam 20 mg orally as premedication. There was a significant reduction in high frequency power and total power, and an increase in the ratio of low to high frequency power after induction of anaesthesia with either propofol or thiopentone. Patients who had received temazepam premedication had significantly greater low frequency, high frequency and total power than those who were not premedicated. There was no significant difference between premedicated and unpremedicated patients in the ratio of low to ventilatory frequency power.

Autonomic control of the heart and peripheral vessels in human septic shock

OBJECTIVE

Circulating endotoxin impairs the sympathetic regulation of the cardiovascular system in animals. We studied the changes in the autonomic control of the heart and circulation during septic shock in humans.

DESIGN

12 patients (age 43.0 +/- 6, 17-83 years) were investigated during septic shock (mean duration: 3.5 +/- 0.5 days) and during recovery, fluctuations in R-R interval, invasive arterial pressure (AP) and peripheral arteriolar circulation (PC, photoplethysmography) were evaluated by spectral analysis as a validated noninvasive measure of sympathovagal tone. Apache II score was adopted as the disease severity index. Low frequency components (0.03-0.15 Hz) of the frequency spectra were expressed as relative to the overall variability (LFnu) for each cardiovascular variable.

RESULTS

LFnu were low or absent during shock but, in the 10 patients who recovered, increased by the time of discharge (post-shock). R-R LFnu increased from 17 +/- 6 to 47 +/- 9 (p < 0.03), AP LFnu from 6 +/- 3 to 35 +/- 4 (p < 0.02) and PC LFnu from 18 +/- 3 to 66 +/- 4 (p < 0.001). Apache II fell from 23.1 +/- 1, at admission, to 14.8 +/- 1.8 at discharge (p < 0.005). Two patients died showing no LFnu increase.

CONCLUSION

Reduced LF components of the variability of cardiovascular signals are characteristic of septic shock, confirming the presence of abnormal autonomic control. Restored sympathetic (LF) modulation seems to be associated with a favourable prognosis.

Microbiological lung surveillance in mechanically ventilated patients, using non-directed bronchial lavage and quantitative culture

We surveyed bronchial microflora by alternate-day, non-directed bronchial lavage (NBL) in 150 patients requiring mechanical ventilation on an intensive care unit. This simple technique uses a 20 ml non-bronchoscopic lung lavage, then quantitative bacterial culture. NBL bacteriological findings were identical to those obtained by same-day bronchoscopic broncho-alveolar lavage on 16/20 occasions. Using serial NBLs, the bronchial bacterial population was characterized during 65 episodes of pneumonia defined by clinical and retrospective criteria. Mean bacterial colony counts increased significantly during the 2 days preceding the clinical onset of pneumonia, from < or = 10(3) cfu/ml to > or = 10(5) cfu/ml (p < 0.05). In 51 patients showing a clinical response to antibiotic treatment, mean colony counts fell significantly after antibiotic initiation (p < 0.05). By contrast, in 14 patients who showed progressive clinical deterioration or relapse, there was no significant fall in NBL counts, and serial NBLs revealed antibiotic resistance or superinfection. The surveillance data altered clinical management in 42% of patients. Positive NBLs guided the choice of antibiotics, whilst negative NBLs encouraged the withholding of antibiotics, or detection of alternative pathology. We propose routine bacteriological lung surveillance of mechanically ventilated patients using this simple, inexpensive and safe technique.

Ventilatory support in asthma

Mechanical ventilation in acute asthma is associated with significant morbidity and mortality, and maximal medical therapy should prevent it being used inappropriately. We review current standards of medical therapy in acute asthma, the indications for mechanical ventilation and its management.

Spectral analysis of heart rate variability in the sepsis syndrome

Sympathetic and parasympathetic activity was evaluated on 39 occasions in 17 patients with the sepsis syndrome, by measurement of the variation in resting heart rate using frequency spectrum analysis. Heart rate was recorded by electrocardiography and respiratory rate by impedance plethysmography. The sepsis syndrome was established on the basis of established clinical and physiological criteria. Subjects were studied, whenever possible, during the period of sepsis and during recovery. Spectral density of the beat-to-beat heart rate was measured within the low frequency band 0.04 to 0.10 Hz (low frequency power, LFP) modulated by sympathetic and parasympathetic activity, and within a 0.12 Hz band width at the respiratory frequency mode (respiratory frequency power, RFP) modulated by parasympathetic activity. Results were expressed as the total variability (total area beneath the power spectrum), as the spectral components normalized to the total power (LFPn, RFPn) or as the ratio of LFP/RFP. During the sepsis syndrome, total heart rate variability and the sympathetically mediated component, LFPn were significantly lower than during the following recovery phase (ANOVA, p < 0.0001, p < 0.01 respectively). Both APACHE II (Acute Physiological and Chronic Health Evaluation) and TISS (Therapeutic Intervention Scoring System) scores showed an inverse correlation with total heart rate variability, logLFP, LFPn and the LFP/RFP ratio (p < 0.002 to 0.0001). Sympathetically mediated heart rate variability was significantly lower during the sepsis syndrome and was inversely proportional to disease severity.

Spectral analysis of heart rate variability in bronchial asthma

Sympathetic and parasympathetic activity was evaluated in ten healthy controls, nine asymptomatic, untreated asthmatic subjects and ten asthmatic patients during treatment for acute asthma, by measurement of the variation in resting heart rate using frequency spectrum analysis. Heart rate was recorded by ECG and respiratory rate by impedance plethysmography. Spectral density of the beat-to-beat heart rate was measured within the low frequency band 0.04 to 0.10 Hz (low frequency power) modulated by sympathetic and parasympathetic activity, and within a 0.12 Hz band width at the respiratory frequency mode (respiratory frequency power) modulated by parasympathetic activity. Acute asthmatics had higher heart rates than either of the other two groups; this was probably related to the effects of beta-adrenoceptor agonist medication. Sympathetically mediated heart rate variability (normalized low frequency power) was significantly lower in both asymptomatic (p less than 0.002) and acute (p less than 0.02) asthma subjects compared to controls. This is consistent with altered sympathetic/parasympathetic regulation of heart rate in subjects with bronchial asthma.

Lung mucociliary transport in asymptomatic asthma: effects of inhaled histamine

Bronchial mucociliary clearance (CB) and tracheal mucus velocity (TMV) were measured during the course of repeated inhalations of histamine in six subjects with asthma who had no symptoms in a double-blind, crossover study with a radioaerosol technique. Subjects inhaled a technetium 99m-labeled ferric oxide aerosol with an aerodynamic diameter of approximately 8 microns. CB was recorded for 2.5 hours with a gamma camera, and TMV measured with a multidetector probe situated over the extrathoracic trachea. Histamine was administered repeatedly in concentrations previously shown to produce a 20% fall in forced expired volume in 1 second and at intervals allowing 90% recovery of pulmonary function. Histamine produced a 28% increase in CB (p less than 0.001, analysis of variance) and an 87% increase in TMV (p less than 0.001, analysis of variance) above control values, which was not significantly different from that previously observed in normal subjects receiving significantly higher concentrations of histamine. We conclude that histamine stimulates the mucus transport mechanism in subjects with asthma and that there is a relative hypersensitivity to histamine when these subjects are compared with normal subjects.

The effect of inhaled histamine on human tracheal mucus velocity and bronchial mucociliary clearance

The effect of inhaled histamine on human tracheal mucus velocity (TMV) and bronchial mucociliary clearance (CB) was investigated in six healthy subjects using radioaerosol techniques in a randomized double-blind crossover study. Subjects inhaled repeated doses of either phosphate-buffered saline (PBS) or histamine, immediately after the inhalation of a radioaerosol and during the subsequent 2.5-h clearance measurements. Histamine was administered in concentrations previously demonstrated to induce a 20% fall in FEV1 at intervals permitting 90% recovery (mean recovery time = 25 min). Both TMV and CB were significantly increased by inhaled histamine (p less than 0.001). Average TMV throughout the 2.5-h studies increased from 4.9 +/- 1.3 to 8.4 +/- 1.6 mm/min. The increase in TMV above control values became apparent from 5 to 20 min after the first histamine administration. The percentage of aerosol clearance in 60 min increased 33%. The enhancement of CB became statistically significant at 21 min and persisted throughout the 2.5-h measurements (p less than 0.05). The increase in CB could not be attributed to differences in aerosol deposition because measurements of aerosol penetration were not significantly different between PBS and histamine studies. These data indicate that the bronchoconstriction caused by histamine is accompanied by an increase in tracheal and bronchial mucus transport. Release of histamine, as part of an inflammatory response, may alter mucociliary clearance in humans.

Sources of error and variability in the determination of anaerobic threshold in healthy humans

The derivation of anaerobic threshold (AT) from the ventilatory responses to incremental exercise is associated with several sources of variability including true biological variability and the error attributable to the observer interpretation of data. To define and quantitate the sources of variability in AT determination we exercised 6 healthy volunteers 6 times and submitted plots of ventilation (Ve), CO2 production (VCO2), respiratory exchange ratio (R) and the ventilatory equivalent for oxygen (Ve/VO2) in random order to 4 independent observers. Within-subject variability in AT ranged from 7 to 55% depending on the subject, ventilatory parameter and observer with an overall mean coefficient of variability of 24%. Significant day-to-day variability was demonstrated in 4 of the 6 subjects using AT values derived from at least one of the ventilatory parameters (Anova, p less than 0.05-p less than 0.001). Mean values for AT obtained with the Ve and VCO2 plots (1.91 and 1.69 liter/min VO2) were significantly lower than those obtained from R and Ve/VO2 plots (2.28 and 2.6 liters/min VO2; p less than 0.001, Anova). Anova showed significant differences in AT values derived by one of the observers compared to the other three (p less than 0.001). A significant observer/ventilation parameter interaction was also found (p less than 0.001) due to one observer consistently estimating higher values of AT from the R plots. The observer error in deriving AT from each exercise test using the Ve plots = 24%, for VCO2 = 19% for R = 29% and for Ve/VO2 = 15%.(ABSTRACT TRUNCATED AT 250 WORDS)

The reproducibility of the respiratory responses to maximum exercise

In order to investigate the reproducibility of maximum progressive exercise we studied 6 healthy volunteers twice a day for 3 nonconsecutive days. The average within-subject coefficients of variation for the maximal ventilatory and heart rate (HR) responses ranged from 3.8% for HR to 12% for minute ventilation (Ve) and anaerobic threshold. Three-way analysis of variance revealed no significant variations in exercise parameters for the group as a whole except for the carbon dioxide output (VCO2) and respiratory exchange ratio (R) which was significantly greater in the afternoon (p less than 0.05). This was attributed to dietary effects of carbohydrate loading upon VCO2 and was associated in 1 subject with a significant increase in Ve (p less than 0.01). In another subject, morning values of Ve were consistently and significantly (p less than 0.05) greater than the afternoon values which could only be attributed to increased effort as indicated by an increased oxygen uptake. Tests of resting pulmonary mechanics (FEV1, FVC, FRC, RAW) measured before each exercise procedure showed no significant diurnal or day-to-day variations. Results indicate that while the maximal responses to progressive exercise are generally reproducible and the first exercise procedure can usually be considered representative, diurnal variations in R, VCO2 and Ve may occur which can be best avoided on repeated testing by exercising subjects at a standardized time of day.

Modifications of lung clearance mechanisms by acute influenza A infection

Four volunteers with naturally acquired, culture-proved influenza A infection inhaled a radiolabeled aerosol to permit investigation of lung mucociliary clearance mechanisms during and after symptomatic illness. Mucus transport in the trachea was undetectable when monitored with an external multidetector probe within 48 hours of the onset of the illness, but was found at a normal velocity (4.9 +/- 1.9 mm/min) by 1 week in three of the four subjects. In two volunteers who coughed 23 to 48 times during the 4.5-hour observation period, whole lung clearance was as fast within the first 48 hours of illness as during health 3 months later in spite of the absence of measurable tracheal mucus transport. Conversely, in spite of the return 1 week later of mucus transport at velocities expected in the trachea, whole lung clearance for the 4.5-hour period was slowed in two volunteers who coughed less than once an hour. The data offer evidence that cough is important in maintaining lung clearance for at least several days after symptomatic influenza A infection when other mechanisms that depend on ciliary function are severely deficient.

Aerosol penetration and mucociliary transport in the healthy human lung. Effect of low serum theophylline levels

The effect of theophylline on the penetration of an inhaled radioaerosol in the lung, bronchial clearance, and tracheal mucociliary transport rate (TMTR) was investigated in 13 healthy volunteers. Following a randomized, double-blind, crossover protocol, subjects ingested 4 mg/kg twice daily of theophylline or placebo for three days which resulted in stable, low therapeutic serum levels. Aerosol penetration, assessed by the skew of the initial distribution of lung radioactivity, was more peripheral (p less than 0.025) with theophylline, indicating bronchodilation that was not detectable by standard pulmonary function tests. The TMTR increased in ten of 13 subjects after theophylline, but not to a significant level. Bronchial clearance was not significantly different with theophylline despite the longer clearance pathway created by the increased peripheral aerosol deposition. This finding suggests that mucus transport rates in the intrapulmonary airways were increased by theophylline.

A mathematical model of particle retention in the air-spaces of human lungs

Knowledge of the total and regional lung retention of particles inhaled continuously by man over long periods can be useful in understanding the potential role of inhaled particles in the pathogenesis of lung diseases. Owing to practical and ethical considerations, however, little or no experimental information exists. A mathematical model of particle retention simulating environmental and occupational exposures has therefore been developed that takes into account particle deposition, tracheobronchial clearance, and two phases of alveolar clearance in the Weibel A anatomical lung model. The derived equations of retention kinetics predict retention of particles as a function of exposure time. For a continuous exposure (simulating environmental conditions) to 4 microns particles, the model predicts that retained particles approach an equilibrium between deposited and cleared particles with the 95% level being reached in 293 days. For an intermittent exposure (simulating occupational conditions) equilibrium is approached in five years. The whole lung burden of particles is predicted to be 9% of the total mass that entered the lung after a one-year environmental exposure and 1.5% after a 25-year occupational exposure. The equilibrium surface concentration and integrated dose of particles per airway generation predict enhanced risk to the pathogenic effects of inhaled particles in the large airways and respiratory bronchioles.

Theoretic analysis of sites of aerosol deposition in the human lung

A new model of aerosol deposition in the human lungs has been developed. It incorporates the deposition probability equations of Landahl and Findeisen in the Horsfield Asymmetric Model 1 of the lung. The deposition model takes into account the regional distribution of ventilation by incorporating principles of ventilatory mechanics into the determination of flow distribution in the lung. Calculations are performed for a 4 micrometers aerosol inhaled with a breathing pattern consisting of a 1,000 ml tidal volume and an average inspiratory flow of 500 ml/sec. A ventilation gradient from base to apex of 1.10 is assigned. The results show that deposition by impaction dominates in the large airways, while deposition by sedimentation dominates in the small airways and alveoli. Calculations of surface concentrations of particles deposited in the airways reveal that the segmental and subsegmental bronchi receive the highest concentrations. The gradient of particles deposited per unit lung volume from base to apex equals 1.13 which is very close to the ventilation gradient. The new model is the first attempt to assess the distribution of deposited particles in an asymmetric model of the lung, using a realistic distribution of ventilation.

The characterization of radioaerosol deposition in the healthy lung by histogram distribution analysis

Thirteen healthy nonsmoking volunteers inhaled an 8.1 micrometers (MMAD) radioaerosol on two occasions. Aerosol deposition pattern within the right lung, as recorded by a gamma camera, was expressed as the 3rd and 4th moments of the distribution histogram (skew and kurtosis) of radioactivity during the first ten minutes after aerosol inhalation. Deposition pattern was also expressed as the percentage of deposited activity retained within the lung at 24 hr (24 hr % retention) and found to be significantly correlated with measures of skew (P less than 0.001). Tests of pulmonary function (FEV1, FVC, and MMFR) were significantly correlated with skew. Correlations were also demonstrated for these pulmonary function tests with 24 hr % retention but at lower levels of significance. Results indicate that changes in measures of forced expiratory airflow in healthy human volunteers influence deposition pattern and that the skew of the distribution of inhaled radioactivity may provide an acceptable index of deposition pattern.

Analysis of aerosol deposition in the healthy human lung

Wide variation in the pattern of deposition of inhaled aerosols has previously been described in both healthy and diseased humans. To investigate the factors responsible for such variation, the authors studied a group of 13 healthy nonsmoking subjects. One two occasions each subject inhaled a monodisperse 8.1 mm (mass median aerodynamic diameter) Fe2O3 aerosol labelled with 99mTc using a standardized breathing pattern. Pulmonary function was defined by tests of forced expiratory airflow. Total activity in the right lung at 0 hr and at 24 hr (24-hr percent retention) was measured using a gamma camera. Numerical indices of deposition pattern were derived in several ways from the initial gamma camera image of the right lung by comparing the ratio of activity within a mid- and peripheral lung region of interest, by analyzing the profile of radioactivity within a horizontal band across the right lung from the midline to the lung edge, and by analysis of a distribution histogram of activity within the whole lung (skew and kurtosis). The 24-hr percent retention of aerosol showed considerable intrasubject variability unlike the deposition indices. The various deposition indices were found to correlate with the 24-hr percent retention, FEV1.0, FEV1.0/FVC%, and MMFR at varying levels of significance. Results indicate that the pattern of aerosol deposition in healthy humans is influenced by mild degrees of obstruction to airflow, as reflected by tests of forced expiratory airflow, increasing airways obstruction being associated with more central deposition of the inhaled aerosol. Deposition indices derived from the initial pattern of aerosol distribution within the lung may prove to be more reliable and sensitive than measurements of 24-hr percent retention in defining aerosol deposition pattern.

Pattern of carbon dioxide stimulated breathing in patients with chronic airway obstruction

The pattern of stimulated breathing during carbon dioxide inhalation was studied in a group of 21 patients with severe irreversible airways obstruction (mean FEV1 = 0.9 litre, mean FEV1/FVC% = 50%). Carbon dioxide rebreathing experiments were performed, the ventilatory response being defined in terms of total ventilation (V) and CO2 sensitivity (S). Breathing pattern was defined by the changes in tidal volume (delta VT) and respiratory frequency (delta f) and the maximum VT achieved (VTmax). Contrary to some previous studied no significant relationship could be demonstrated between the severity of airway obstruction (FEV1/FVC%, Raw) and the ventilatory response to rebreathing (V, S, delta VT, delta f, VTmax). However, measurements of dynamic lung volume (FEV1, FVC, IC) were found to be significantly correlated with the breathing pattern variables (delta VT, delta f, VTmax). Resting PaO2 and PaCO2 were significantly correlated with delta VT but not delta f. Results indicate that the degree of airway obstruction does not dictate the ventilatory or breathing pattern response to carbon dioxide induced hyperpnoea. In contrast it is the restriction of dynamic lung volume, by limiting the VT response, that appears to determine the ventilatory and breathing pattern response in patients with severe airway obstruction.

The pattern of breathing in patients with chronic airflow obstruction

1. The pattern of breathing in 12 patients with severe irreversible airflow obstruction has been studied during ventilatory stimulation by rebreathing CO2. Mean maximum tidal volume response was only 1.23 +/- 0.30 litres (mean +/- SD); this represented 65% of mean measured vital capacity and 82% of mean measured inspiratory capacity. During the course of rebreathing mean total breath duration was reduced from 3.48 +/- 0.93 to 2.44 +/- 0.48 s. 2. End-expiratory thoracic gas volume (FRC) was elevated at rest in all subjects and increased significantly by a further 0.50 +/- 1.90 litres during ventilatory stimulation in 10 of the 12 subjects. The maximum increase in FRC was proportional to the degree of airflow obstruction afforded by the airways in each subject. 3. It is suggested that the increase in FRC during ventilatory stimulation is responsible for the diminished tidal volume response and is an important determinant of breathing pattern and symptomatology in patients with airflow obstruction.

The effects of expiratory positive airway pressure on functional residual capacity in normal subjects

Functional residual capacity (FRC) was determined by constant volume, whole body plethysmography in seven normal subjects under resting conditions and following the addition of increasing levels of expiratory positive airway pressure (EPAP). There was a significant increase in FRC in six of the seven subjects studied. Grouped data showed a progressive increase in FRC with increasing EPAP (p less than 0.01). The highest level of EPAP (15 cm H2O) was associated with a 20% increase in FRC. We have been able to confirm that EPAP provides a simple and effective method of increasing FRC which could be applied to the treatment of conditions characterized by temporary and reversible reduction in lung volume.

The pattern of stimulated breathing in man during non-elastic expiratory loading

1. The pattern of breathing expressed as the relationship between tidal volume and the components of breath interval was studied in normal subjects during CO2 rebreathing, both under unloaded conditions and following the introduction of a non-elastic expiratory resistance. 2. Under unloaded conditions end-expiratory thoracic gas volume (FRC) measured plethysmographically did not alter during the course of the rebreathing experiment. Maximum tidal volume attained (VT, max.) was equal to or just less than the inspiratory capacity of the subject measured at rest. Expiratory reserve volume was not encroached upon even at the highest levels of ventilation. 3. Under loaded conditions the pattern of breathing was altered. VT, max. was diminished in all subjects and FRC showed a progressive rise during rebreathing which was proportional to the resistive load afforded by the artificial resistance. There were no consistent differences in the components of breath duration either at rest or on maximal ventilatory stimulation between the loaded and unloaded states. 4. It is suggested that the pattern of breathing adopted under conditions of expiratory non-elastic loading is influenced more by the secondary effects of breathing at an elevated lung volume, than by the effect of the non-elastic load per se.