Dose of digoxin prescribed in the UK compared with France and the USA.

BACKGROUND: Two trials in patients with heart failure showed that some patients grew worse if digoxin was withdrawn. The median daily dose of digoxin in these trials was 375 micrograms. We suspected that doses used in the UK were much lower. METHODS: We studied the prescription of digoxin to 119 patients who were discharged from St George's Hospital, London, UK, in April and May, 1994. We calculated the appropriate digoxin prescription dose by Jelliffe's formula. Appropriate doses were put into a scale of six levels: 62.5 micrograms, 125 micrograms, 187.5 micrograms, 250 micrograms, 375 micrograms, or 500 micrograms. We compared our findings with information from a database of prescription records. These records, gathered during 1993-94, came from the UK (1085 prescriptions). France (1148), and the USA (2303). FINDINGS: In the St George's Hospital series, the median daily dose of digoxin was 125 micrograms (mean 170 [SD 80] micrograms). The dose was significantly higher in patients who had heart failure than those who did not, by a mean of 40 micrograms. Among the 100 patients for whom appropriate dose was calculated, the dose was as predicted in 28, one level too low in 34, two or more levels too low in 32, and one level too high in six. There was no difference between St George's Hospital and UK practice as a whole (p > 0.05), but the dosage was significantly higher in the USA and France than in the UK (p < 0.001 for both) and significantly higher in France than in the USA (p < 0.001). INTERPRETATION: Differences in dosage may be correlated to the range of tablet strengths available in each country in our analysis. Underdosing can be detected with certainty only if plasma digoxin concentration is measured. However, our study provides strong circumstantial evidence that many patients in the UK are receiving too little digoxin.

Effect of adenosine infusion on oxygen induced carbon dioxide retention in severe chronic obstructive pulmonary disease.

BACKGROUND: In normal subjects intravenous adenosine infusion has been shown to stimulate ventilation with a consequent fall in arterial partial pressure of carbon dioxide (Paco2), probably by an action on the carotid bodies. The objective of this study was to determine whether the increase in Paco2 seen when patients with ventilatory failure secondary to chronic obstructive pulmonary disease (COPD) are given a high concentration of oxygen to breathe might be ameliorated by an intravenous infusion of adenosine. METHODS: Eight subjects with chronic stable ventilatory failure secondary to COPD were studied. Their mean (SE) forced expiratory volume in one second (FEV1) was 0.63 (0.12) 1 with forced vital capacity (FVC) of 1.63 (0.21) 1. They received continuous intravenous infusions of saline and adenosine in random order, double blind. The infusions were administered for two minutes at 20 micrograms/kg/min, increasing in increments of 20 micrograms/kg/min every two minutes to a maximum infusion rate of 80 micrograms/kg/min adenosine (or an equivalent saline infusion rate), or until side effects supervened. The infusions were continued at that rate for five minutes, after which the fractional inspired oxygen (FIO2) was raised to 0.50 during a further 20 minutes of the infusion at that rate. Haemoglobin oxygen saturation (SaO2) and transcutaneous PCO2 (PtcCO2) were monitored throughout the procedure. Spirometric tests were performed before and after each infusion. RESULTS: Adenosine infusion was accompanied by a fall in PtcCO2 from a mean (SE) of 7.29 (0.42) kPa to 6.95 (0.48) kPa: mean difference -0.34 (95% confidence interval, -0.56 to -0.11) kPa. During saline infusion oxygen administration resulted in an increase in transcutaneous PtcCO2 from 7.35 (0.34) kPa to 7.88 (0.28) kPa: mean difference 0.53 (95% CI 0.20 to 0.85) kPa. PtcCO2 did not rise above baseline levels when oxygen was administered during the adenosine infusion. A small fall in FVC was seen following adenosine infusion. CONCLUSIONS: The increase in PtcCO2 seen when patients with stable ventilatory failure secondary to severe COPD are given a high concentration of oxygen to breathe is counteracted by a continuous intravenous infusion of adenosine.

Alterations in gene expression associated with changes in the state of endothelial differentiation.

The endothelium maintains a developmental plasticity which allows rapid phenotypic change in response to extracellular signals during normal processes, such as corpus luteum formation and wound healing, and in pathologic processes, such as tumor angiogenesis. Endothelial cells (EC) in culture have been very useful for investigating various aspects of endothelial growth and behavior. In spite of documented similarities between EC in vitro and the endothelium in vivo, many characteristics of the vessel endothelium are lost when the cells are placed into culture. We have undertaken to identify differences in gene expression between differentiated vessel endothelium and dedifferentiated EC. We utilized a new technique called differential display which compares polymerase chain reaction (PCR)-amplified mRNA from two (or more) cell populations. Endothelium scraped directly from freshly obtained aortas, and demonstrated to be free of contaminants, were used as the source of differentiated RNA, whereas proliferating, primary explanted EC grown for five days in the presence of basic fibroblast growth factor (bFGF) provided a pool of 'dedifferentiated' RNA. Using differential display, we have observed numerous reproducible differences in gene expression. To confirm that the expression differences visualized by differential display represented actual differences in gene expression, we isolated vessel-specific and culture-specific cDNA tags for additional analysis. Three cDNA tags specific to vessel endothelium were cloned and sequenced, and compared to nucleotide and protein databases. Two of the clones (A1 and 2.5) displayed no significant sequence similarity, whereas a third clone (A2) is nearly identical to a human expressed sequence tag (EST) and has significant sequence similarities to a plant and Xenopus ubiquitin-like protein. Northern and/or in situ hybridization analysis of the A1 and A2 genes confirmed their restricted expression to the vessel endothelium. The expression of A1 by the endothelium in vivo is not simply a function of growth state, as cultured cells did not express A1 even when grown to postconfluence. One other cDNA fragment, selected as a culture-induced gene, was identified by sequence analysis as the bovine homologue of laminin B1, and Northern analysis confirmed that expression was induced upon culturing of EC. Use of differential display to study endothelial gene expression will allow us to investigate the molecular mechanisms that underlie initiation and maintenance of endothelial differentiation.

Ventilatory sensitivity to single breaths of CO2 around the control point in man.

We used single inspiratory capacity breaths of 5, 6 or 8% CO2 in air to obtain ventilatory responses in normal subjects, with ensemble averaging of repeated runs to define stimulus and response (Protocol 1). We also compared the effect of an inspiratory capacity (IC) breath of 8% CO2 with that of two tidal volumes (TV) at the same concentration (Protocol 2). The ventilatory response was defined first as the ratio of peak changes in ventilation and end-tidal PCO2, and secondly by the ratio of their integrals. We obtained group mean values of 0.21 L min-1 mmHg-1 for the peak method and 0.80 L min-1 mmHg-1 for integrals (Protocol 1). There was no significant difference between IC and TV response values (Protocol 2) either by the peak method (0.17 vs 0.19 L min-1 mmHg-1) or by integrals (0.47 vs 0.53 L min-1 mmHg-1). A significant decrease in ventilation was seen in the second tidal volume 8% CO2 breath, even though the stimulus was unperceived by four out of five subjects. CO2 responses can be obtained from these techniques, but the necessary analysis is too cumbersome for general use. Taking a deep breath had no detectable separate effect, but CO2 in the airway may depress ventilation even at concentrations which the subject cannot detect.

Effects of deep breaths on subsequent ventilation in man during rest and exercise.

1. We examined the effects of twenty-four to thirty inspiratory capacity (IC), expiratory capacity (EC) and vital capacity (VC) breaths on subsequent breathing pattern in five normal subjects at rest. 2. During IC breaths and following EC and VC breaths at rest, end-tidal CO2 pressure (PET,CO2) fell by 7.5, 8.5 and 9.5 mmHg, respectively. In the group analysis significant inhibition of ventilation of 1.5 l min-1 was seen after the IC breath but not after EC or VC breaths. 3. We repeated the study with five normal subjects under conditions of higher ventilatory drive, namely 50 W exercise (one subject was common to both groups). 4. During exercise, the drop in PET,CO2 was smaller (4.0, 3.5 and 4.0 mmHg, respectively, with IC, EC and VC breaths) but ventilation was inhibited to a greater extent. Ventilatory undershoot was seen after all three types of deep breaths. 5. We propose that the expiration to residual volume in EC and VC breaths abolished the hypocapnic inhibition of ventilation at rest, possibly by a deflation reflex which was not sufficiently powerful to overcome the ventilatory undershoot during exercise. Our results also support the view that the slope of the CO2 response curve is steeper near the control point during exercise.

Spontaneous improvement in a patient with the hepatopulmonary syndrome assessed by serial exercise tests.

A 37 year old patient with chronic active hepatitis progressing to cirrhosis presented with increasing breathlessness and was found to be hypoxic with finger clubbing. A progressive exercise study with measurement of oxygen saturation (SaO2) showed abnormally high ventilation and desaturation to 81% at 100 W. Serial studies over nearly two years showed, first, deterioration, then improvement with lower ventilation and higher saturation levels at all work loads. This could not be correlated with any change in treatment with azathioprine, prednisolone, or propranolol.

Isocapnic and small hypercapnic single-breath stimuli: evidence for an inhibitory inflation reflex in conscious man.

We wished to find out if a deep inspiration had any influence on subsequent breathing which was mediated by neural rather than chemical stimuli. We therefore compared the effect on ventilation of a deep isocapnic breath with that of a similar breath containing 6% CO2, and with the effect of two successive tidal volume breaths of 6% CO2. We studied five normal subjects, each of whom repeated the three manoeuvres 20 times, and we used ensemble averaging to increase the signal-to-noise ratio. The isocapnic deep inspiration was followed by a significant inhibition of ventilation in the group in the second post-stimulus breath, and in 4 of the 5 subjects in first and second post-stimulus breaths. This was due to an increase in both inspiratory and expiratory time, with a variable effect on tidal volume. A similar initial ventilatory inhibition was seen in the response to a deep breath of 6% CO2. When the isocapnic response was subtracted from the hypercapnic response, the result was similar to that observed from two tidal volume breaths of 6% CO2. We conclude that a single deep inflation of the lungs in awake man inhibits subsequent ventilation by a neural mechanism, and that this may affect the CO2 response measured by single-breath techniques using such manoeuvres.

A role for the epidermal growth factor-like domain of P-selectin in ligand recognition and cell adhesion.

The selectin family of adhesion molecules mediates the initial interactions of leukocytes with endothelium. The extracellular region of each selectin contains an amino-terminal C-type lectin domain, followed by an EGF-like domain and multiple short consensus repeat units (SCR). Previous studies have indirectly suggested a role for each of the extracellular domains of the selectins in cell adhesion. In this study, a panel of chimeric selectins created by exchange of domains between L- and P-selectin was used to directly examine the role of the extracellular domains in cell adhesion. Exchange of only the lectin domains between L- and P-selectin conferred the adhesive and ligand recognition functions of the lectin domain of the parent molecule. However, chimeric selectins which contained both the lectin domain of L-selectin and the EGF-like domain of P-selectin exhibited dual ligand-binding specificity. These chimeric proteins supported adhesion both to myeloid cells and to high endothelial venules (HEV) of lymph nodes and mesenteric venules in vivo. Exchange of the SCR domains had no detectable effect on receptor function or specificity. Thus, the EGF-like domain of P-selectin may play a direct role in ligand recognition and leukocyte adhesion mediated by P-selectin, with the lectin plus EGF-like domains collectively forming a functional ligand recognition unit.

Chemoreceptor drives and short sleep-wake cycles during hypoxia: a simulation study.

We used a Grodins-type mathematical model of the cardio-pulmonary system to investigate the cycling behaviour of the respiratory system during hypoxia in response to changes of state between waking and sleeping, namely a diminution of respiratory chemosensitivity during sleep. Shifts between waking and sleeping were triggered by various combinations of threshold values for PAO2. Mild or moderate hypoxia was simulated by values of inspired O2 concentration between 13% and 16% (normal 21%). In mild or moderate hypoxia, reductions of overall respiratory gain from about 2 to 0.8 l.min-1 mmHg-1 at sleep onset will produce falls in PAO2 likely to cause sleep-wake cycles with oscillations in PAO2. The higher the arousal threshold (in relation to steady-state PAO2 during sleep), the shorter and more stable the sleep-wake cycles. As the arousal threshold is raised, and as hypoxia is exacerbated from mild (FIO2 = 16%) to moderate (FIO2 = 13%), the sleep-wake cycle length tends to converge to a value around one minute. The level and determinants of the "back-to-sleep" threshold are hard to define from presently available experimental data, but the level is not important in determining the length of the sleep-wake cycle compared to the arousal threshold. Alinearities in chemoreceptor feedback were introduced first by incorporating "drive" thresholds, to simulate central or obstructive apnoea. This produced larger oscillations in respiratory variables, but no change in cycle length. Chemoreceptor thresholds for PCO2 at the level of 38-39 mmHg did produce shorter ventilation cycles, down to about 20 s in length, but these were not related in any simple way to the resulting sleep-wake cycles. The combination of sleep state changes and chemoreceptor feedback alinearities can produce short sleep-wake cycles despite the diminution in chemoreceptor gain occurring in sleep.

Estimates of mean alveolar PCO2 during steady-state exercise in man: a theoretical study.

The partial pressure of carbon dioxide in arterial blood is an important operator in the control of breathing, by actions on peripheral and central chemoreceptors. In experiments on man we must often assume that lung alveolar PCO2 equals arterial PCO2 and obtain estimates of the former derived from measurements in expired gas sampled at the mouth. This paper explores the potential errors of such estimates, which are magnified during exercise. We used a published model of the cardiopulmonary system to simulate various levels of exercise up to 300 W. We tested three methods of estimating mean alveolar PCO2 (PACO2) against the true value derived from a time average of the within-breath oscillation in steady-state exercise. We used both sinusoidal and square-wave ventilatory flow wave forms. Over the range 33-133 W end-tidal PCO2 (P(et)CO2) overestimated PACO2 progressively with increasing workload, by about 4 mmHg at 133 W with normal respiratory rate for that load. PCO2 by a graphical approximation technique (PgCO2; "graphical method") underestimated PACO2 by 1-2 mmHg. PCO2 from an experimentally obtained empirical equation (PnjCO2; "empirical method") overestimated PACO2 by 0.5-1.0 mmHg. Graphical and empirical methods were insensitive to alterations in cardiac output or respiratory rate. End-tidal PCO2 was markedly affected by respiratory rate during exercise, the overestimate of PACO2 increasing if respiratory rate was slowed. An increase in anatomical dead space with exercise tends to decrease the error in P(et)CO2 and increase the error in the graphical method. Changes in the proportion of each breath taken up by inspiration make no important difference, and changes in functional residual capacity, while important in principle, are too small to have any major effect on the estimates. Changes in overall alveolar ventilation which alter steady-state PACO2 over a range of 30-50 mmHg have no important effect. At heavy work loads (200-300 W), P(et)CO2 grossly overestimates by 6-9 mmHg. The graphical method progressively underestimates, by about 5 mmHg at 300 W. A simulated CO2 response (the relation between ventilation and increasing PCO2) performed at 100 W suggests that a response slope close to the true one can be obtained by using any of the three methods. The graphical method gave results closest to the true absolute values. Either graphical or empirical methods should be satisfactory for detecting experimentally produced changes in PACO2 during steady-state exercise, to make comparisons between different steady-state exercise loads, and to assess CO2 response in exercise.(ABSTRACT TRUNCATED AT 400 WORDS)

An in vitro model for cell-cell interactions.

Heterotypic cell-cell interactions appear to be involved in the control of development and function in a wide variety of tissues. In the vasculature, endothelial cells and mural cells (smooth muscle cells or pericytes) make frequent contacts, suggesting a role for intercellular interactions in the regulation of vascular growth and function. We have previously grown endothelial cells and mural cells together in mixed cultures and found that heterocellular contact led to endothelial growth inhibition. However, this mixed culture system does not lend itself to the examination of the effects of contact on the phenotype of the individual cell types. We have therefore developed a co-culture system in which cells can be co-cultured across a porous membrane, permitting intercellular contact while maintaining pure cell populations. Co-culture of endothelial cells and smooth muscle cells across membranes with pore sizes of 0.02, 0.4, 0.6, and 0.8 microns maintained the two cell types as homogeneous populations, whereas smooth muscle cells migrated across the membrane through pores of 2.0 microns. Vascular cell co-culture across membranes with 0.8-microns pores resulted in the inhibition of endothelial cell proliferation and the generation of conditioned media which inhibited endothelial cell growth The arrangement of the cells in this co-culture system mimics the in vivo orientation of vascular cells in which mural cells are separated from the abluminal surface of the endothelium by a fenestrated internal elastic lamina or basement membrane. Because this co-culture system maintains separable populations of cells in contact or close proximity allowing for biochemical and molecular analyses of pure populations, it should prove useful for the study of cell-cell interactions in a variety of systems.

Ventilatory responsiveness to carbon dioxide below the normal control point in conscious normoxic humans.

A recently developed CO2 pulse technique was used to test for ventilatory sensitivity to CO2 in four normal men following 2 min voluntary hyperventilation down to an end-tidal CO2 tension (PETCO2) of 20 mmHg (2.7 kPa). Pure CO2 was injected into the inspiratory limb of a breathing circuit at 0.4 l.min-1 for 30 s and any small ventilatory response was detected against background noise by ensemble-averaging of multiple runs. Following hyperventilation, ventilation was initially often above control and apnoea was not seen. In one subject, the ventilatory response to the CO2 pulse was barely detectable either before or after hyperventilation. In another subject, there was a response to pulses given before hyperventilation and 3 and 5.5 min after hyperventilation but not 30 s after hyperventilation when PETCO2 was about 25 mmHg (3.3 kPa) and rising. In the two remaining subjects ventilatory responses were seen to CO2 pulses started 30 s after hyperventilation, although PETCO2 following the pulse remained some 5 mmHg (0.7 kPa) below baseline. We conclude that in some subjects the PETCO2 threshold lies well below the normal PETCO2. The technique is tedious for the experimental subject because of the large number of repetitions required and, therefore, unsuitable for a study on a large number of subjects.