The Reservoir Function of the Left Atrium

During Ventricular Systole*

An Angiocardiographic Study of Atrial Stroke Volume and Work

COLIN GRANT, M.B., t IVAN L. BUNNELL, M.D. and DAVID G. GREENE, M.D.

Bufalo, New York

M OST INVESTIGATORS have regarded the atria as muscular contractile chambers whose

purpose is to aid ventricular filling [ 7-31, adding energy to their contents by an active systole. Many elegant experiments, some using a fibrillating atrium as the control preparation [4,5], have been carried out to study the process. In these experiments the fibrillating atrium is tacitly assumed to have failed and to be a paralyzed functionless structure located up- stream from the atrioventricular valve. In the corresponding clinical situation a patient with atria1 fibrillation experiences no obvious ill effects provided his pulse rate is controlled [4,6]. From this observation it is usually deduced that the atria1 contribution is not critically important. However, an alternative explanation could be that the fibrillating atrium is function- ing well. A possible function can be visualized by considering the behavior of a soft balloon placed between the pulmonary veins and mitral valve, able to act as an easily distensible reser- voir. Such a balloon will fill during ventricular systole while the mitral valve is closed and will be able to empty again while the valve is open. Thus, it will be acting as a reservoir for blood when the mitral valve is closed during ven- tricular systole, and without it flow in the pulmonary veins would stop during this time. For this short-term storage function the main operation is filling, and emptying is the necessary preliminary action to refilling. Such a function could be carried out equally well by a muscular contractile chamber, and this communication presents evidence that the left atrium in man behaves in this way.

In addition to storing blood, an atria1 reser- voir chamber stores energy. When the elastic chamber dilates, work is performed in stretching the wall, and energy is absorbed from the blood. This energy is stored in the wall and released as the chamber empties. The energy transfer can be demonstrated from the atria1 pressure- volume diagram, for the area of such a diagram is a measure of the work performed. This measurement is fundamental in establishing the importance of reservoir function relative to that of the contractile function, for the concept of active atria1 contraction implies that the chamber does work on its contents, expelling them with substantially more energy than that with which they arrived. In contrast, an inflow reservoir to the left ventricle which acts as a perfect elastic storage chamber will add no energy to its contents but in emptying will put out all the energy it takes in during filling. The important measurements are energy absorbed by the left atrium as it fills during ventricular systole (work of filling), total energy output of the chamber as it empties (total atria1 work) and the difference between these, the energy added by the chamber to its contents (net atria1 work). (Fig. 1.) All three quantities are displayed in Figure 2, and their relations may be simply expressed:

Work of filling the atrium + net atria1 work = total atria1 work output (1)

For a pure elastic reservoir the second term will be zero; for an active contractile chamber the second term will be large; for an atrium behaving in both ways at the same time, the

* From the Department of Medicine, State University of New York at Buffalo, and the Buffalo General Hospital, Buffalo, New York. This study was supported by Grants HE 07539-01 and HTS 5508 from the U. S. Public Health Service and grants from the New York State Heart Assembly and the Heart Association of Erie County. Manuscript received June 24, 1963.

t Formerly Research Fellow of the American Heart Association.

36 AMERICAN JOURNAL OF MEDICINE

Left Atria1 Function-Grant et al. 37

20

10

Pressure (insi. Hs)

0

-5 r .Thoracic Pressure I I* ,’

0 80 100 130

Volume (ml. )

FIG. 1. Case IV. Pressure-volume line of the left atrium during its filling (during ventricular systole) showing the measurement of energy absorbed (shaded area, work of filling the atrium) which in this case amounts to 1,210 gm. cm. (1.2 X 10s ergs).

relative sizes of the first two terms will give an indication of the comparative importance of reservoir function and contractile function.

MATERIAL AND METHODS

Patients referred for possible cardiac surgery were studied by catheterization and selective angiocardiog- raphy. Adequate data were obtained in twenty-four studies on twenty-three patients, one patient being studied both in atria1 fibrillation and in sinus rhythm three months later.

In Cases I and II (Table I) there were normal hemo- dynamics with a systolic murmur and idiopathic dilatation of the pulmonary artery, respectively. In Case XVI (Table II) there were only minimal valve lesions. In Cases XXI and XXII there was combined

TOTAL ATRIAL WORK 0tA = 1580 gm.cm.

MINUS WORK OF FILLING B tv = 1210 wl.cm.

IT

0 ; I volume (ml.) seconds

FIG. 2. FIG. 3.

FIG. 2. Case IV. Complete pressure-volume figure of the left atrium, showing the method of measuring net atria1 work.

FIG. 3. Case IV. Left atria1 (LA) and left ventricular (LV) volume-time data. Each point is a chamber-volume estimation from measurements on one anteroposterior roentgenograph and the simultaneous lateral roentgenograph.

TABLE I

THE LEFT ATRIUM AS A RESERVOIR FOR BLOOD

ATRlAL AND VENTRICULAR STROKE VOLUME

MEASUREMENTS IN PATIENTS WITHOUT

AORTIC OR MITRAL REGURGITATION -

Case No. Patient Diagnosis

I A. I. Normal II s. S. Normal

III D. V. Coarctation IV G. C. Aortic stenosis V M. P. Aortic stenosis

VI E. w. Aortic stenosis VII R. M. Mitral stenosis

VI11 L. L. Mitral stenosis IX v. s. Mitral stenosis x M. N. Mitral stenosis

XI E. G. Mitral stenosis XII A. C. Mitral stenosis

-__

Average

i-

Average ratio Atria1 Stroke Volume ____ %entricular Stroke Volume Length of Ventricular Systole ~~ ___._ ~ Length of Cardiac Cycle

Stroke Volume (ml.)

Atria1 Ven-

tricular

--

20 90 20 100 48 120 56 105 36 100 30 85 51 75 33 85 23 100 37 75 50 65 30 90

~- _-

38 90

.____-

* Measured from electrocardiograms by the ratio Q-T: R-R.

mitral disease of gross severity. In the remainder there were lesions of intermediate severity, most requiring surgical treatment.

VOL. 37, JULY 1964

38 Left Atria1 Function-Grant et al.

TABLE II

LEFT ATRIUM AS A RESERVOIR FOR ENERGY

MEASUREMENTS OF ATRIAL WORK FROM THE PRESSURE-VOLUME DIAGRAM

Work of Total Atria1 Net Ratio of Case

Patient Diagnosis Filling Work Atria1 Work of Filling :

No. the Atrium * output t Work t Total Work Output (gm. cm.) (pm. cm.) (gm. cm.) (%)

In Sinus Rhythm

I

III

IV

V

XIII

XIV

XV

XVI

VII

VIII

IX

X

XVII

XXIII 1

-

-

A. I. D. V. G. C. M. P. N. W. P. N. K. S. M. B.

R. M. L. L. v. s. M. N. H. W. B. H.

T

-

-

Normal Coarctation Aortic stenosis Aortic stenosis Aortic regurgitation Aortic regurgitation Aortic regurgitation Mitral stenosis, aortic regurgita-

tion Mitral stenosis Mitral stenosis Mitral stenosis Mitral stenosis Mitral stenosis Mitral and aortic regurgitation

XVIlI M. G. XIX B. Ha.

xx F. B. XXI c. P.

XXII H. E. XXIII$ B. H.

Average

510 540 30 95 640 900 260 71

1,210 1,580 370 77 1,110 1,210 100 92

730 870 140 84 1,520 1,360 -160 112 1,640 1,420 -220 115

960 1,330 370 72

1,400 1,540 140 91 1,090 1,010 -80 108 1,050 950 -100 110 1,320 1,280 -40 103 2,020 1,820 -200 111 1,770 1,770 0 100

___-

1,210 1,250 40

__

-

-

_-

-

97

With Atria1 Fibrillation

Mitral stenosis Mitral stenosis, aortic regurgita-

tion Mitral stenosis and regurgitation Mitral stenosis and regurgitation Mitral stenosis and regurgitation Mitral and aortic regurgitation

Ratio of work output to work of filling 1,710:2,030 (84%)

-

910 880 1,110 1,020

1,630 1,440 3,100 2,480 2,110 1,580 3,330 2,850

-30 -90

-190 -620 -530 -480

2,030 1,710 -320

103 109

113 125 133 117

119

Average energy loss in atria1 fibrillation 16%

* Work of filling the left atrium is energy absorbed by the chamber as it is distended by inflowing blood (area under the filling half of the atria1 pressure-volume figure (Fig. 1)).

t Measured as in Figure 2. $ Patient studied first in atria1 fibrillation and three months later in sinus rhythm.

Selective pulmonary artery angiograms were per- formed in Cases I, II, VIII and XIII, using 75 to 85 ml. of 76 per cent methylglucamine diatrizoate (Renog- rafin@). The left atrium was entered for pressure measurement (and in twenty studies for selective angiography) by the Brockenbrough transseptal

technic. A special four-way stopcock* allowed one catheter to deliver the contrast medium and then immediately to record intra-atria1 pressure during angiographic filming. The rate of contrast injection averaged 22 ml. per second (range 10 to 32),

* Becton-Dickinson Co., Rutherford, New Jersey.

AMERICAN JOURNAL OF MEDIClNE

Left Atria1 Function-Grant et al. 39

i,

I I

I I 1 0 50 100

%

FIG. 4. The ratio of left atria1 stroke volume to left ventricular stroke volume almost equals the ratio of atria1 filling period (duration of ventricular systole, Q-T interval) to cycle-length (R-R interval) as would be expected if the atrium existed to allow uniform forward flow in the pulmonary veins. Average of values in twelve cases. (Table I.)

and the intra-atria1 dose averaged 65 ml. (range 40 to 75).

The biplane angiograms, taken on 30 cm. roll film at six or twelve exposures per second, were used to measure the dimensions of the left atrium and ventricle. Volumes were calculated by Arvidsson’s ellipsoid formula [7], volume-time curves constructed and stroke-volumes estimated for both chambers. Left atria1 pressure-volume figures were constructed from the pressure-time and volume-time curves, allowing 0.02 seconds for delay in the catheter-manometer system. In three subjects an esophageal balloon- tipped catheter was passed, to allow measurement of intrathoracic pressure during the procedure [8]. The reference level for pressure measurements was 10 cm. above the table top.

From the atria1 pressure-volume diagrams, total atria1 work output was calculated as the area under the emptying half of the pressure-volume figure and above the intrathoracic pressure. (Fig. 2.) When this pressure was not measured, a value of minus 5 mm. Hg (relative to atmospheric pressure) was assumed [9]. Work of filling was similarly estimated, and the difference between the two measurements was taken as net atria1 work.

RESULTS

Figure 3 is an example of the volume-time curves used to measure atria1 and ventricular stroke volume; it shows that atria1 enlargement occurs only during ventricular systole and that atria1 emptying occurs as the ventricle becomes enlarged. This reciprocal size change has been a constant feature of the atria1 and ventricular volume-time curves so that the left atrium may be correctly described as a chamber that fills while the ventricle contracts and empties while

VOL. 37, JULY 1964

Atria1 Stroke Volume Pipe-Flow Volume

42%

Ventricular Stroke Volm~

FIG. 5. The left atrium may be treated as if it were a rigid pipe in parallel with a pulsatilc chamber. When the mitral valve is closed no flow takes place through the atrium acting as a pipe, but the pulsatile chamber fills. When the valve opens, the pipe transmits 58 per cent of the ventricular stroke volume, and the pulsatile chamber discharges its 42 per cent. .4verage of values in twelve cases. (Table I.)

the ventricle is filling. In Table I are shown the measurements of atria1 stroke volume and ventricular stroke volume. Since atria1 filling takes place during ventricular systole, these data show that on an average 42 per cent. of the total filling on the left side of the heart (pulmonary vein flow) takes place while the mitral valve is closed. If this flow were uniform throughout the cardiac cycle, the ratio of stroke volumes (atria1 stroke volume to ventricular stroke volume) would be the same as the ratio of ventricular systole to cardiac cycle length. Using the un- corrected (1-T interval of the electrocardiogram to estimate duration of ventricular systole, the ratio Q-T : R-R in the cases in Table I averages 43 per cent (range 36 to 47 per cent). (Fig. 4.) This is close to the figure of 42 per cent for the ratio of stroke volumes (Table I), so atria1 storage volume is of the order expected by simple theory. The variability of the ratio of stroke volumes is large (22 to 77 per cent) and may reflect variations in the phasic pattern of pulmo- nary vein flow. During diastole the atrium discharges its stroke volume, and the ventricle accepts its larger stroke volume. The extra volume must flow through the atrium, which is open at both ends and acts in this respect as a pipe. For many purposes it is convenient to consider these two components of ventricular filling separately, as if there were a rigid pipe in parallel with a pulsatile chamber. The com- ponents correspond to pulmonary vein flow during two phases of the heart cycle, pipe flow function transporting venous return while the mitral valve is open and the atria1 pulsatile function accepting venous return when the valve is shut. (Fig. 5.)

40 Left Atria1 Function-Grant et al.

TABLE III

CONTRIBUTIONS OF ACTIVE CONTRACTION AND RESERVOIR FUNCTION TO TOTAL WORK OUTPUT OF

THE LEFT ATRIUM CALCULATED FROM AN APPROXIMATE CORRECTION FOR ENERGY LOSS*

Case No.

I

III I” ”

XIII XIV xv

XVI vn

VIII IX X

XVII XXIII

-

-_

-

Average

Patient Diagnosis

A. I. Normal D. V. Coarctation G. C. Aortic stenosis M. P. Aortic stenosis N. W. Aortic regurgitation P. N. Aortic regurgitation K. S. Aortic regurgitation M. B. Mitral stenosis, aortic regurgitation R. M. Mitral stenosis L. L. Mitral stenosis v. s. Mitral stenosis M. N. Mitral stenosis H. W. Mitral stenosis B. H. Mitral and aortic regurgitation

-

Per cent

-

__

--

-

Contractile Reservoir Effect Contribution Contribution

(pm. cm.) (gm. cm.)

Contractile Contribution

(%)

_ __-

100 440 19 350 550 39 550 1,030 35 260 950 20 250 620 29

70 1,290 5 30 1,390 2

510 820 38 350 1,190 23

80 930 8 60 890 6

150 1,130 13 100 1,720 6 270 1,500 15

-

220 1,030 -

18 82 .

NOTE: Data derived from Table n (first section), assuming atria1 efficiency as an energy reservoir is 85 per cent. Reservoir effect contribution = 85 per cent of work of filling; contractile contribution = net atria1 work plus 15 per cent of work of filling.

* Patients in sinus rhythm. For patients in atria1 fibrillation (Table II, second section) energy output is entirely due to reservoir effect. so that contractile contribution is zero.

In relation to energy storage the important findings from Table II are twofold: Work of filling the atrium is much larger than net atria1 work, and values for net atria1 work may be negative. The former, as discussed previously, implies that reservoir function is more important than active contraction in providing energy to fill the left ventricle.

The finding of negative values for net atria1 work in the more severe cases of mitral valve disease and aortic regurgitation suggests there may be so much disorganization of atria1 func- tion that more energy is dissipated by irreversible processes (friction, etc.) in the chamber wall than can be added by the muscular contraction. This demonstration that there is energy loss in the system allows equation 1 to be rewritten in a more complete form:

Work of filling the atrium - energy loss + energy of atria1 contraction

= total atria1 work output (2)

when the second and third terms together equal net atria1 work. Since the energy loss cannot be measured, the true energy contribution of active atria1 contraction cannot be calculated. How- ever, an estimate may be made by considering results in atria1 fibrillation. Here, with no coordinated muscle contraction, the energy con- tribution of atria1 contraction must be zero, so the (negative) net atria1 work measures the energy loss. The six subjects with atria1 fibrilla- tion lost 3, 8, 11, 20, 25 and 14 per cent of the work of filling. The highest values were found in two subjects with severe combined mitral valve lesions and thickened diseased atria1 walls. The energy loss probably does not exceed 15 per cent in the atrium of an average person and may be substantially less for a young healthy chamber. The figure of 15 per cent is supported by data on Case XXIII, in which an energy loss of 14 per cent was seen during fibrillation and a zero energy loss three months later when the patient was in sinus rhythm. Figure 6 shows the average energy measurements in this group.

AMERICAN TOURNAL OF MEDICINE

Left Atria1 Function-Grant et al. 41

Work of filling the atrium

Total atria1 work output

Contribution of active atrial contraction (zero)

Energy loss in atria1 wall

% 0 40 100

-

L____i

I

I I ,b

0 1,000 2,000 gm. cm.

FIG. 6. Energy exchanges in the left atrium of patients with atria1 fibrillation (average of six cases, Table II).

The assumption of zero atria1 contractile contribution allows energy loss to be estimated (16 per cent of work of filling or 19 per cent of work output).

The best estimate of the efficiency of the atrium as an energy store is, therefore, about 85 per cent. The corresponding estimates for the contribution of active atria1 contraction (net atria1 work plus 15 per cent of work of filling) are shown in Table III and Figure 7 together with the net contribution of the reservoir func- tion (85 per cent of work of filling). From these approximate data the reservoir function appears to be about four times as important as active atria1 contraction in providing energy for the work of filling the left ventricle.

COMMENTS

The principal thesis of this paper is that the left atrium acts as a short-term storage reservoir during ventricular systole, expanding to receive blood from the right side of the heart and lungs while the mitral valve is shut. The data include measurements on subjects with mitral regurgita- tion, and it is necessary to consider how this can be reconciled with the reservoir function con- cept. In mitral regurgitation the left atrium is filled from the ventricle during ventricular systole so that work of filling the atrium will be performed by left ventricular contraction. Thus the atrium will be storing blood and energy derived from the left ventricle and returning its contents to the same place. This buffer function of the atrium is clearly beneficial since the lungs are protected from the damaging effects of a high-energy regurgitant jet, and the blood and energy are readily available to contribute to ventricular filling. The five openings of the left atrium are normally classified in two groups (mitral valve and four pulmonary veins), but

VOL. 37, JULY 1964

Work of filling the atrium

Total atria1 work output

Net left atria1 work

Energy lass in atria1 wall

Net contribution of atria1 reservoir function

Contribution of active aLria1 contraction

0 500 1,250 gm. cm.

FIG. 7. Energy exchanges in the left atrium of patients in sinus rhythm (average of fifteen cases, Tables II and III). Energy loss in the atria1 wall (15 per cent of work of filling) is not an exact measurement, so the lowest two quantities are approximate measurements.

atria1 function study is facilitated if these are considered together and the chamber simply regarded as a variable volume reservoir whose behavior is substantially described in terms of two processes, filling and emptying. By this approach atria1 function in mild mitral regurgi- tation (forward flow in the pulmonary veins, atria1 filling partly from the lungs) can be studied with the same measurements as are used for severe mitral regurgitation (reversed flow in pulmonary veins, atria1 filling entirely from the left ventricle) and for subjects with normal mitral valves. In mitral regurgitation the atrium stores energy mainly from the left ventricle and returns it to that chamber; with normal mitral valves the atrium stores energy from the right ventricle and later transmits it forward. With left ventricular failure the rise in mean left atria1 pressure will allow the atrium to accept more energy from the right side of the heart and trans- mit it forward to assist the failing chamber. Pulmonary vein backflow from mitral regurgita- tion occurs during ventricular systole; it must be differentiated from another kind of backflow occasionally seen on clinical angiocardiograms occurring during ventricular diastole, which appears to be produced by too forceful an atria1 systole.

The atrium is about 85 per cent efficient as an energy reservoir, but conservation of matter indicates that it must be 100 per cent efficient as a reservoir for blood. However, a more useful meaning for the term “efficiency as a reservoir

42 Left Atria1 Function-Grant et al.

for blood” is obtained if blood forced back into the pulmonary veins by atria1 systole is regarded as wasted; then the efficiency of the volume storage function will be reduced proportionately. Formally for the left atrium:

Efficiency (volume storage during ventricular

systole) = forward flowing volume

atria1 stroke volume

= l- regurgitant volume (3)

atria1 stroke volume

when the regurgitant volume is the volume ejected back by atria1 systole. The other kind of pulmonary vein backflow, that secondary to mitral insufficiency, might conceivably be present in the same heart but represents a reversal of the pipe flow function.

The over-all efficiency of the atrium as an energy store will be reduced in the same way by atriosystolic pulmonary vein backflow. The venoatrial junctions appear to have no valve-like action at all [70], so there is no way in which the adverse effects of an overforceful atria1 con- traction can be avoided. There is, therefore, an optimal strength of atria1 contraction just suffi- cient to avoid pulmonary vein regurgitation; the latter is rare, judging by our experience with clinical angiocardiography, so that the force of atria1 contraction appears to be regulated so as not to exceed this optimum.

From these considerations there emerges a view of the left atrium different from that of the classic booster-pump schema. The chamber behaves as a reservoir to accept and store blood (and the energy which the blood stream carries with it) while the mitral valve is closed. It can behave in this way whether it is fibrillating or beating normally. The atria1 contraction, when present, represents an extension and improve- ment of the elastic properties of the atria1 wall, allowing the atrium to empty itself more com- pletely and hence to refill more easily during the succeeding period. Atria1 contraction is carried out gently so as to minimize the disturbance to venous return from the lungs.

Clinical data [77] suggest that in left ven- tricular hypertrophy the booster pump action may become an important factor in filling the ventricle, and our own observations support this. The patients whose atria contract most force- fully (Tables II and III) all have left ventricular

hypertrophy. Angiographically we have not seen atria1 systole produce pulmonary vein backflow unless left ventricular hypertrophy was present. It appears that the need for help in filling the stiffened ventricle makes a forceful atria1 systole beneficial, even at the price of producing pulmonary vein backflow.

A more complete analysis than this one would consider the effects of inertial factors in blood and tissues [72]. This is not feasible at present but would be expected to modify the results only slightly.

Although the average ratio of atria1 stroke volume to ventricular stroke volume (42 per cent) agrees excellently with the average ratio of Q-T to R-R interval (ventricular systole to cycle length, 43 per cent) the correspondence in individual cases is poor. The Q-T: R-R ratios all fall between 36 and 47 per cent, while the stroke volume ratios range from 22 to 77 per cent. This may reflect the variability of pulmo- nary vein flow patterns, which at rest are cer- tainly not uniform throughout the cardiac cycle. In high-output states such as maximal exertion the inertial effect of torrential blood flow is likely to make pulmonary venous return more nearly constant. It is under these conditions that the smoothing action of the atrium on pulmo- nary vein flow is likely to be most important, for the energy needed to accelerate blood in pulmonary veins could only be provided at the cost of raising pulmonary capillary pressure. On the right side of the heart the problem of finding energy to produce pulsatile venous return in high-output states is probably even more difficult, and this suggests that reservoir function may be equally important in the right atrium. No data are available to test this hypothesis.

SUMMARY AND CONCLUSIONS

The left atrium has an important function as an inlet reservoir to the left ventricle. It en- larges to store blood and energy while the mitral valve is closed so that pulmonary vein flow is not halted at this time. This reservoir function has been studied in twenty-three patients by volume angiocardiography, using the pressure- volume diagram to measure atria1 work.

Left atria1 stroke volume (volume change during the cardiac cycle) averaged 42 per cent of left ventricular stroke volume, and atria1

AMERICAN JOURNAL OF MEDICINE

Left Atria1 Function-Grant et al.

filling occurred only while the mitral valve was closed so that 42 per cent of flow in the pulmo- nary veins took place during this period. The period occupied about 43 per cent of the cardiac cycle. The remaining three fifths of total pulmo- nary vein flow occurred while the mitral valve was open, the atrium acting as a pipe to convey blood directly into the ventricle. This pipe-flow function was performed concurrently with the atria1 emptying.

The pressure-volume measurements showed that most of the energy released by atria1 emptying had been stored in the chamber wall as it stretched during filling. Acting in this way the chamber was approximately 85 per cent efficient as an energy reservoir. The additional energy contribution of active atria1 muscle con- traction, measured by an approximate method, ranged from 2 to 39 per cent of total atria1 work in different clinical conditions, the highest values occurring in patients with left ventricular hypertrophy.

In atria1 fibrillation the left atrium could still perform the reservoir function, distending and then partly emptying by virtue of its elastic properties. The absence of a coordinated muscle contraction limited its usefulness only slightly.

The unexpected feebleness of the normal atria1 contraction may be a regulatory mechanism to avoid regurgitation in the pulmonary veins.

REFERENCES

1. HARVEY, W. Anatomical Studies on the Motion of the Heart and Blood, 3rd ed., p. 40. Springfield, Ill., 1949. Charles C Thomas.

2. SARNOFF, S. J. and MITCHELL, J. H. The regulation of the performance of the heart. Am. J. Med., 30: 747, lG1.

3. GUYTON, A. C. Textbook of Medical Physiology. Philadeluhia. 1956. W. B. Saunders & Co.

4. GRAETTIN~ER, J. S., CARLETON, R. A. and MUEN- STER, J. J. Circulatory consequences of changes in cardiac rhythm produced in patients by trans- thoracic direct-current shock. J. Clin. Invest., 42: 938, 1963.

5. SKINNER, N. S., MITCHELL, J. H. and WALLACE, A. G. Hemodynamic effects of simulated atria1 fibrillation at constant ventricular rates. Fed. Pm., 21: 136, 1962.

6. FRIEDBERG, C. Diseases of the Heart, 2nd ed., p. 359. Philadeluhia. 1956. W. B. Saunders & Co.

7. ARV~~SON: H. ’ Angiocardiographic observations in mitral disease. Acta Radial., 158 (supp.): 38, 1958.

8. MEAD, J., MCILROY, M. B., SELVERSTONE, N. J. and KREITE, B. C. Measurement of intraesophageal pressure. J. A!#. Physiol., 7: 491, 1955.

9. DITTMER, D. S. and GREBE, R. M. Handbook of Respiration, p. 135. Philadelphia, 1958. W. B. Saunders & Co.

10. GRANT, C. Function at the veno-atria1 junctions. Fed. Pm., 22: 459, 1963.

11. BRAUNWALD, E. and FRAHM, C. J. Observations on the hemodynamic functions of the left atrium in men. Circulation, 24: 633, 1961.

12. COTTON, K. L. and GABE, I. Fluid inductance and its measurement in rigid tubes. Physics Med. B Biol., 6: 87, 1961.

VOL. 37 , ,,ULY 1964