Changes In Other Variables Research Articles

Effects of Training on the Tolerance to High-Intensity Exercise in Patients with Severe COPD

Background: There has been no systemic investigation of the effects of training on endurance at different high-intensity work rates in patients with COPD to date. Objectives: We wanted to determine the effects of intense endurance training on tolerance to several high-intensity work rates and to establish the relationship between power (WR) and its tolerable duration (t) (t = W’/(WR – CP), being W’, the curvature constant, thought to be reflective of anaerobic energy availability and/or tolerance to the uncomfortable sensations associated with the high-intensity exercise, and CP the critical power. Methods: We studied 27 patients: age 62 ± 5 years; FEV₁ 1.2 ± 0.2 liters. Before and after the intervention, the subjects randomly underwent 4 high-intensity constant WR exercise tests. The endurance times of the highest 3 tests were used. Results: Ventilation reached approximately the same level in each of the tests. In response to the training, the average peak increased by 9 ± 5% (p < 0.0001), CP by 14 ± 12% (p < 0.0001) and W’ 18 ± 22% (p = 0.002). In 67% of the patients, CP increased, accompanied by changes in other related physiological variables of aerobic capacity. In the remaining 33%, W’ increased but not CP. Smaller or no changes in the other variables of aerobic capacity were found in those subjects. This latter group was significantly older and had more hyperinflation (i.e. higher RV/TLC%). Conclusions: Our results suggest that neither an incremental nor a single endurance test at constant WR provides an adequate characterisation of exercise tolerance at other WRs. CP and W’ appear to be important parameters characterizing exercise tolerance over a range high-intensity work rates and identifying two distinct types of response to training.

Early stroke: a dynamic process.

In the acutely infarcted brain, cytokines are released into cerebrospinal fluid and blood. Interleukin (IL)-1β, IL-6, and tumor necrosis factor (TNF)-α, among others, may play a role in the acute increases in plasma adrenocorticotropic hormone, cortisol, epinephrine, norepinephrine, and vasopressin.1–6 Increases in cytokines and hormones may in turn induce changes in other variables. When recorded on admission in acute stroke patients, many variables have been found to be associated with poor outcome in acute stroke. Body temperature7 is one such variable, blood glucose another,8–18 and C-reactive protein (CRP)19–22 and white blood cell count (WBC)20,23 serum cortisol and ferritin24 are further examples. Elevated plasma and cerebrospinal fluid levels of glutamate, glycine,25 ferritin,26 and IL-627 were also associated with deteriorating stroke. It seems plausible that these variables generally are unaffected at stroke onset and then increase in the early hours after onset, depending on the severity of the stroke. We found this to be the case with temperature,28 which in a large series of patients was normal when measured within 2 hours of stroke onset, but which rose at 4 to 6 hours after stroke onset in patients with severe neurological deficits. At 8 to 10 hours after stroke onset, elevated temperature was associated with poor outcome. This association arose several hours after onset of severe stroke. Thus, the initial severity of the stroke …

The effect of velocity on the solid particle erosion rate of alloys

Velocity is a critical test variable in erosion, and can easily overshadow changes in other variables, such as target material, impact angle, etc. The effect of velocity on erosion rate was studied in 70–30 brass (cold worked and annealed) and Fe–C martensite (as-quenched and tempered) and it was found, as previously shown, that erosion rate is dependent on velocity by a power law, given by ER= kV n . However, the velocity exponent n was found to be target material independent and is governed by test conditions, including particle characteristics and the erosion test apparatus. In addition, n is not dependent on the erosion mechanism. Results from tested as-quenched martensite and tempered martensite showed that the exponent is approximately 2.9 for both materials, even though martensite eroded by a brittle cracking mechanism, while tempered martensite eroded by a plastic deformation mechanism. No difference in the erosion rate relationship was found between the fully annealed and the 70% cold worked brass. The exponent n was found to change over time with nominally the same erosion test conditions, indicating that n is very sensitive to slight changes in erodent particles and/or the test apparatus, and that it must be measured periodically if erosion results generated at different times are to be compared. 1 Prime Novelty: The velocity exponent n is independent of both target material and erosion mechanism and is highly dependent on erosion test conditions for the materials tested. 1

Comparison of the myocardial effects of desflurane and isoflurane in healthy patients: assessment by continuous oesophageal aortic blood flow echo-Doppler

Experimentally, desflurane causes a moderate positive inotropic effect and a transient increase in arterial pressure with rapid increases in concentration compared with isoflurane. We used a continuous oesophageal aortic blood flow echo Doppler device to study the myocardial effects of equi-anaesthetic concentrations of isoflurane and desflurane in 32 healthy patients undergoing superficial surgery. After induction of anaesthesia with midazolam, etomidate and fentanyl general anaesthesia was maintained in 16 patients with 0.6% end-expired concentration of isoflurane and in 16 patients with 3% end expired concentration of desflurane. Isoflurane induced a rapid decrease in aortic blood flow (ABF) which remained almost stable whereas desflurane induced an early, moderate and transient increase in ABF (1 min after introduction of the halogenated agent, mean ABF was 107 (SD 3)% in the desflurane group vs 95 (9)% in isoflurane group compared with control values before introduction of the inhalation agent; P = 0.005), followed by a marked secondary decrease in ABF. The maximal decrease in ABF reached 71 (15)% of its initial value in the desflurane group compared with 80 (14)% in the isoflurane group (ns). Neither agent caused significant changes in other variables except for PE'CO2 which decreased in both groups. Continuous ABF echo-Doppler monitoring demonstrated an early transient positive inotropic effect of desflurane.

Evaluation of treatment with a pulsed electromagnetic field on wound healing, clinicopathologic variables, and central nervous system activity of dogs

Abstract Objective To evaluate effects of treatment with a pulsed electromagnetic field (PEMF) on healing of open and sutured wounds, clinicopathologic variables, and CNS activity of dogs. Animals 12 adult female Beagles. Procedure Open and sutured wounds were created in the skin of the trunk of the dogs. Dogs were divided into 2 groups. One group received PEMF treatment and 1 group served as untreated (control) dogs. The PEMF-treated dogs received treatment twice a day starting the day before surgery and lasting through day 21 after surgery. Wounds were evaluated by use of tensiometry, planimetry, laser Doppler perfusion imaging, and histologic examination. Clinicopathologic variables and electroencephalographic tracings were also evaluated. Results Use of PEMF treatment resulted in significantly enhanced epithelialization of open wounds 10 and 15 days after surgery. Five days after surgery, wounds of control dogs had a negative value for wound contraction, whereas PEMF-treated wounds had a positive value. The PEMF treatment did not cause significant changes in short-term planimetric, perfusion, tensiometric, histologic, clinicopathologic, or electroencephalographic results. Conclusions The PEMF treatment enhanced wound epithelialization in open cutaneous wounds and provided indications of early contraction without significant short-term changes in other variables. (Am J Vet Res 1998;59:1177-1181)

Waiting time in an urban accident and emergency department--a way to improve it.

OBJECTIVE: To determine how the introduction of a new small team consultation system could reduce the average waiting time of patients in the busy accident and emergency (A&E) department of...

REPRODUCIBILITY OF RESPONSES TO EXHAUSTIVE CYCLE ERGOMETER EXERCISE TESTS352

Seven men of mean (±SD) age 27 ± 5 yr, ht 185 ± 7 cm, and mass 81.6 ± 4.3 kg, volunteered to participate. They performed two incremental cycle ergometer tests and two exhaustive tests at the work rate at which ˙VO2max was elicited in the first incremental test (Wmax; 266± 20 W). During tests, expired gases were analyzed using a MedGraphics CardiO2. Mean values for ˙VO2max in the incremental tests were not different (43.8 ± 5.7 ml·kg-1·min-1; 44.1 ± 5.9 ml·kg-1·min-1; p = 0.83) and highly correlated (r= 0.88, p ≤ 0.05). Similarly, means for ˙VO2max in tests at Wmax were not different (41.7 ± 4.4 ml·kg-1·min-1; 42.2 ± 5.5 ml·kg-1·min-1; p = 0.70) and highly correlated (r= 0.85, p ≤ 0.05). Time to exhaustion (TE), time to achieve˙VO2max (TTmax), time that ˙VO2max was sustained(TAmax), and O2 deficit were measured only in the constant-power tests. Trial-to-trial differences were not significant despite ranging from 9 to 17%. Means (± SD): Table There were relationships between trial-to-trial differences in TE, TTmax, and ˙VO2max, suggesting that the measure of TTmax as a descriptor of ˙VO2 kinetics may be influenced by even small changes in other variables. It is concluded that, while ˙VO2max seems quite robust, other exercise responses are subject to greater amounts of day-to-day variability and/or learning effects.

Long-term effects of clodronate on growing rat bone.

For evaluating the long-term effects of the bisphosphonate compound clodronate on the rat skeleton, 100 female rats were given subcutaneous injections of clodronate at doses of 0 (vehicle), 4 (low), or 12 (high) mg/kg per week, or 50 mg/kg every fourth week (cyclical). The treatment was started at 3 months of age and was continued for 6 months. The mechanical strength of bones was studied by torsion of the tibia, three-point bending of the femur, axial compression of the femoral neck, and compression of a lumbar vertebra. Quantitative histomorphometric variables were determined from distal femur and lumbar vertebra, and variables reflecting bone metabolism were measured in serum and urine. Bone mass, indicated by ash weight of the tibia, was increased with the low and high clodronate doses compared with the vehicle. The maximum load in vertebra compression was increased with the low dose of clodronate compared with the vehicle, whereas changes in other variables concerning bone strength were not significant. In bone histomorphometry, clodronate treatment induced more pronounced changes in cancellous bone volume in distal femur than in lumbar vertebra, the differences not being statistically significant between the groups at either site, however. The longitudinal growth rate of the femur, measured by double-fluorochrome labeling for 1 week at the end of the treatment period, was significantly decreased in the high-dose clodronate group compared with the other groups. Serum values for calcium, tartrate-resistant acid phosphatase, and alkaline phosphatase did not differ between the groups. However, serum osteocalcin was significantly lower in the high-dose group compared with the vehicle group. Urinary calcium, hydroxyproline, and hydroxylysylpyridinoline were decreased at all the clodronate doses administered. In conclusion, the beneficial effects of long-term clodronate treatment on bone mass and strength were observed at the lowest dose used. A high dose of clodronate decreased bone growth rate, which was, however, not reflected in the mechanical quality of bone.

The influence of extracorporeal membrane oxygenation on cerebral oxygenation and hemodynamics in normoxemic and hypoxemic piglets.

The objective of this study was to compare the effect of extracorporeal membrane oxygenation (ECMO) on cerebral oxygenation and hemodynamics in normoxemic and hypoxemic piglets. Six hypoxemic and six normoxemic piglets were put on venoarterial ECMO after cannulation of the right common carotid artery and external jugular vein with careful priming to avoid hemodilution. Changes in cerebral concentrations of oxyhemoglobin (cO2Hb), deoxyhemoglobin (cHHb), (oxidized-reduced) cytochrome aa3 (cCyt.aa3), and blood volume (CBV) were continuously measured by near infrared spectrophotometry. Heart rate, arterial O2 saturation (SaO2), arterial blood pressure, pulsatility ratio of systemic circulation (calculated as systolic-diastolic/mean arterial blood pressure), central venous pressure, intracranial pressure, and left common carotid artery blood flow (LCaBF) were simultaneously measured. We found that the cannulation procedure resulted in increased CBV, cHHb, and LCaBF in both groups. At 60 and 120 min after starting ECMO, the values of cO2Hb, CBV, and LCaBF in both groups were significantly higher than precannulation values, while the pulsatility ratio decreased. In the hypoxemic groups cHHb decreased and SaO2 increased as well. No significant changes of other variables were found. Between hypoxemic and normoxemic groups no significant differences in the response of CBV and LCaBF at 60 and 120 min were found. We conclude that in piglets cannulation for ECMO resulted in cerebral venous congestion and compensated increase in LCaBF. After starting ECMO, the cerebral O2 supply increased due to increased arterial O2 content. It was accompanied by similar increase of CBV in both groups, probably as a result of hyperperfusion, which seems to be related to the ECMO procedure itself.

The changing consumer in Denmark

Major changes have taken place in the Danish economy throughout the eighties and the early nineties, with a strong impact on consumers. A survey of economic factors and of changes in other variables influencing the consumer, such as demographic, educational and cultural trends, is presented. The impact of these factors on consumption structure, on consumption and time use, and on trends in the composition of the ‘macro’ marketing mix are discussed.

Treatment of surgical and non-surgical septic multiorgan failure with bicarbonate hemodialysis and sequential hemofiltration

Hospital mortality of patients with septic multiorgan failure (MOF) is still around 95%. The present study investigates whether this high mortality could be significantly reduced by the addition of sequential hemofiltration (SH) with bicarbonate hemodialysis (HD) to the currently used life supportive measures. 35 (18 surgical and 17 nonsurgical) patients, with 3 or more organ failures, had daily sessions of zero balance SH, for periods ranging from 2-22 days. SH induced significant improvement of PaO2/100 FIO2, Apache II score, MAP, as well as blood chemistry in survivors. Dying patients had less marked improvement of blood oxygenation, non-significant changes in other variables, in addition to low MAP before and after SH, as well as marked hemodynamic unstability during the procedure. The observed hospital mortality was 38% for the surgical group, and 35.3% for the medical patients (n.s.). Mortality observed in this retrospective, uncontrolled study was significantly lower than that currently observed with conventional supportive therapy, with or without the addition of other forms of blood purification, e.g. CAVH and CAVHD. This improvement in results appears to be related to the property of SH to completely clear 90% of the blood from mediators of inflammation in only one passage through the hemofilter, and to better tolerance of HD done using bicarbonate buffer. A definite evaluation of this technique will be eventually reached by a programmed, appropriate sample size study, which is out of reach for one individual ICU.

Effects of repeated indomethacin administration on cerebral oxygenation and haemodynamics in preterm infants: Combined near infrared spectrophotometry and Doppler ultrasound study

The objectives of this study were to evaluate the effect of repeated indomethacin administration on cerebral oxygenation in relation to changes in cerebral blood flow velocity (CBFV) and other relevant physiological variables. Fourteen preterm infants with patent ductus arteriosus were studied during three subsequent indomethacin bolus administrations with intervals of 12 and 24 h. Changes in concentration of oxyhaemoglobin (cO2Hb), deoxyhaemoglobin (cHHb) and oxidized cytochrome aa3 (cCyt.aa3) in cerebral tissue and changes in cerebral blood volume (CBV) were measured by near infrared spectrophotometry; changes in mean CBFV in the internal carotid artery were measured by pulsed Doppler ultrasound. Simultaneously heart rate, transcutaneous pO2 and pCO2, arterial O2 saturation and blood pressure were measured. All variables were continuously recorded until 60 min after indomethacin administration. Within 5 min after each indomethacin administration, significant decreases in CBFV, CBV and cO2Hb and cCyt.aa3 were observed which persisted for at least 60 min, while cHHb increased or did not change at all. There were no changes in the other variables recorded. These data demonstrate that indomethacin administration is accompanied by a reduction in cerebral tissue oxygenation due to decreased cerebral blood flow. Therefore, low arterial oxygen content, either caused by low arterial O2 saturation or by low haemoglobin concentration, may be a contraindication for indomethacin treatment in preterm infants.

Peak flow variation in childhood asthma: A three-year analysis

Measuring peak expiratory flow (PEF) variation has been suggested as a indicator of asthma disease severity and also of nonspecific bronchial hyperreactivity. To test these assumptions, we examined the relationships between PEF variation, methacholine reactivity, symptom scores, and medication requirements in 74 children with tightly controlled allergic asthma. The level of mean diurnal variation (MDV) for the group was 7.1%, which is generally regarded as normal. We found statistically significant correlations between MDV and both methacholine reactivity ( r = 0.43, p = 0.0001) and symptom scores ( r = 0.28, p = 0.016). These asthma variables were analyzed longitudinally in 33 children who were followed up at 6-month intervals for at least 36 months. Visit-to-visit changes in MDV were generally not reflective of changes in other variables. However, group levels of MDV gradually decreased over time, especially in children with initial MDV of more than 8%. This reduction in group MDV coincided with similar reductions in group medication requirements and methacholine reactivity. We conclude that children with moderately severe asthma that is tightly controlled may have normal levels of PEF variation. The correlation between PEF variation and other asthma variables is statistically significant but too weak to be useful in the treatment of individual patients. In contrast, measurement of MDV may be a useful indicator of disease severity in group studies of asthma.

Hormonal and Hemodynamic Changes Following Single-Dose Felodipine Treatment During Mental Stress

Mental stress induces changes in hemodynamic variables and in the plasma level of many hormones and plasma peptides. These changes can be modulated by various drugs, eg, beta-blockers. In a double-blind, placebo-controlled crossover study, the authors evaluated the hormonal and hemodynamic changes during psychological stress and the effect of felodipine 10 mg (plain tablet). Eight male volunteers participated. Heart rate, blood pressures, and stroke volume were measured by ECG, mercury sphygmomanometer, and impedance cardiography. Catecholamines and atrial natriuretic factor in plasma were measured by electrochemical and radioimmunoassay techniques. A single dose of felodipine, 10 mg, exaggerates the heart rate decreases the left ventricular ejection time and augments the plasma level increment of noradrenaline. The stress-induced changes in other variables were not influenced by felodipine treatment. In conclusion, acute felodipine treatment influences the reflex activation of the sympathetic nervous system during psychological stress. In the treatment of patients, especially patients with heart disease, these findings could be important but further investigations in patients need to be done.

An examination of the covariability of subclinical albuminuria in insulin-dependent diabetes mellitus: implications for monitoring microalbuminuria

We examined the covariability of subclinical albuminuria in a cohort of 160 insulin-dependent diabetics who underwent simultaneous measurements of overnight urinary albumin excretion rate (U AV) and several clinical variables on 7 occasions over a period of 18 months. After allowing for within-patient correlation between visits, significant within-patient associations were found between U AV and variations in tobacco consumption (regression coefficient = 0.006, S.E. = 0.002, P = 0.02), insulin dose (regression coefficient = 0.003, S.E. = 0.001, P = 0.01) and creatinine clearance (regression coefficient = 0.776, S.E. = 0.047, P < 0.0001); the association between U AV and creatinine clearance was not influenced by adjusting for urine flow rate. No significnat associations were found between U AV and changes in blood pressure, glycaemic control or other variables. We then focused on 33 patients with intermittent microalbuminuria, defined as a U AV > 30 μg/min on at least one occasion, but not exceeding 3 consecutive occasions. These patients had a total of 52 episodes of microalbuminuria which were significantly associated at the 5% level with increases in creatinine clearance ( P = 0.02), but not with changes in other variables. Our findings did not differ in patients with intermittent microalbuminuria defined with reference to a cut-off U AV of 20 μg/min. We conclude that changes in glomerular filtration rate (as reflected by creatinine clearance) may partly account for the within-patient variability in subclinical albuminuria in insulin-dependent diabetics, and that variables routinely measured in the clinic are not useful for deciding when to monitor patients for microalbuminuria.

Effects of Inspiratory Flow Rate Alterations on Gas Exchange During Mechanical Ventilation in Normal Lungs: Efficiency of End-inspiratory Pause

The influence of inspiratory flow rate (TI), without changing respiratory frequency, tidal volume, and FIO2, was investigated in 11 normal lungs in patients undergoing mechanical ventilation because of central respiratory failure due to stable coma. The patients were anesthetized and paralyzed. They first received a conventional ventilation (TI = 25 percent, pause = 10 percent) and then, were submitted to four different TI values, randomly administered without any end-inspiratory pause (EIP) (TI = 20 percent; TI = 33 percent; TI = 50 percent; TI = 67 percent). In the middle and at the end of the procedure, a return to basal conditions was introduced. At each ventilator setting, the following were obtained: respiratory flow (Pneumotachograph Fleish No. 2), airway pressure, FRC changes (inductive plethysmography), arterial and mixed venous blood gases, hemodynamic data, and VA/Q ratios distribution using multiple inert gases technique. EIP suppression provides a significant increase in VA/Q mismatch (until TI = 50 percent) and in shunt effect (between 3 and 9 percent of cardiac output [QT]). The absence of simultaneous PaO2 change is due to increasing PVO2 linked to a higher QT. The shorter the TI, the higher the PaCO2 connected with a relative alveolar hypoventilation. However, increasing TI without EIP significantly decreases ventilation distribution inequalities. This improvement is concomitant with a rise in FRC (FRC67-FRC20 = 0.340 +/- 0.450, p < 0.05) without any change in other variables or auto-PEEP production. In summary, in subjects with very slight mechanical lung impairment (peak inspiratory pressure = 20.5 +/- 5.3 cm H2O at TI = 20 percent and 15.2 +/- 3.3 cm H2O at TI = 67 percent), this study confirms the deleterious effect of EIP suppression and TI decrease. One can compensate for this effect of EIP absence by increasing TI as soon as it reaches TI = 67 percent, ie, inverse ratio ventilation.

Photoperiod-driven changes in reproductive function in male rhesus monkeys.

Environmental lighting, which regulates seasonal breeding in many animal species, has not been examined as a regulator of reproduction or puberty in man or nonhuman primates. In this study we examined the effects of controlled alternating long and short daily photoperiods, independent of other environmental variables, on testicular size and function in rhesus monkeys. Sixteen animals, some pubertal, others prepubertal, were individually caged indoors in light-controlled rooms. They were subjected to a 32-week "year" with alternating 16-week cycles of long (16 h of light and 8 h of darkness) or short (8 h of light and 16 h of darkness) days. Animals were examined every 2 weeks over four 32-week "years". Body weight, testicular diameter, and testicular volume were measured, and blood was collected for testosterone and PRL determinations. We found that although short days did not trigger testicular development in prepubertal animals, testicular growth was markedly enhanced by short days in postpubertal animals, accompanied by increased plasma testosterone levels and reduced PRL levels. In long days, testes regressed, testosterone levels fell, and PRL levels rose. The periodicity of testicular size, as determined by spectral analysis, showed a strong signal at a cycle length of 31 +/- 13 weeks, but no signal at intervals close to the natural 52-week year, indicating that the observed periodicity is induced by the changes in lighting, rather than by circannual changes in other variables. These studies establish that changes in photo-period alone can modulate reproductive function in a higher primate and suggest that the onset of puberty is not directly driven by seasonal fluctuations in day length.

Central Effects of Atrial Natriuretic Peptide on Plasma Catecholamines, Vasopressin, Renin and Beta-Endorphin and on Renal Excretory Functions in the Dog

Atrial natriuretic peptide (ANP) has been identified in the central nervous system and its participation in regulation of various regulatory brain functions has been postulated. To elucidate whether central ANP influences endocrine systems related to blood pressure regulation and renal excretory functions, effects of infusion of ANP at a rate of 120 ng.min-1 into the third cerebral ventricle on plasma level of epinephrine (E), norepinephrine (NE), renin, vasopressin and beta-endorphin as well as on excretion of urine, sodium, potassium (UKV) solutes and free water (CH2O) were investigated in conscious dogs. Significant decrease of plasma E from 77.6 +/- 7.0 to 62.1 +/- 4.8 pg.ml-1 and of NE from 345.5 +/- 20.7 to 286.4 +/- 15.0 pg.ml-1 was found at the end of 30 min lasting ANP infusion. Significant elevation of PRA and UKV and a decrease in CH2O were found 60 min after ANP infusion. No significant changes in other variables were found. In time control experiments plasma hormones concentration and renal excretory functions were not significantly influenced. The results suggest that central ANP may affect the sympatho-adrenal outflow.

Surveillance continue de la S [formula omitted] au cours de l'anesthésie pour chirurgie pulmonaire

The multiplicity of potential causes of variations in mixed venous oxygen saturation (Sv̄ 2) during one lung ventilation (OLV), including a constant ventilation/perfusion mismatch, explains that it has been suggested as a routine monitoring procedure. To assess its usefulness, 12 adults undergoing OLV were monitored during surgery with an Oximetrix ® pulmonary catheter, placed on the side opposite to the surgical field under fluoroscopic control. Seventy two complete sets of haemodynamic measurements were obtained at 6 different times during surgery. We studied the ability of changes in Sv̄ 2 to predict changes in arterial oxygen saturation (Sao 2), cardiac output (CO), and venous admixture (VA) by calculating sensitivities (Se), specificities (Sp) and predictive values with regard to these variables. There were no complications due to the protocol. However left-sided catheter placement failed in four cases. Correlation between optical and measured Sv̄ 2 was very strong (r=0.94; p<0.001). Sv̄ 2, oxygen consumption ( V ˙ o 2 ) and the rate of oxygen extraction remained constant throughout the procedure, even when CO, mean arterial pressure, VA, Sao 2 and Pao 2 varied. Clamping the pulmonary artery returned VA, Sao 2 and Pao 2 values to those found before OLV, but produced a significant decrease in CO. Sv̄ 2 had low Se and Sp for changes in other variables (CO: 76±7, 48±9; Pao 2: 79±6, 59±9; VA: 54±7, 48±7 respectively). In this type of surgery, alterations in variables related to oxygen are probably balanced by haemodynamic changes. In fact, according to Fick's formula, Sv̄ 2 is almost completely determined by Sao 2 and CO, when V ˙ o 2 and haemoglobin remain stable. However, a decrease in Sv̄ 2 should alert the clinician to a potential change in haemodynamic indices or to a problem in some component in the oxygen delivery system. Sv̄ 2 values do not add to or correlate well with other data, but facilitate their continuous monitoring.

Comments on Professor Freedman’s Paper

Professor Freedman makes a number of useful comments on path analysis. Perhaps their reiteration by an eminent statistician will have more of an effect than similar statements by others have had. For example, Keynes (1939, 1940), Baumrind (1983), and Cliff (1983) seem to have had little effect on the volume of such naive applications of path analysis. I am somewhat concerned, though, that careless reading of his will serve to reinforce the naive interpretations of path analysis that he is trying to counteract. A superficial reading of its preliminaries leads to the impression that causality is established by drawing straight lines from one thing to another. This may lead the whimsical or literal-minded reader to establish some surprising causal relations with the aid of a crayon. More fundamental to possible misunderstanding is the formulation of the theorem itself. This seems to prove that the correlation between variables is the sum of products of path coefficients and correlations with proximate causes. This will seem to the path analyst to reflect exactly what he or she has been doing. I believe that Professor Freedman's purpose in the paper is to discourage the practice of treating correlations in this way, but in the context he seems to be accepting it. That is, correlational data are rarely sufficient to demonstrate causal relations, but the theorem seems to condone the use of correlations to establish causality. A point that he emphasizes that may not have received due attention in earlier statements of this kind is some of the fallacies in the interventionist interpretation of a path model. What many users of path analysis would like to conclude, or perhaps would like others to conclude, is that if there is a path between X and Y as revealed by a path analysis, then intervening to change X will result in the change in Y that is shown by the coefficient in the model. Such hopeful interpretations are unsophisticated for the following reasons: 1. It is hard to tell whether there will be a change in other variables correlated with X or not. (A promotion of the father of a particular child will change the father's place on the occupational scale. Will it also change the numerous other variables that are correlated with occupation?) 2. Close examination of almost all applications will reveal that the variables that furnish the numbers actually used in a path analysis are surro-