The heart shadows in the runners in the American Marathon , who were examined radiologically immediately after the race , were not increased at all .sx In fact , they were smaller than they were on the following day .sx During severe exercise in normal subjects , Clark-Kennedy and Owen ( 2 ) have shown that there is a simultaneous breakdown of both the circulatory and the respiratory functions , with a complete failure to eliminate the carbon dioxide and the lactic acid products formed by the muscles .sx This causes a cessation of the exercise , and so protects the heart from damage .sx In diseased hearts the protection may not be sufficient .sx With them the reserve power , which is so great a factor in the normal heart , is lessened .sx Exercise may cause the limit to be exceeded , and may precipitate cardiac failure .sx The Circulatory Rate .sx Since the function of the heart is to maintain the circulation , it is important to know what happens to the rate of the circulation when the heart fails .sx METHODS OF ASCERTAINING THE CIRCULATORY RATE .sx Many methods of obtaining the circulatory rate have been devised for animal experiments , but only a few which could be used in the human subject .sx These had inherent defects which caused their results to differ within rather wide limits , and they all involved the intelligent co-operation of the patient , besides a laborious technique for gas analysis .sx Recently three methods have been devised which have led to an important advance in our knowledge of the dynamics of circulatory failure .sx The circulatory rate may be expressed by calculating the time taken for the blood to move from point to point in the body .sx This is called the velocity of the blood flow .sx It is not a constant factor in the body , since the velocity decreases from the aorta to the periphery , owing to the widening of the total stream-bed .sx If , however , corresponding points are taken in different individuals , it is a valuable method of ascertaining the exact rate of the blood flow .sx The difficulty has been to obtain some substance which , when introduced into one part of the blood stream , wouldhe capable of being detected instantaneously upon its arrival in another .sx Another method of ascertaining the circulatory rate is to estimate the total volume of blood passing a certain point within a given time .sx The only place where this can be done in the human subject is in the lungs .sx The principle is that laid down by Fick .sx If the amount of oxygen required to change a fixed quantity of venous blood into arterial blood is known , and the amount of oxygen which is used up in the lungs within a given period is ascertained , then the quantity of blood which has passed through the lungs in that period can be calculated .sx The result gives the actual volume of flow , and is expressed in litres per minute .sx It is called the volume flow .sx VELOCITY OF BLOOD FLOW .sx Blumgart and Yens ( 3 ) and Weiss ( 4 ) have introduced a method of obtaining the velocity of the blood flow , which , although it requires a technique too elaborate for any general application , has yet afforded a valuable insight into the effects of cardiac disease upon the circulation .sx Radium Method .sx They expose a solution of sodium chloride to the emanations of radium , and then inject it into the median basilic vein .sx They are careful to keep the arm at the same level as the right auricle .sx At about the corresponding position of the other arm they place a radium detector which shows the presence of the radium emanations as soon as they reach the arteries at the bend of the forearm .sx The emanations are prevented from reaching the detector , except by means of the blood stream , by a lead screen which is interposed immediately proximally to the detector .sx This cuts off all stray rays from the other parts of the body .sx The time taken by the emanations to travel from one arm to the other is measured with a stop watch , and the result is expressed simply in seconds .sx Arm-to-Arm Times .sx In a number of normal subjects the time varied from fifteen to twenty seconds .sx The flow was somewhat rapid in children .sx Various groups of cardiac lesions were examined , and in general they found that the times were greatly increased in cardiac failure alone .sx Thus they were normal or moderately prolonged according to the clinical condition , in groups of rheumatic cases with and without valve lesions , where heart failure was absent .sx The same was found in syphilitic disease .sx In auricular fibrillation they noted that the velocity was always low , but they do not appear to have examined any cases where some signs of heart failure were not present .sx When fibrillation was superadded to a heart already extensively diseased , the rate was extremely slow .sx In high blood pressure the rate was sometimes slow , in other cases it was normal ; it was never rapid .sx In cardiac failure a great increase in the time was invariably found .sx Instead of the normal fifteen to twenty seconds , the readings ranged from fifty to seventy , and their length was in .sx direct proportion to the amount of failure , as shown by dyspnoea and oedema .sx Pulmonary Times .sx By a modification of their method they .sx have been able to estimate the time taken by the blood stream to .sx pass through the pulmonary circuit .sx They placed a detector over the skin corresponding to the position of the right auricle .sx By this means they obtained the time taken by the blood to traverse the veins from the ante-cubital fossa to the right auricle .sx This they called the arm-to-heart time , and it was a register of the velocity of the blood in the veins .sx The actual time spent in the pulmonary circulation was estimated by the following calculation .sx In travelling from the right auricle to the arteries at the forearm , the blood has to pass first through the right side of the heart , then through the lungs to the left side of the heart , and from the left ventricle it is sent down the arteries .sx They accepted a previously ascertained calculation that the time spent in the heart was a constant of one second , and estimated that the velocity in the arteries was double that in the veins .sx In normal subjects they found that the average arm-to-heart time was seven seconds .sx The time in the arteries was therefore three and a half seconds .sx The total arm-to-arm velocity varied , as we have stated , from fifteen to twenty seconds .sx Taking the average to be eighteen , and sub-tracting from this seven seconds for the arm-to-heart time , three and a half for the time in the arteries , and one second for that in the heart , an average pulmonary circulation time is obtained of six and a half seconds .sx An increase in the ventricular rate was accompanied by a slight but definite increase in the velocity of the pulmonary flow .sx Variations in the systemic pressure had no effect upon it .sx Patients with emphysema were found to have normal velocities .sx The few exceptions had signs of circulatory failure in addition to their emphysema .sx In syphilitic aortic regurgitation the pulmonary times were increased , although the arm-to-heart times were usually normal .sx They suggest that in this condition the left side of the heart fails before the right ; the result is that the pulmonary circulation is slowed while the venous return to the right side of the heart is still efficiently dealt with .sx This view is important , since it implies that an inefficient left ventricle is associated with pulmonary congestion .sx The left ventricle fails to deal with the blood coming to it , and so the pressure is increased behind it .sx Cases of myocardial degeneration with normal rhythm were also examined .sx When there had been no history of failure , the pulmonary circulation times were increased up to 50 per cent .sx , but the arm-to-heart times were normal .sx In those who had a history of previous failure , the pulmonary circulation times were doubled , and , in addition , the arm-to-heart times were increased 80 per cent .sx In some , heart failure was actually present at the time of the test , and both the pulmonary and the arm-to-heart times were increased 400 per cent .sx , so that the blood stream was only travelling at a quarter the usual speed .sx These figures are of interest , since , in a condition in which the stress might be expected to fall chiefly upon the left ventricle , the pulmonary times always exceeded the arm-to-heart times until the stage of cardiac failure had definitely been reached .sx They also found generally that there was no oedema in those in whom the arm-to-heart time was under fifteen seconds , which is double the normal time , but oedema was usually present in those in whom the time was more than eighteen seconds .sx Conclusions .sx Their work affords valuable evidence that in .sx cardiac failure the circulation rate is slowed :sx that the slowing is present both in the systemic and the pulmonary circulations ; that the speed of the circulation is unaltered in other conditions causing dyspnoea , such as emphysema ; and that oedema may be expected when the velocity of the returning venous blood is less than half the normal rate .sx It further suggests that in those conditions in which the left ventricle is likely to fail first , the stream is in fact slowed more in the pulmonary circulation than in the systemic .sx Histamine Method .sx Weiss .sx Blumgart and Robb ( 5 ) have devised another method of obtaining the same information which is well adapted for clinical use .sx It depends upon the fact that histamine dilates the skin vessels .sx They inject suddenly into the antecubital vein 0.001 mg .sx of histamine per kilo .sx of body weight in a 1 :sx 5,000 or 1 :sx 10,000 solution , and time with a stop watch the appearance of the flush in the face .sx The patient usually experiences at the same time a metallic taste in the mouth , which is also noted .sx The average time taken in normal subjects was twenty-three seconds .sx This is five seconds longer than the average normal arm-to-arm times obtained by the radium method .sx The difference is due to the fact that the histamine method includes the time taken by the blood stream to pass through the small vessels in which the current is slower .sx The figures in circulatory disorders corresponded with those obtained with the radium method .sx Thus , in hypertension and in emphysema , the results were normal .sx In hyperthyroidism and in pernicious anaemia the velocity was increased .sx In cardiac failure the velocity was greatly diminished .sx They noted that by this method the times were sometimes normal in pulmonary oedema and syphilitic aortic regurgitation where severe derangement of the circulation was evident .sx This they thought might be due to a slight disproportion in the output of the two sides , even though the circulation was being maintained at a normal speed .sx THE VOLUME FLOW .sx The method of Henderson and Haggard ( 6 ) does not furnish such valuable information as the radium method , since it is impossible by it to distinguish between the rates in the systemic and pulmonary circuits .sx The apparatus issimilar to that required for the estimation of the basal metabolic rate , but it is considerably easier than the latter test for a dyspnoeic patient to manage .sx Ethyl Iodide Method .sx Ethyl iodide is a liquid which is easily vaporised , and when taken in through the lungs is entirely destroyed in the tissues , being converted into sodium iodide and ethyl alcohol .sx The amount which is absorbed from the lungs will travel to the tissues , but none of it will return .sx The arterial blood absorbs twice as much ethyl iodide per litre as there is in the alveolar air .sx The number of litres of blood that pass through the lungs in a minute equals , therefore , the quantity of ethyl iodide retained in the lungs during a minute divided by twice the amount in each litre of alveolar air .sx Samples of the inspired , expired and the alveolar air are collected by means of a special spirometer , and are analysed .sx The amount of ethyl iodide in each litre of inspired air , less that in each litre of expired air , multiplied by the number of litres breathed in a minute , furnishes the quantity of the gas that has been left in the lungs during the minute .sx