Relationship of pulmonary diffusing capacity (D L ) and cardiac output (\dot Q_c ) in exercise

The present study was designed to investigate the interrelationships of pulmonary diffusing capacity for CO ( \(D_{L_{{\text{CO}}} } \) ), pulmonary capillary blood flow ( \(\dot Q_c \) ), oxygen uptake ( \(\dot V_{{\text{O}}_{\text{2}} } \) ), and related functions in exercise. Six young adult men were tested on a bicycle ergometer on 9–20 occasions at various intensities of exercise up to the maximal level that could be sustained for 5 min. Measurements at each exercise level included work load (kgm/min), heart rate (HR), minute ventilation (V _I ), \(\dot Q_c \) , \(D_{L_{{\text{CO}}} } \) , and \(\dot V_{{\text{O}}_{\text{2}} } \) . Using regression analysis, it was established that \(\dot Q_c \) and D _{L CO} increased linearly with \(\dot V_{{\text{O}}_{\text{2}} } \) throughout the work range in each subject and no tendency toward a plateau was observed. While the maximal value varied from subject to subject, there was no difference between individuals in the coefffcient describing the relationship of D _L and \(\dot Q_c \) to \(\dot V_{{\text{O}}_{\text{2}} } \) . Combining all subjects, D _L was found to increase linearly with \(\dot Q_c \) the regression equation being: $$D_L = 26.4 + 1.03{\text{ }}\dot Q_c ,{\text{ }}r = .79$$ These results suggest that high-intensity short-duration exercise (5 min) is probably not limited by either of these functions in normals.     

Untersuchung zur Tagesperiodik verschiedener Kreislauf-und Atemgrößen bei submaximalen und maximalen Leistungen am Fahrradergometer

The maximal aerobic power of six highly trained young cyclist, mean age 16.3 years and mean \(\dot V_{O_2 \max } \) 4.9 l/min, was directly measured at intervals of 4 hrs. A Latin square design was used for the test order. At submaximal work of O₂-consumption 2.4 to 4.4 l/min no circadian variation of any single function was found. However, at maximal work load the differences between the maxima and minima values were 12.4% for maximal work output ( \(\dot W_{\max } \) ), 7.8% for expiratory minute volume ( \(\dot V_{E\max } \) ), 5.7% for maximal aerobic power ( \(\dot V_{O_2 \max } \) ) and 3.4% for maximal heart rate (HR _max). All the functions—with the exception of \(\dot V_{O_2 \max } \) —had their minima at 0300 hrs; the minima of \(\dot V_{O_2 \max } \) was reached already at 2300 hours. The maxima-values of \(\dot V_{E\max } \) and \(\dot V_{O_2 \max } \) were measured at 1500 hrs, of \(\dot W_{\max } \) andHR _max at 0700 and ofHR _rest at 1900 hrs correspondingly. A one-tailed test showed significant differences between the maxima and minima values of all variables (P<0.05). The results suggest a decreased cardiopulmonary working capacity at night. However, this impairment is only of practical importance if the work will be done near the limit of endurance capacity. Besides it will suggest, that the indirect methods for assessing the cardiopulmonary capacity based on \(\dot V_{O_2 \max } \) and \(\dot W_{170} \) are not useful at nighttime, because the presuppositions for these methods are limited of the time of day.     

Experimental study of the performance of competition swimmers

The purpose of this study was to consider which characteristics are related to a high velocity in water during swimming. The study was performed on 13 of the best competition swimmers of the region of Lyon (France). The collected data were: \(\dot V_{O_2 } \) max, measured during leg-work ( \(\dot V_{O_2 } \) max LW), during arm-work ( \(\dot V_{O_2 } \) max AW), and the hydrodynamic resistances measured when the swimmers were towed at the speed of 1.80 m/sec. The cardiac output was measured for four of the subjects. The swimmers were characterized by a mean \(\dot V_{O_2 } \) max LW of 56 ml/kg/min and by a high value of the ratio \(\frac{{\dot V_{O_2 } \max {\text{ }}AW}}{{\dot V_{O_2 } \max {\text{ }}LW}}\) which reached 99% for three of them. There was a very high correlation (r = 0.90 and 0.91) between the mean speed calculated from the best performances in competition swims of 400 m and 1500 m and a value which is thought to express the aerobic energy available to the swimmer: $$\dot V_{O_2 } \max water:\dot V_{O_2 } \max {\text{ }}AW + \frac{{\dot V_{O_2 } \max {\text{ }}LW - \dot V_{O_2 } \max {\text{ }}AW}}{6}$$      

Maximal steady state versus state of conditioning

Criteria for the identification of maximal steady state as related to state of conditioning were evaluated. 13 volunteers walked and/or ran during a series of 15 min tests on a treadmill. The speeds ranged from mild to exhaustive. Heart rate was monitored continuously; \(\dot V_{O_2 } \) was determined from 6 min to 9 min; and venous blood was obtained at 10 min and 15 min for lactate analyses. Max \(\dot V_{O_2 } \) was established for each subject. Subjects were classified on level of conditioning according to the quantity and quality of their activity record for the previous 6 months. The 10 min heart rate associated with a blood lactate level of 2.2 mM/L (MSSHR) was the best predictor of conditioning. The relative \(\dot V_{O_2 } \) (% of max \(\dot V_{O_2 } \) ) found with a 10 min blood lactate concentration of 2.2 mM/L (RMSS \(\dot V_{O_2 } \) ) was almost as accurate as MSSHR in predicting state of conditioning. Changes in blood lactate levels between 10 min and 15 min were not significantly related to conditioning.     

Energy cost of arm stroke,leg kick,and the whole stroke in competitive swimming styles

In male elite swimmers \(\dot V_{{\text{O}}_{\text{2}} } \) at a given velocity in freestyle and backstroke was on average 1 to 2 l x min^?1 lower as compared with breaststroke and butterfly. Except for breaststroke, swimming with arm strokes only demanded a lower \(\dot V_{{\text{O}}_{\text{2}} } \) at a given submaximal velocity than the whole stroke. In freestyle and backstroke the submaximal \(\dot V_{{\text{O}}_{\text{2}} } \) of leg kick at a given velocity was clearly higher than the whole stroke. The highest velocity during maximal swimming was always attained with the whole stroke, and the lowest with the leg kick, except for breast stroke, where the leg kick was most powerful. At a given submaximal \(\dot V_{{\text{O}}_{\text{2}} } \) , heart rate and \(\dot V_{\text{E}} :\dot V_{{\text{O}}_{\text{2}} } \) tended to be higher during swimming with arm strokes only as compared with the whole stroke. Highest values for \(\dot V_{{\text{O}}_{\text{2}} } \) , heart rate and blood lactate during maximal exercise were almost always attained when swimming the whole stroke, and lowest when swimming with arm strokes only. At higher velocities body drag was 0.5 to 0.9 kp lower when arms or legs were supported by a cork plate as compared with body drag without support.     

Assessment of aerobic and anaerobic capacity of athletes in treadmill running tests

The criteria of max \(\dot V_{O_2 } \) and max O₂ D which are traditionally used in studying aerobic and anaerobic work capacity, have the different dimensions. While max \(\dot V_{O_2 } \) is an index of the power of aerobic energy output, max O₂ D assesses the capacity of anaerobic sources. For a comprehensive assessment of physical working capacity of athletes, both aerobic and anaerobic capabilities should be represented in three dimensions,i.e. in indexes of power, capacity and efficiency. Experimental procedures have been developed for assessing these three parameters in treadmill running tests. It is proposed to assess anaerobic power by measuring excess CO₂, concurrently with determination of max \(\dot V_{O_2 } \) . Maximal aerobic capacity is established as the product of max \(\dot V_{O_2 } \) by the time of max \(\dot V_{O_2 } \) maintenance determined in a special test with running at critical speed. The ergometric criteria derived on the basis of the tests proposed, may be used for systematization of various physical work loads.     

Ammonia production following maximal exercise: Treadmill vs. bicycle testing

From a population of 20 healthy male volunteers, half performed constant speed, incremental load maximal aerobic capacity ( \(\dot V_{O_2 \max } \) ) tests on a motor-driven treadmill, while the other half performed similar \(\dot V_{O_2 \max } \) tests on a bicycle ergometer. The two groups, matched for size and age, showed no significant differences in \(\dot V_{O_2 \max } \) , maximum heart rate, or in post-exercise (4 min) peripheral venous blood concentrations of lactate or pyruvate. However, post-exercise peripheral venous blood ammonia levels were significantly higher in the group tested on the bicycle ergometer than in the treadmill group.     

Plasma catecholamine concentration during dynamic exercise involving different muscle groups

The change in plasma catecholamine concentration (ΔC) has been studied in four healthy male subjects during work, involving different muscle groups, whilst breathing air and 45% oxygen. The results show that during arm and (1- and 2-) leg(s) work ΔC was more closely associated with relative (expressed as a % of \(\dot V_{{\text{O}}_{{\text{2max}}} }\) ) than absolute work load; a rise in C occurred at ～ 60% \(\dot V_{{\text{O}}_{{\text{2max}}} }\) in all 3 forms of exercise. However, in arm and 1-leg work the curves relating ΔC to % \(\dot V_{{\text{O}}_{{\text{2max}}} }\) were displaced to the right indicating the independence of the two variables. Further, breathing 45% oxygen reduced ΔC but was without effect on either \(\dot V_{O_2 } \) at a given work load or \(\dot V_{{\text{O}}_{{\text{2max}}} }\) . For a given \(\dot V_{O_2 } \) , ΔC was inversely related to the effective muscle (plus bone) volume used to perform the work and associated with change of blood lactic acid (LA) concentration, but again the use of exercise involving different muscle groups indicated that the changes in C and LA were essentially independent. This was also true of the changes of C with cardiac output but not cardiac frequency (f _H ). Plasma C changed as a curvilinear function of f _H , the association between the two variables being independent of type of exercise and inspired O₂ concentration within the range used in this study. This suggests that the rise in C and f _H in exercise may be closely related to circulatory stress and may reflect the degree of vasoconstriction present in ‘non-active’ tissues and efficacy of the body's ability to maintain the integrity of systemic blood pressure in the face of increased demands of the exercising muscles for blood and the transport of oxygen.     

Interaction of central and peripheral respiratory drives in cats II. Peripheral and central interaction of hypoxia and hypercapnia

In cats anesthetized with chloralose-urethane the blood supply to the right carotid body was performed by an extracorporal circuit containing a supporting pump and a membrane oxygenator. Inlet and outlet of the circuit were connected to the central and the peripheral end, resp., of the dissected right common carotid artery. By exposure of the circuit blood to different gas mixtures in the membrane oxygenator, \(P_{O_2 } \) and \(P_{CO_2 } \) (referred to as \(P_{gl,O_2 } \) and \(P_{gl,CO_2 } \) ) could be adjusted independently from the gas tensions in the systemic blood. The animals breathed oxygen containing different concentrations of CO₂ in order to obtain selected levels of \(P_{a,CO_2 } \) . The left carotid nerve was dissected. The steady state respiratory responses to isolated and combined hypoxic and hypercapnic stimuli were evaluated.

1. The ventilatory response to severe peripheral hypoxia ( \(P_{gl,CO_2 } \) 33 mmHg) decreased significantly, when \(P_{gl,CO_2 } \) was raised from 30–35 mmHg to 55–60mmHg.
2. This decrease was slightly but not significantly enhanced when \(P_{gl,CO_2 } \) was raised from 21–42mmHg.
3. At constant \(P_{a,CO_2 } \) (28–36mmHg in different animals), the ventilatory response to peripheral hypoxia ( \(P_{gl,CO_2 } \) 63, 47, or 33 mmHg) in general slightly decreased when \(P_{gl,CO_2 } \) was stepwise increased (21–56mmHg).

Under the experimental conditions described above a hypo-additive interaction of respiratory drives could be demonstrated at the level of integration of peripheral and central afferents. At the level of the peripheral chemoreceptors alone a hyper-additive interaction of hypoxic and hypercapnic stimuli could not be confirmed.     

Effects of hypoxic training on normoxic maximal aerobic power output

The responses to 1-leg submaximal and maximal exercise have been studied in four male subjects before and after a 5 week training programme. One leg was trained under normoxic conditions and the other under hypoxic ( \(F_{IO_2 } \) =0.12) conditions for 30 min/day, 3 times/week at a fixed absolute work load which approximated to 75% of the limb's normoxic \(\dot V_{O_2 \max } \) . Before and after training both limbs were measured in normoxia, one limb was additionally measured in hypoxia. The aim of the experiments being to use each subject as his own control and to try and elucidate the effects of hypoxia per se as a training stimulus to the improvement of maximal aerobic power output ( \(\dot V_{O_2 \max } \) ) measured in normoxia. The results showed that before training the responses to exercise at submaximal and maximal levels were identical in each limb; the effects of hypoxia being to raise V _E1.5 and f _H1.5, to reduce \(\dot V_{O_2 \max } \) and to leave \(\dot V_{O_2 450} \) unchanged. The effects of the two types of training were to reduce \(\dot V_{O_2 450} \) , decrease f _H1.5 and increase \(\dot V_{O_2 \max } \) , the effects being independent of the \(F_{IO_2 } \) . The changes in \(\dot V_{O_2 \max } \) of the hypoxic and normoxic trained legs were related to the initial \(\dot V_{O_2 \max } \) of each subjects' limb. It was concluded that our investigation lends no support to the view that hypoxia has either an additive or potentiating effect with exercise during a training programme on the improvement of aerobic power output measured under normoxic conditions.     

A quantitative analysis of the Na+-dependence of\dot V_{max} of the fast action potential in mammalian ventricular myocardium

In microelectrode experiments on papillary muscles of guinea pigs, the quantitative dependence of \(\dot V_{{\text{max}}} \) of the fast action potential, taken as a measure of I_Na, on external Na⁺ concentration has been analyzed under different experimental conditions including the presence of antiarrhythmic drugs such as lidocaine, procaine and propafenone.

1. External Na⁺ concentration changes between 225 mmol/l and 45 mmol/l led to a non-linear response of \(\dot V_{{\text{max}}} \) in that the \(\dot V_{{\text{max}}} \) changes obtained experimentally were significantly smaller than predicted theoretically from a linear dependence of \(\dot V_{{\text{max}}} \) on [Na⁺] _o .
2. Each individual \(\dot V_{{\text{max}}} \) relationship exhibited saturation characteristics. In Lineweaver-Burk plots, \(\dot V_{{\text{max}}} \) correlated extremely well to 1/[Na⁺] _o with correlation coefficients between 0.995 and 0.999. In 16 experiments, the apparentK _m for Na⁺ varied within a range from 170 to 455 mmol/l. Values of 450 V/s–900 V/s were calculated for the saturated \(\dot V_{{\text{max}}} \) (at an infinitely large [Na⁺] _o ).
3. The apparentK _m for Na⁺ rose from 232.0±24.7 mmol/l to 544.0±50.2 mmol/l when the K⁺ concentration of the medium was increased from 5.4 to 10 mmol/l and, thus, resting potential declined from ?90.5±2.5 mV to ?76.1±1.8 mV.
4. Alkalization (pH 9.0) of the medium lowered the apparentK _m for Na⁺ and, simultaneously, reduced the saturated \(\dot V_{{\text{max}}} \) . This typical shift of the straight relating \(\dot V_{{\text{max}}} \) to 1/[Na⁺] _o in the Lineweaver-Burk plot excludes a competitive interaction between Na⁺ and H⁺ ions.
5. Lidocaine (5×10^?5–2×10^?4 mol/l), procaine (2×10^?4 mol/l) and propafenone (0.5–3×10^?5 mol/l) depressed \(\dot V_{{\text{max}}} \) the stronger the lower [Na⁺] _o was. The changes of the \(\dot V_{{\text{max}}} \) relationship induced by these drugs indicated neither a competitive nor a non-competitive interaction of these compounds with Na⁺.
6. As tested with propafenone, external Na⁺ changes modulated the tonic and the phasic block of \(\dot V_{{\text{max}}} \) . The Na⁺ sensitivity of both types of block differed considerably. In a Na⁺-poor (50 mmol/l) medium, the apparentK _m for the tonic block declined from 5×10^?5 to 1.4×10^?5 mol/l and the apparentK _m for the phasic block from 3.8×10^?5 to 2.3×10^?5 mol/l.
7. Na⁺ is not the sole cation that determines the strength of a drug-induced blockade of \(\dot V_{{\text{max}}} \) as it can be substituted by the permeant Li⁺.
8. In the presence of drugs like propafenone which are capable of shifting the steady state inactivation (h _∞) of I_Na to more negative potentials, the voltage-dependence ofh _∞ can be modified by external Na⁺ variations. Na⁺ withdrawal led to a considerable shift in the hyperpolarizing direction.

     

Central leptin replacement enhances chemorespiratory responses in leptin-deficient mice independent of changes in body weight

Previous studies showed that leptin-deficient (ob/ob) mice develop obesity and impaired ventilatory responses to CO₂ $ \left( {{{\dot{V}}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}}} \right) $ . In this study, we examined if leptin replacement improves chemorespiratory responses to hypercapnia (7?% CO₂) in ob/ob mice and if these effects were due to changes in body weight or to the direct effects of leptin in the central nervous system (CNS). $ {\dot{V}_{{{\text{E}}\,}}}{\text{ - C}}{{\text{O}}_{{2}}} $ was measured via plethysmography in obese leptin-deficient- (ob/ob) and wild-type- (WT) mice before and after leptin (10???g/2???l?day) or vehicle (phosphate buffer solution) were microinjected into the fourth ventricle for four consecutive days. Although baseline $ {\dot{V}_{\text{E}}} $ was similar between groups, obese ob/ob mice exhibited attenuated $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ compared to WT mice (134?±?9 versus 196?±?10?ml?min^?1). Fourth ventricle leptin treatment in obese ob/ob mice significantly improved $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ (from 131 ± 15 to 197 ± 10?ml?min^?1) by increasing tidal volume (from 0.38?±?0.03 to 0.55?±?0.02?ml, vehicle and leptin, respectively). Subcutaneous leptin administration at the same dose administered centrally did not change $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ in ob/ob mice. Central leptin treatment in WT had no effect on $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ . Since the fourth ventricle leptin treatment decreased body weight in ob/ob mice, we also examined $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ in lean pair-weighted ob/ob mice and found it to be impaired compared to WT mice. Thus, leptin deficiency, rather than obesity, is the main cause of impaired $ {\dot{V}_{{{\text{E}}\,}}}{ - }\,{\text{C}}{{\text{O}}_{{2}}} $ in ob/ob mice and leptin appears to play an important role in regulating chemorespiratory response by its direct actions on the CNS.     

Indirect determination of maximal aerobic power output during work with one or two limbs

The cardiac frequency (f _H ) and oxygen intake ( \(\dot V_{O_2 } \) ) responses to submaximal and maximal work with 1- and 2-arms and 1- and 2-legs on suitably modified bicycle ergometers in relation to the prediction of maximal aerobic power output ( \(\dot V_{{\text{O}}_{{\text{2max}}} }\) ) have been examined in 12 healthy male subjects. The results showed that the physiological responses to the different forms of submaximal and maximal exercise were distinct and dependent upon the effective muscle mass used to perform the work. The observed \(\dot V_{{\text{O}}_{{\text{2max}}} }\) of 1-limb could be converted to respective 2-limb value with a coefficient of variation ranging from 4 to 7%, but maximal work with the arms gave no guide to that of the legs. Extrapolation of the \(\dot V_{O_2 } \) /f _H curve to the f _{H max} in 1-leg (175 beats/min) and 2-arm (165 beats/min) resulted in an overestimation of \(\dot V_{{\text{O}}_{{\text{2max}}} }\) of +70 ± 200 ml · min^?1 and +70 ± 240 ml · min^?1; but in 1-arm work (153 beats/min), \(\dot V_{{\text{O}}_{{\text{2max}}} }\) was underestimated by ?70 ± 270 ml · min^?1. The bias in predictions for the 3 forms of exercise could be removed by applying the appropriate regression equations relating predicted to observed \(\dot V_{{\text{O}}_{{\text{2max}}} }\) , but the overall accuracy of the extrapolation method was limited to ± 8%, ± 15% and ± 23% in 1-leg, 2-arm and 1-arm work respectively. It was concluded that maximal work with the upper and lower limbs should be treated separately and if an accuracy of greater than ± 8 to ± 23% is required in situations where through injury, two limb exercise cannot be performed, attempts should be made to ascertain the \(\dot V_{{\text{O}}_{{\text{2max}}} }\) of a single limb directly.     

The effect of increased body temperature due to exercise on the heart rate and on the maximal aerobic power

The effect of an increased body temperature (T _r) elicited by prolonged heavy exercise at normal ambient temperature in absence of any heat stress, on the maximal aerobic power ( \(\dot V_{O_2 \max } \) ) and on heart rate (HR) has been studied. The prolonged exercise consisted in running for 1 hr on a motor driven treadmill, this leading to an average increase of T _r of 1.2° C. Oxygen consumption ( \(\dot V_{O_2 } \) ), ventilation (V _I ), HR and T _r were measured at rest and every 10 min during the prolonged exercise. Before and after this exercise indirect measurement of \(\dot V_{O_2 \max } \) were made. After the exercise, HR in submaximal exercise was increased, the increase being less pronounced the heavier the exercise. The HR increment was 17.5 beats/min per 1° C rise in T _r in the exercise involving an oxygen comsumption of 22 ml/kg·min and it dropped to 7.5 b/min · °C when the O₂ consumption increased to 32.4 ml/kg · · min. \(\dot V_{O_2 \max } \) as calculated indirectly from HR values in submaximal exercise remained essentially the same before and after the treadmill run.     

A Numerical Study of Heat and Water Vapor Transfer in MDCT-Based Human Airway Models

A three-dimensional (3D) thermo-fluid model is developed to study regional distributions of temperature and water vapor in three multi-detector row computed-tomography-based human airways with minute ventilations of 6, 15 and 30 L/min. A one-dimensional (1D) model is also solved to provide necessary initial and boundary conditions for the 3D model. Both 3D and 1D predicted temperature distributions agree well with available in vivo measurement data. On inspiration, the 3D cold high-speed air stream is split at the bifurcation to form secondary flows, with its cold regions biased toward the inner wall. The cold air flowing along the wall is warmed up more rapidly than the air in the lumen center. The repeated splitting pattern of air streams caused by bifurcations acts as an effective mechanism for rapid heat and mass transfer in 3D. This provides a key difference from the 1D model, where heating relies largely on diffusion in the radial direction, thus significantly affecting gradient-dependent variables, such as energy flux and water loss rate. We then propose the correlations for respective heat and mass transfer in the airways of up to 6 generations: \(Nu = 3.504\left( {Re\frac{{D_{\text{a}} }}{{D_{\text{t}} }}} \right)^{0.277} , \quad R = 0.841\) and \(Sh = 3.652\left( {Re\frac{{D_{\text{a}} }}{{D_{\text{t}} }}} \right)^{0.268} , \quad R = 0.825\) , where Nu is the Nusselt number, Sh is the Sherwood number, Re is the branch Reynolds number, D _a is the airway equivalent diameter, and \(D_{\text{t}}\) is the tracheal equivalent diameter.     

Determination of heart dipole vector\bar P ‘electrically’ and ‘magnetically’. Comparison‘electrically’ and ‘magnetically’. Comparison

Even though the electrical activity of the heart varies very slowly with time, a complete and thorough investigation of the heart activity in terms of time-varying potentials and fields has been studied. Starting from Maxwell's equations in their vector partial differential equation form, a vector partial differential equation of the electric Hertzian vector potential \(\bar \Pi \) is set up in terms of the assumed electric dipole moment per unit volume of the electrical heart activityP. The equation is solved in terms of an infinite series in \(\bar \Pi \) and then approximated only to its first term since it converges extremely fast. The potential distribution inside the body ?, the electric field intensity \(\bar E\) , and the magnetic field intensity \(\bar H\) , are derived from \(\bar \Pi \) in terms of \(\bar P\) . Thus ?, \(\bar E\) and \(\bar H\) are known in terms of \(\bar P\) , the source. Next, the inverse problem is investigated since it is of unique importance i.e. to determine \(\bar P\) knowing ?, \(\bar E\) or \(\bar H\) . This is done in two different ways: the ‘electric’, ECG, and the ‘magnetic’, MCG. A comparison between the ECG and MCG follows and, finally, ways of determining \(\bar P\) ‘electrically’ and ‘magnetically’ are given.     

Maximum oxygen consumption and catecholamines in thyroidectomized dogs

Comparisons have been made in 7 dogs between maximum oxygen consumption recorded before (N dogs) and after thyroidectomy (T dogs). The comparisons were performed under two conditions 1) during severe cold stress (C \(V_{O_2 }\) max), 2) during a short period of exhaustive work (Ex \(\dot V_{O_2 }\) max). Heart rate, plasma catecholamine and substrate concentrations (glucose, lactic acid, FFA) were measured under each condition.

1. Thyroidectomy induced a more substantial decrease in C \(\dot V_{O_2 }\) max than in Ex \(\dot V_{O_2 }\) max.
2. At C \(\dot V_{O_2 }\) max, average plasma epinephrine and norepinephrine concentrations rose to a higher level in T dogs than in N dogs. In T dogs, correlations were found between plasma epinephrine concentrations and C \(\dot V_{O_2 }\) max values, and between plasma norepinephrine concentrations and C \(\dot V_{O_2 }\) max values. At Ex \(V_{O_2 }\) max, average plasma norepinephrine concentrations were similar in N dogs and in T dogs, and average plasma epinephrine concentrations were not significantly different from each other.
3. At Ex \(\dot V_{O_2 }\) max, average plasma concentrations of the various substrates were not significantly different in N dogs and T dogs. At C \(\dot V_{O_2 }\) max, plasma FFA levels were higher in T dogs.

It may be concluded that in dogs, thyroidectomy affects mechanisms which are more specifically involved in heat production than in muscular exercise. The increased catecholamine secretion in response to cold which occured in T dogs appeared merely to limit the decrease in heat production. It seems possible that increased catecholamine secretion compensates for the decreased sensitivity of β receptors to catecholamine but it cannot fully account for the effects of thyroidectomy.     

Effects of carbon monoxide or low oxygen gas mixture inhalation on regional oxygenation,blood flow,and small vessel blood content of the rabbit heart

The effects of lowering arterial O₂ content, approximately 30%, by inspiration of low O₂ or CO gas mixtures on regional myocardial relative tissue \(P_{O_2 } \) , perfusion and small vessel blood content were studied in anesthetized, thoracotomized New Zealand white rabbits. Relative tissue \(P_{O_2 } \) and perfusion were determined polarographically.⁵⁹FeCl₃ was used to determine small vessel blood content. In control, relative tissue \(P_{O_2 } \) , perfusion and small vessel blood content averaged 33.1 mm Hg, 64.9 ml/min/100 g and 4.3 ml/100 g respectively in the subepicardium (EPI) and 22.7, 53.6 and 4.2 in the subendocardium (ENDO) of the left ventricle. Both hypoxic conditions increased regional blood flow, but to a lesser extent in the ENDO. Relative ENDO tissue \(P_{O_2 } \) fell more markedly than EPI in both conditions. Small vessel blood content increased more with CO than low O₂. Regional O₂ consumption, calculated by Krogh analysis, increased under both conditions. The response to lowered O₂ content is thus an increase in flow, metabolic rate and the number of open capillaries with a lowered driving pressure for O₂. The effects of these types of hypoxia appear more severe in the ENDO.     

Tonic and phasicI Na blockade by antiarrhythmics

Drug-induced depression ofI _Na was studied in microelectrode experiments on papillary muscles of guinea pigs by taking \(\dot V_{\max } \) of the fast Na^ü action potential as an estimate forg _max thereby systematically discriminating between tonic and phasic blockade.

1. The antiarrhythmic compound propafenone and its derivatives butafenone and Sa 76, which are structurally related to local anesthetics, exerted a tonic and phasic \(\dot V_{\max } \) blockade. At a given drug concentration, the strength as well as the kinetics of development and removal of phasic block are functions of the interstimulus interval. The slope of the curve relating the strength to the interstimulus interval indicates the sensitivity of phasic block to changes in stimulation rate. It depends on the molecular structure of the drug and can be also affected by the resting potential. With increasingly shorter interstimulation intervals, phasic inhibition became more marked and developed faster. However, when the development kinetics are considered as an event-dependent process, the fraction of complete phasic block that is installed per single excitation decreases.
2. Each type of drug-induced \(\dot V_{\max } \) block possesses its own individual apparent dissociation constant and apparentn _H coefficient. In the case of propafenone, for instance, the apparentK _m's for tonic and phasic inhibition (at a frequency of 120/min) were 4.4×10^?5 mol/l and 1×10^?5 mol/l, respectively, and the apparentn _H's amounted to 1.76 and 0.84, respectively. The apparentK _m for the phasic block depends on the interstimulus interval and decreases with an increase in stimulation rate. The stoichiometry of the underlying drugreceptor interaction, however, remains virtually unaffected since the apparentn _H of the phasic \(\dot V_{\max } \) block proved insensitive to frequency changes.
3. Propafenone wash-out experiments revealed that tonic and phasic \(\dot V_{\max } \) block disappeared with individual time constants differing by a factor of 3–5 from each other.
4. TTX (5×10^?6 mol/l) enhanced the propafenone action and intensified tonic and phasic \(\dot V_{\max } \) blockade. Each block exhibited an individual quantitative response. Tonic blockade became stronger by a factor of 3.1±0.72, but phasic block by a factor of 1.9±0.1.
5. Chemical channel modification induced by formaldehyde increased the inhibitory efficacy of propafenone. Again, tonic \(\dot V_{\max } \) blockade responded more sensitive than phasic blockade and the former rose by a factor of 6.9 but the latter by a factor of only 1.5.
6. These results can be satisfactorily accounted for in terms of the modulated single receptor hypothesis (HHK model).

     

Methodische Untersuchung zur indirekten Bestimmung der maximalen O2-Aufnahme

The predictability of maximum oxygen uptake ( \(\dot V_{O_2 }\) max) was tested on 78 male subjects taken from four different age-groups (ages: 9 to 10, 13 to 14, 17 to 18, 20 to 52). \(\dot V_{O_2 }\) max and W₁₇₀ were measured during the same experiment, both with stepwise respectively continuously increasing loads on the bicycle ergometer. Oxygen uptake and heart rate were measured at submaximal and maximal workloads. We examined the predictability of \(\dot V_{O_2 }\) max by following the methods described by åstrand and Ryhming (1954), Margaria et al. (1965), and Rutenfranz (1971). We also examined the W₁₇₀-test which gives a direct information on submaximal criteria and requires no further extrapolation and the equation given by Müller (1961 a, b), which allows to compare results of the LPI-method for assessing the endurance limit at submaximal work with the method of I. åstrand (1960) to determine the endurance limit as percentage of maximal aerobic power. When we used the methods mentioned above we observed a considerable underestimation of the measured \(\dot V_{O_2 }\) max. The results obtained by Müller's equation, however, deviated unsystematically from measured \(\dot V_{O_2 }\) max. By means of age-correcting factors according to I. åstrand (1960) respectively to von Döbeln et al. (1967) it is possible to adjust the prediction of \(\dot V_{O_2 }\) max without inquiring into the sources of the observed deviations. By means of special factors to correct extrapolation we were, however, able to avoid an error that is typical of all methods which determine \(\dot V_{O_2 }\) max based on the linear relationship between pulse frequency and oxygen uptake during work by extrapolation to maximal heart rate. This error is due to an underestimation of \(\dot V_{O_2 }\) max as there is no linear relationship between heart rate and O₂-consumption in the upper performance section. In this section O₂-consumption increases more with increasing workload than heart rate. In order to compare the above mentioned procedures we transformed the original formulas and adapted them to the conditions we met in our own experiments. The coefficients of validity with the exception of those calculated according to Müller show that there do not exist any striking differences. The cardio-vascular performance capacity was estimated equally well by means of W₁₇₀. In predicting \(\dot V_{O_2 }\) max the standard deviation was 7 to 10,5%. The mean error of a single determination was 4 to 7%, whether \(\dot V_{O_2 }\) max was measured or predicted.