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Stanley asked whether a tree is determined up to isomorphism by its chromatic symmetric function. We approach Stanley's problem by studying the relationship between the chromatic symmetric function and other invariants. First, we prove Crew's conjecture that the chromatic symmetric function of a tree determines its generalized degree sequence, which enumerates vertex subsets by cardinality and the numbers of internal and external edges. Second, we prove that the restriction of the generalized degree sequence to subtrees contains exactly the same information as the subtree polynomial, which enumerates subtrees by cardinality and number of leaves. Third, we construct arbitrarily large families of trees sharing the same subtree polynomial, proving and generalizing a conjecture of Eisenstat and Gordon., Comment: 19 pages, 7 figures. Final version, to appear in Bulletin of the London Mathematical Society. One open question from previous version has been resolved by Michael Tang; other changes are minor

Large, high-purity, germanium (HPGe) detectors are needed for neutrinoless double-beta decay and dark matter experiments. Currently, large (> 4 inches in diameter) HPGe crystals can be grown at the University of South Dakota (USD). We verify that the quality of the grown crystals is sufficient for use in large detectors by fabricating and characterizing smaller HPGe detectors made from those crystals. We report the results from eight detectors fabricated over six months using crystals grown at USD. Amorphous germanium (a-Ge) contacts are used for blocking both electrons and holes. Two types of geometry were used to fabricate HPGe detectors. As a result, the fabrication process of small planar detectors at USD is discussed in great detail. The impact of the procedure and geometry on the detector performance was analyzed for eight detectors. We characterized the detectors by measuring the leakage current, capacitance, and energy resolution at 662 keV with a Cs-137 source. Four detectors show good performance, which indicates that crystals grown at USD are suitable for making HPGe detectors., Comment: 18 pages and 13 figures

Fuzzing is a commonly used technique designed to test software by automatically crafting program inputs. Currently, the most successful fuzzing algorithms emphasize simple, low-overhead strategies with the ability to efficiently monitor program state during execution. Through compile-time instrumentation, these approaches have access to numerous aspects of program state including coverage, data flow, and heterogeneous fault detection and classification. However, existing approaches utilize blind random mutation strategies when generating test inputs. We present a different approach that uses this state information to optimize mutation operators using reinforcement learning (RL). By integrating OpenAI Gym with libFuzzer we are able to simultaneously leverage advancements in reinforcement learning as well as fuzzing to achieve deeper coverage across several varied benchmarks. Our technique connects the rich, efficient program monitors provided by LLVM Santizers with a deep neural net to learn mutation selection strategies directly from the input data. The cross-language, asynchronous architecture we developed enables us to apply any OpenAI Gym compatible deep reinforcement learning algorithm to any fuzzing problem with minimal slowdown.

The \emph{simplicial rook graph} SR(d,n) is the graph whose vertices are the lattice points in the $n$th dilate of the standard simplex in $\mathbb{R}^d$, with two vertices adjacent if they differ in exactly two coordinates. We prove that the adjacency and Laplacian matrices of SR(3,n) have integral spectrum for every $n$. The proof proceeds by calculating an explicit eigenbasis. We conjecture that SR(d,n) is integral for all $d$ and $n$, and present evidence in support of this conjecture. For $n<\binom{d}{2}$, the evidence indicates that the smallest eigenvalue of the adjacency matrix is $-n$, and that the corresponding eigenspace has dimension given by the Mahonian numbers, which enumerate permutations by number of inversions., Comment: Minor revisions. Final version, to appear in Graphs and Combinatorics

Let $I_n$ be the ideal of all algebraic relations on the slopes of the $\binom{n}{2}$ lines formed by placing $n$ points in a plane and connecting each pair of points with a line. Under each of two natural term orders, the initial ideal of $I_n$ is generated by monomials corresponding to permutations satisfying a certain pattern-avoidance condition. We show bijectively that these permutations are enumerated by the updown (or Euler) numbers, thereby obtaining a formula for the number of generators of the initial ideal of $I_n$ in each degree., Comment: 7 pages; 1 figure

Let $T$ be an unrooted tree. The \emph{chromatic symmetric function} $X_T$, introduced by Stanley, is a sum of monomial symmetric functions corresponding to proper colorings of $T$. The \emph{subtree polynomial} $S_T$, first considered under a different name by Chaudhary and Gordon, is the bivariate generating function for subtrees of $T$ by their numbers of edges and leaves. We prove that $S_T = <\Phi,X_T>$, where $<\cdot,\cdot>$ is the Hall inner product on symmetric functions and $\Phi$ is a certain symmetric function that does not depend on $T$. Thus the chromatic symmetric function is a stronger isomorphism invariant than the subtree polynomial. As a corollary, the path and degree sequences of a tree can be obtained from its chromatic symmetric function. As another application, we exhibit two infinite families of trees (\emph{spiders} and some \emph{caterpillars}), and one family of unicyclic graphs (\emph{squids}) whose members are determined completely by their chromatic symmetric functions., Comment: 16 pages, 3 figures. Added references [2], [13], and [15]

We study the construction of premonoidal categories, where the pentagon relation fails, through representations of finite group algebras and their quantum doubles. Both finite group algebras and their quantum doubles have a finite number of irreducible representations. We show that in each case there are at least $2^{n-1}$ inequivalent premonoidal categories of representations, where $n$ is the number of irreducible representations. By construction, for the case of finite group algebras the categories are symmetric whereas for the quantum doubles the categories are braided.

This paper discusses interactivity enabled by two-way technologies providing real-time exchanges of audio, video, text, and graphical information among distributed participants, as one of distance learning's primary identifying characteristics. Background is provided on interaction agents, including three types of instructional interactions--interactions between the learner and the instructor, interactions among learners, and interactions between learners and content. Interaction outcomes are summarized; the two purposes of interactions in this context are to change learners and to move learners toward an action state of goal attainment. The following types of categories that focus on what learners are to achieve as a result of an interaction are then described: participation; communication; feedback; elaboration; learner control/self-regulation; motivation; negotiation; team-building; discovery; exploration; clarification; and closure. The paper concludes with a discussion of applying interaction in practice. (DLS)

Classroom teachers in the 1990s are encountering more and more students who are uninterested in learning. In fact, some students begrudge even opening up their books whether it be to read, spell or solve math problems. These negative attitudes are believed by many teachers to have a significant bearing on the students' ability to perform well in any given subject area. A study therefore investigated the correlation between attitudes toward reading and performance in reading. Subjects, 35 eighth-grade students out of a class of 360 predominantly white, lower middle class, urban junior high students in Ohio were given the Wisconsin Reading Attitude Inventory Form which assessed their attitudes toward reading. The scores on the inventory were then compared to the grades the students received in their seventh grade reading class last year to see if a correlational relationship existed. The results were determined by establishing the mean, standard deviation, median, Cronbach's reliability of each part of the inventory and how they correlated with the grades students received. For this group of readers, the results revealed that a low correlation existed between any attitude score or a combination of scores and grades in seventh grade reading class. (Contains seven references and one table.) (Author/TB)

Issues and trends currently affecting the enterprise of education are identified, particularly as they relate to continuing education. To date, the national education reform agenda has featured the major components of national standards, a national curriculum, national tests, improved teacher training, and teacher empowerment combined with site-based management. The six national education goals define the conceptual parameters for a comprehensive educational reform agenda. Many educators recognize that, given the changing nature of what students need to know and do, traditional curricula and assessments are inadequate. A performance-based approach will address the needs of learners to have meaningful and relevant instructional experiences. Recommendations for transformed educational practice made by the Western Interstate Commission for Higher Education include: (1) an agenda for action at the state level; (2) commitment to greater collaboration in the public interest; (3) the pursuit of excellence in higher education; (4) creating a new environment for racial and ethnic diversity by fostering successful multicultural environments; (5) going beyond the traditional curriculum in technology use; and (6) faculty leadership toward education goals. Continuing educators are in an excellent position to facilitate educational reform by formulating strategies to meet the needs of the public. (Contains 11 references.) (SLD)

An introduction to instructional technology systems used for distance learning applications is provided, with a compilation of technical information in straightforward terms. Broadcast communications theory is discussed, and several models of communications are considered, including transport mechanics and network facilities. The most prevalent feature of the technology applications associated with distance learning is teleconferencing. The four primary modes of teleconferencing are audioconferences, audiographic teleconferences, computer conferences, and video teleconferences. Interactive distance learning technologies are categorized as narrow-band or broadband systems, both of which are described. The distinctions between analog and digital transmission are identified. Nontechnical users of distance learning systems will benefit from the rudimentary information in this paper. Five charts illustrate models and means of distance education. (Contains seven references.) (SLD)

The methodology developed for the Western Cooperative for Educational Telecommunications' evaluation of the "New Pathways to a Degree" projects funded by the Annenberg/CPB Project is described. This methodology provides an approach for conducting a valid, reliable evaluation effort that is sensitive to specific institutional issues and contingencies, while providing generalizable results for cross-institutional discussions. Also included is a description of an environmental scan process that reveals the data points found most useful in evaluating distance learning projects. Seven projects were funded through the New Pathways initiative. The key to consistency in data collection and reporting within and across sites in this evaluation was to establish a framework within which the evaluations would occur. The evaluation approach used protocols for quantitative and qualitative investigation that facilitated systematic decision making with regard to: (1) context evaluation; (2) input evaluation; (3) process evaluation; and (4) product evaluation. Nine categories of information were identified as areas in which evaluative data were likely to be collected. The environmental scan provided evaluators with a way to determine the information actually being collected. Thirteen attachments illustrate the evaluation approach. (SLD)

Following a detailed review of present tuition and financial aid policies, the Committee on Tuition and Other Student Costs focused on recommended changes in policy and the implication of such policies on the costs to students of access to higher education opportunities. The committee affirmed its belief that an important goal of the Board of Higher Education should be the "availability of educational opportunity without regard to financial status and the elimination of financial barriers to higher education." To obtain this goal, the objectives of student choice, student access, and student opportunity must be met. Sections two and three of this study summarize the recommendation of the committee. (Author)

The use of Learner Centered Psychological Principles in instruction and training emphasizes self-motivated, self-directed, and interactive learning. Topics include traditional versus learner-centered instructional approaches, the Learner-Centered Psychological Principles, the challenge of designing learner-centered instruction, and learner-centered designs for distance education. (AEF)

Discusses several systems models of interaction in distance education and relates them to the contexts of instructional delivery, instructional design, instructional theory, and learning theory in order to establish conceptual parameters for the function of interaction. Models covered include those developed by Dick and Carey, Shannon and Weaver, and Shannon and Schramm. (Contains 41 references.) (KRN)

Discusses variables that affect the success of distance education programs in elementary, secondary, and postsecondary education or corporate training environments. Topics addressed include needs assessment, audience analysis, instructional design, course selection and reconfiguration, instructional delivery systems, adapting media and other materials, student and faculty support services, and program evaluations. (nine references) (LRW)

Operation Everest II was designed to examine the physiological responses to gradual decompression simulating an ascent of Mt Everest (8,848 m) to an inspired PO2 of 43 mmHg. The principal studies conducted were cardiovascular, respiratory, muscular-skeletal and metabolic responses to exercise. Eight healthy males aged 21-31 years began the "ascent" and six successfully reached the "summit", where their resting arterial blood gases were PO2 = 30 mmHg and PCO2 = 11 mmHg, pH = 7.56. Their maximal oxygen uptake decreased from 3.98 +/- 0.2 L/min at sea level to 1.17 +/- 0.08 L/min at PIO2 43 mmHg. The principal factors responsible for oxygen transport from the atmosphere to tissues were (1) Alveolar ventilation--a four fold increase. (2) Diffusion from the alveolus to end capillary blood--unchanged. (3) Cardiac function (assessed by hemodynamics, echocardiography and electrocardiography)--normal--although maximum cardiac output and heart rate were reduced. (4) Oxygen extraction--maximal with PvO2 14.8 +/- 1 mmHg. With increasing altitude maximal blood and muscle lactate progressively declined although at any submaximal intensity blood and muscle lactate was higher at higher altitudes.

It is not known whether the asymptotic behavior of whole body O2 consumption (VO2) at maximal work rates (WR) is explained by similar behavior of VO2 in the exercising legs. To resolve this question, simultaneous measurements of body and leg VO2 were made at submaximal and maximal levels of effort breathing normoxic and hypoxic gases in seven trained male cyclists (maximal VO2, 64.7 +/- 2.7 ml O2.min-1.kg-1), each of whom demonstrated a reproducible VO2-WR asymptote during fatiguing incremental cycle ergometry. Left leg blood flow was measured by constant-infusion thermodilution, and total leg VO2 was calculated as the product of twice leg flow and radial arterial-femoral venous O2 concentration difference. The VO2-WR relationships determined at submaximal WR's were extrapolated to maximal WR as a basis for assessing the body and leg VO2 responses. The differences between measured and extrapolated maximal VO2 were 235 +/- 45 (body) and 203 +/- 70 (leg) ml O2/min (not significantly different). Plateauing of leg VO2 was associated with, and explained by, plateauing of both leg blood flow and O2 extraction and hence of leg VO2. We conclude that the asymptotic behavior of whole body VO2 at maximal WRs is a direct reflection of the VO2 profile at the exercising legs.

To quantify the relative contributions of convective and peripheral diffusive components of O2 transport to the increase in leg O2 uptake (VO2leg) at maximum O2 uptake (VO2max) after 9 wk of endurance training, 12 sedentary subjects (age 21.8 +/- 3.4 yr, VO2max 36.9 +/- 5.9 ml.min-1.kg-1) were studied. VO2max, leg blood flow (Qleg), and arterial and femoral venous PO2, and thus VO2leg, were measured while the subjects breathed room air, 15% O2, and 12% O2. The sequence of the three inspirates was balanced. After training, VO2max and VO2leg increased at each inspired O2 concentration [FIO2; mean over the 3 FIO2 values 25.2 +/- 17.8 and 36.5 +/- 33% (SD), respectively]. Before training, VO2leg and mean capillary PO2 were linearly related through the origin during hypoxia but not during room air breathing, suggesting that, at 21% O2, VO2max was not limited by O2 supply. After training, VO2leg and mean capillary PO2 at each FIO2 fell along a straight line with zero intercept, just as in athletes (Roca et al. J. Appl. Physiol. 67: 291-299, 1989). Calculated muscle O2 diffusing capacity (DO2) rose 34% while Qleg increased 19%. The relatively greater rise in DO2 increased the DO2/Qleg, which led to 9.9% greater O2 extraction. By numerical analysis, the increase in Qleg alone (constant DO2) would have raised VO2leg by 35 ml/min (mean), but that of DO2 (constant Qleg) would have increased VO2leg by 85 ml/min, more than twice as much. The sum of these individual effects (120 ml/min) was less (P = 0.013) than the observed rise of 164 ml/min (mean). This synergism (explained by the increase in DO2/Qleg) seems to be an important contribution to increases in VO2max with training.

Most models of metabolic control concentrate on the regulation of ATP production and largely ignore the regulation of ATP demand. We describe a model, based on the results of Hogan et al. (J. Appl. Physiol. 73: 728-736, 1992), that incorporates the effects of ATP demand. The model is developed from the premise that a unique set of intracellular conditions can be measured at each level of ATP turnover and that this relationship is best described by energetic state. Current concepts suggest that cells are capable of maintaining oxygen consumption in the face of declines in the concentration of oxygen through compensatory changes in cellular metabolites. We show that these compensatory changes can cause significant declines in ATP demand and result in a decline in oxygen consumption and ATP turnover. Furthermore we find that hypoxia does not directly affect the rate of anaerobic ATP synthesis and associated lactate production. Rather, lactate production appears to be related to energetic state, whatever the PO2. The model is used to describe the interaction between ATP demand and ATP supply in determining final ATP turnover.

This study was designed to investigate the role of tissue oxygenation in some of the factors that are thought to regulate muscle respiration and metabolism. Tissue oxygenation was altered by reductions in O2 delivery (muscle blood flow x arterial O2 content), induced by decreases in arterial PO2 (PaO2). O2 uptake (VO2) was measured in isolated in situ canine gastrocnemius at rest and while working at two stimulation intensities (isometric tetanic contractions at 0.5 and 1 contractions/s) on three separate occasions, with only the level of PaO2 (78, 30, and 21 Torr) being different for each occasion. Muscle blood flow was held constant (pump perfusion) at each work intensity for the three different levels of PaO2. Muscle biopsies were obtained at the end of each rest and work period. Muscle VO2 was significantly less (P less than 0.05) at both stimulation intensities for the hypoxemic conditions, whereas [ATP] was reduced only during the highest work intensity during both hypoxemic conditions (31% reduction at 21 Torr PaO2 and 17% at 30 Torr). For each level of PaO2, the relationships between the changes that occurred in VO2 and levels of phosphocreatine, ADP, and ATP/ADP.P(i) as the stimulation intensity was increased were significantly correlated; however, the slopes and intercepts of these lines were significantly different for each PaO2. Thus a greater change in any of the proposed regulators of tissue respiration (e.g., phosphocreatine, ADP) was required to achieve a given VO2 as PaO2 was decreased.(ABSTRACT TRUNCATED AT 250 WORDS).

In normal subjects, exercise widens the alveolar-arterial PO2 difference (P[A-a]O2) despite a more uniform topographic distribution of ventilation-perfusion (VA/Q) ratios. While part of the increase in P(A-a)O2 (especially during heavy exercise) is due to diffusion limitation, a considerable amount is caused by an increase in VA/Q mismatch as detected by the multiple inert gas elimination technique. Why this occurs is unknown, but circumstantial evidence suggests it may be related to interstitial pulmonary edema rather than to factors dependent on ventilation, airway gas mixing, airway muscle tone, or pulmonary vascular tone. In patients with lung disease, the gas exchange consequences of exercise are variable. Thus, arterial PO2 may increase, remain the same, or fall. In general, patients with advanced chronic obstructive pulmonary disease (COPD) or interstitial fibrosis who exercise show a fall in PO2. This is usually not due to worsening VA/Q relationships but mostly to the well-known fall in mixed venous PO2, which itself results from a relatively smaller increase in cardiac output than VO2. However, in interstitial fibrosis (but not COPD), there is good evidence that a part of the fall in PO2 on exercise is caused by alveolar-capillary diffusion limitation of O2 transport; in COPD (but not interstitial fibrosis), a frequent additional contributing factor to the hypoxemia of exercise is an inadequate ventilatory response, such that minute ventilation does not rise as much as does CO2 production or O2 uptake, causing arterial PCO2 to increase and PO2 to fall.

Ventilation-perfusion (VA/Q) inequality has been shown to increase with exercise. Potential mechanisms for this increase include nonuniform pulmonary vasoconstriction, ventilatory time constant inequality, reduced large airway gas mixing, and development of interstitial pulmonary edema. We hypothesized that persistence of VA/Q mismatch after ventilation and cardiac output subside during recovery would be consistent with edema; however, rapid resolution would suggest mechanisms related to changes in ventilation and blood flow per se. Thirteen healthy males performed near-maximal cycle ergometry at an inspiratory PO2 of 91 Torr (because hypoxia accentuates VA/Q mismatch on exercise). Cardiorespiratory variables and inert gas elimination patterns were measured at rest, during exercise, and between 2 and 30 min of recovery. Two profiles of VA/Q distribution behavior emerged during heavy exercise: in group 1 an increase in VA/Q mismatch (log SDQ of 0.35 +/- 0.02 at rest and 0.44 +/- 0.02 at exercise; P less than 0.05, n = 7) and in group 2 no change in VA/Q mismatch (n = 6). There were no differences in anthropometric data, work rate, O2 uptake, or ventilation during heavy exercise between groups. Group 1 demonstrated significantly greater VA/Q inequality, lower vital capacity, and higher forced expiratory flow at 25-75% of forced vital capacity for the first 20 min during recovery than group 2. Cardiac index was higher in group 1 both during heavy exercise and 4 and 6 min postexercise. However, both ventilation and cardiac output returned toward baseline values more rapidly than did VA/Q relationships. Arterial pH was lower in group 1 during exercise and recovery. We conclude that greater VA/Q inequality in group 1 and its persistence during recovery are consistent with the hypothesis that edema occurs and contributes to the increase in VA/Q inequality during exercise. This is supported by observation of greater blood flows and acidosis and, presumably therefore, higher pulmonary vascular pressures in such subjects.

In a recent study by Tsukimoto et al. (J. Appl. Physiol. 68: 2488-2493, 1990), CO2 inhalation appeared to reduce the size of the high ventilation-perfusion ratio (VA/Q) mode commonly observed in anesthetized mechanically air-ventilated dogs. In that study, large tidal volumes (VT) were used during CO2 inhalation to preserve normocapnia. To separate the influences of CO2 and high VT on the VA/Q distribution in the present study, we examined the effect of inspired CO2 on the high VA/Q mode using eight mechanically ventilated dogs (4 given CO2, 4 controls). The VA/Q distribution was measured first with normal VT and then with increased VT. In the CO2 group at high VT, data were collected before, during, and after CO2 inhalation. With normal VT, there was no difference in the size of the high VA/Q mode between groups [10.5 +/- 3.5% (SE) of ventilation in the CO2 group, 11.8 +/- 5.2% in the control group]. Unexpectedly, the size of the high VA/Q mode decreased similarly in both groups over time, independently of the inspired PCO2, at a rate similar to the fall in cardiac output over time. The reduction in the high VA/Q mode together with a simultaneous increase in alveolar dead space (estimated by the difference between inert gas dead space and Fowler dead space) suggests that poorly perfused high VA/Q areas became unperfused over time. A possible mechanism is that elevated alveolar pressure and decreased cardiac output eliminate blood flow from corner vessels in nondependent high VA/Q regions.

Insights into muscle energetics during exercise (e.g., muscular efficiency) are often inferred from measurements of pulmonary gas exchange. This procedure presupposes that changes of pulmonary O2 (VO2) associated with increases of external work reflect accurately the increased muscle VO2. The present investigation addressed this issue directly by making simultaneous determinations of pulmonary and leg VO2 over a range of work rates calculated to elicit 20-90% of maximum VO2 on the basis of prior incremental (25 or 30 W/min) cycle ergometry. VO2 for both legs was calculated as the product of twice one-leg blood flow (constant-infusion thermodilution) and arteriovenous O2 content difference across the leg. Measurements were made 3-5 min after each work rate imposition to avoid incorporation of the VO2 slow component above the lactate threshold. For all 17 subjects, the slope of pulmonary VO2 (9.9 +/- 0.2 ml O2.W-1.min-1) was not different (P greater than 0.05) from that for leg VO2 (9.2 +/- 0.6 ml O2.W-1.min-1). Estimation of "delta" efficiency (i.e., delta work accomplished divided by delta energy expended, calculated from slope of VO2 vs. work rate and a caloric equivalent for O2 of 4.985 cal/ml) using pulmonary VO2 measurements (29.1 +/- 0.6%) was likewise not significantly different (P greater than 0.05) from that made using leg VO2 measurements (33.7 +/- 2.4%). These data suggest that the net VO2 cost of metabolic "support" processes outside the exercising legs changes little over a relatively broad range of exercise intensities. Thus, under the conditions of this investigation, changes of VO2 measured from expired gas reflected closely those occurring within the exercising legs.

Summarizing results of a study of the educational programme developed by the United Nations Relief and Works Agency (UNRWA) for Palestine Refugees in the Near East in cooperation with Unesco, the document's specific focus is on aspects of the UNRWA/Unesco programme of interest to others concerned with refugee education. Chapters 1 and 2 provide basic background information and describe the UNRWA/Unesco Educational System. Chapter 3 reviews the programme's development and outlines basic prerequisites for initiation of broad-based educational programmes in emergency camp environments. Chapter 4 is concerned with a discussion of educational planning and decentralization of the planning process. Chapter 5 includes qualitative aspects of school education, basic statistical information, and emphasizes the coherency between school education, in-service teacher training, and supervision. Chapter 6 examines the structure of the supervisory system and focuses on the importance of maintaining such a system, providing guidance to teachers, and assessing teaching performance. Chapter 7 reviews the development of the in-service teacher-training programme and deals with components of its approach to teacher training, with a view to the possibility of applying these components in training programmes in other settings. Chapter 8 presents a summary, conclusions, and general recommendations. (Author/AN)

Two assessment endeavors were undertaken to determine the relative impact of alphanumeric and graphic instructional mediators upon intentional and incidental learning outcomes in applied instructional contexts. The intent of these investigations was to determine the feasibility of embedding strategic organizational cues within instruction to improve intentional learning outcomes in contexts where direct interactive feedback opportunities are limited. Results indicated that alphanumeric and graphic displays both provided organizational frameworks for subjects to use in immediate recall exercises, and that facilitation effects for alphanumeric and graphic displays were not maintained in delayed recall exercises. (18 references) (Author)

The resolution of the technique considered is sufficient to describe smooth distributions containing blood flow to unventilated regions (shunt), ventilation to unperfused regions (dead space), and up to three additional modes over the range of finite ventilation-perfusion ratios. In particular, areas whose ventilation-perfusion ratios are low can be separated from unventilated regions and those whose ventilation-perfusion ratios are high can similarly be distinguished from unperfused areas.

Carbon monoxide (CO) is difficult to measure with a respiratory mass spectrometer because of the similar mass numbers of CO and nitrogen, but this is possible using carbon-18 monoxide. The mass resolution, signal-to-noise ratio, linearity, and background were all found to be adequate. The measurement of the single-breath diffusing capacity was examined. Unless the mean alveolar volume during breath holding is used in the calculation, the value for Dco obtained depends on which portion of the alveolar sample is analyzed. The anatomic dead space for CO was found to be almost the same as that for argon suggesting that the diffusion rate at the dead space-alveolar gas interface was not greatly affected by the alveolar concentration of the gas.

Respiratory gas exchange, end-tidal gas tensions, alveolar ventilation, respiratory frequency, cardiac output, and pulse rate were measured in four healthy adult males at rest and during mild and moderate exercise in air at 18-22 C and underwater at 35.0-35.5 C. Immersion was associated with a 10% increase in pulse rate and cardiac output at all levels of exercise. There were no changes in end-tidal CO2 tension or alveolar ventilation. It is concluded that horizontal subjects breathing at eupneic pressures and working against mild and moderate loads in warm water show the same responses to exercise as in air.

Simultaneous time courses for O2 and CO2 exchange along the capillary have been calculated for homogeneous lungs, allowing for O2-CO2 interactions, dissolved O2, and chemical reaction rates. As diffusing capacity (Dl) was reduced, the transfer of CO2 and O2 was impaired by similar amounts, in spite of the 20-fold greater diffusing capacity for CO2. The reason why CO2 is affected so much is that the slope of the content against partial pressure is so much greater in blood than tissue for this gas. Because of the shapes of their respective dissociation curves, O2 transfer was most affected at normal ventilation-perfusion ratios, whereas CO2 was most affected at high ratios. Exercise exaggerated the impairment of transfer of both gases.