The relationship between the timing of atrial systole and the useful work of the left ventricle in man

The relationship of atrial systole

between the timing and the useful work

the left ventricle

of

in man

Robert E. Snell, M.D.* Peter C. Luchsinger, M.D.** Gerald I. Shugoll, M.D.*** Washington, D. C.

T

he prominent phasic variations in arterial pressure noted in patients with an idioventricular cardiac rhythm have been related to alterations in the temporal relationship between atria1 and ventricular contractian.‘v2 Presumably, accompanying changes occur in the left ventricular volume output and in the useful stroke work. It has not been possible to quantitate these changes in man, however, with methods previously available. The computed pressure gradient technique has recently been applied to the measurement of instantaneous blood flow in the human ascending aorta.3 If aortic end-diastolic flow is assumed to be zero, the volume output of the left ventricle can be estimated for individual beats. Such estimates, together with measurements of instantaneous aortic pressure, have been used to calculate left ventricular work and power in man under various conditions.* The present report extends these observaFrom

tions to 3 subjects with acquired, chronic, complete heart block of unknown etiology. The patients were studied during idioventricular pacing, and the results were analyzed to determine the effect of variations in the timing of atria1 systole on useful ventricular work and power. Method The patients were studied at rest, without premeditation. All had acquired, complete heart block and were judged to be suitable candidates for implantation of an artificial pacemaker. Prior to the actual operation, an electrode cathetert was passed into the right ventricle and set to produce a stimulus rate near 60 beats per minute. The R-R and P-P intervals in each subject are shown in Table I. A specially designed dual-lumen catheter-1 was then introduced through the surgically exposed femoral artery and advanced to the ascending aorta, just distal to the aortic valve.

the Cardiopulmonary Research Laboratory, Veterans Administration Hospital, and the Department of Medicine, Georgetown University School of Medicine. Washinrrton. D. C. Received for publication Jan. 17, 1966. *Clinical Investigator, Veterans Administration Hospital, and Instructor in Medicine, Georgetown University School of Medicine. Address: Veterans Administration Hospital. 50 Irving Street, N.W., Washington, D.C., 20&q. **Chief, Cardiopulmonary Research Laboratory, Veterans Administration Hospital, and Associate Professor of Medicine, Georgetown University School of Medicine. ***Chief, Cardiology Section, Veterans Administration Hospital, and Assistant Professor of Medicine, Howard University College of Medicine. tMode1 353-PT, U. S. Catheter and Instrument Corp., Glens Falls, N. Y. 1U.S. Catheter and Instrument Corp.. Glens Falls, N. Y.

653

I\lcasurcmcnts 0i instantaneous ln-essure, velocity, and tlow were then made as previously described.3 In one patient, the pacing frequency was increased and measurements were repeated at a second rate. Recordings of pressure and velocity (flow) were displayed on a photographic recorder,* together with Lead II of an electrocardiogram, at a paper speed of 200 mm. per second. An average of 50 paired pressure and flow complexes were then selected at random for analysis in each subject. Ordinate values were measured at l-mm. intervals by a Gerber automatic scanner-i with punch card output. Left ventricular stroke work and power were then calculated from Equation 1, using digital computation:

where LVSW is left ventricular stroke work (Gm.M. per beat); P is instantaneous aortic pressure (cm. HzO); Q is instantaneous aortic blood flow (c.c./sec.); v is instantaneous aortic blood velocity (cm./ sec.); p is blood density (taken as 1.057 Gm./c.c.); g is the force of gravity (980 cnl./sec.p); and ti-to is the systolic ejection period (sec.). The aortic cross-sectional area, necessary to convert velocity to flow, was obtained from measurements of the average vessel radius as determined by angiography.” Values for pressure, stroke volume, and stroke work in each subject were then grouped and averaged according to the associated P-R interval: group a consisted of beats with an interval between 0 and 0.09 sec., and group b consisted of those with an interval between 0.10 and 0.19 sec., etc. The seventh group consisted of beats with a P-R interval greater than 0.60 sec. Results

The gross beat-to-beat changes in pressure, flow, and stroke volume found in one patient are shown in Figs. 1 and 2. At a pacing rate of 60 beats per minute (Fig. 1) *Model 560, Sanborn tModc1 GDDRS-3B. ford. Corm.

Co., Waltham, Mass. Gerber Scientific Instrument

Co..

Hart-

I

I

R-R Subject 53 65 81

69 63 53

interval (sec.) 1.010 1.005 1.005

P-P

interval (ser.) 0.680 0.590 0.800

there are variations in both peak and mean aortic pressure. The peak flow values remain relatively constant at this rate, however, and the changes in stroke volume (the area under the flow curve) result from beat-to-beat differences in the ejection time. At a rate of IO5 beats per minute (Fig. 2) there are more prominent variations in both peak and mean pressure and flOW.

Average mean systolic pressure, stroke volume, and stroke work in each patient at rates near 60 beats per minute are shown in Table II, and the relationship between the P-R interval and the useful work is illustrated in Fig. 3. Work maxima are seen in each subject; in two, when the interval is between 0.10 and 0.19 sec., and in the third when it is between 0.20 and 0.29 sec. Beyond this optimum time, pressure and stroke volume both diminish, with work output decreasing an average of 20 per cent from the peak value. Further lengthening of the P-R interval, however, does not significantly alter the work done until the time exceeds 0.60 sec. At this limit, when the atria1 contraction would be expected to have occurred during the preceding ventricular systole, a further drop in external work is seen in the two patients with the slower atria1 rates. The effect on this relationship of an increase in the pacing rate is shown in Fig. 4. Comparison with values for the same individual in Fig. 2 shows that the work done at any given P-R interval has diminished. Most striking at the higher rate is the almost 100 per cent drop in the work accomplished when the interval exceeds 0.30 sec., and there is no effective atria1 contraction during the preceding ventricular diastole.

Timing of atria1 systole relative to useful work oj’ LT’

P

Fig. 1. Measurements of pressure, aorta during transvenous cardiac

Fig. 2. Measurements

made

P

P

P

flow, and Lead II of an ECG made in one subject pacing at a rate of 60 beats per minute.

in the same individual

as in Fig.

The difference between two consecutive beats in the temporal pattern of work performance is shown in Fig. 5. During the early portions of the ejection period, power output is about the same in both beats. At the shorter P-R interval, however, there is both a marked increase in values for instantaneous power output in late

1 during

65.5

pacing

(No.

53) in the ascending

at a rate of 105 beats

per minute.

systole and a prolongation of the ejection period, resulting in a larger total work output. Discussion

Many investigators, using various experimental models, have confirmed the importance of an appropriately timed

Table II. Average stroke volume, mean systolic pressure, and stroke work at di’erenf .I’-R

interval

--_I

0.3-0.39

P-3 irttervals

(SK.)

Subject 0.0-0.9 53.

6.5.

81.

sv MSP LVSW

k

SEM

52 166 89 *

SV MSP LVSW

70 221 150 *

SV MSP LVSW

51 266 139 *

I

I

0.2-0.29

4

68 189 133 + 3

58 182 107 *

9

68 232 151 *

7

75 228 166 *

8

52 279 14.5 *

9

47 271 126 *

*P-P interval in this subject was 0.59 sec. 57’: Stroke volume. ml. MSP: Mean systolic error of the mean.

Fig. 3. Left ventricular stroke of the preceding P-R interval complete heart block during pacing at rates near 60 beats subject, individual values for first grouped according to the terval as follows: Group a-P-R b-0.10-0.19 sec.; c-0.20-0.29 sec.; e-0.40-0.49 sec.; f-0.50-0.59 sec. The grouped values were each patient. The vertical lines dard error of the mean.

0.1-0.19

pressure,

cm.HzO.

work as a function in 3 subjects with transvenous cardiac per minute. In each stroke work were associated P-R ininterval O-0.09 sec.; sec.; d-0.30-0.39 sec.; g-> 0.60 then averaged for represent the stan-

/

o.,?lr,,l,l,’

2

58 183 108=s

7

67 221 145 * 6

64 222 139 f

10

47 262 124 *

47 259 124 *

LVSW:

Left

6

11

ventricular

>0.6

57 182 106=+= 3

50 171 88 *

--*

9

69 213 146*9

5

44 264 118~

42 245 90 *

work,

Gm.M./beat.

stroke

46 162 71 *

5

2

--

8

SEM;

4

Standard

atria1 contraction to the subsequent ventricular systole. 1,2,6-11 The extent of this atria1 contribution is conditioned by the state of the myocardium and is more pronounced in individuals with congenital heart block than in those with acquired disease.g The results of the present study indicate that useful ventricular work may vary from beat to beat by as much as 100 per cent, depending upon the presence of an atria1 contraction, its placement in time, and the cardiac rate. The actual variations are probably even greater, since average vessel radius was used in determining blood flow in these studies, and changes in aortic diameter were not taken into account. Of interest is the finding that ventricular work does not fall off progressively as the interval between atria1 and ventricular contraction lengthens. Once the optimal P-R interval is exceeded, the useful work output is not significantly lessened by a f,urther lengthening of the interval, as long as the atria1 contraction falls within the diastolic filling period. When atria1 contraction is simultaneous with the preceding ventricular systole, however, there is a further drop in the work accomplished. A possible explanation for this finding can be offered. It is known that left atria1 contraction influences 1)oth ventricular filling

Volztme Number

72 5

Timing of atria1 systole relative to useful work of LV

-RATE

60/min.

ORATE

I05/min.

LVSW GM.M./BEAT 75-

25-

I, , , , , , , D

b

c

P-R

d

e

f

4

INTERVAL

Fig. 4. Left ventricular stroke of the preceding P-R interval two different pacing rates. Work and averaged as in Fig. 3.

work as a function in one subject at values were grouped

and mitral valve closure.‘*J3 Probably the occurrence of atria1 systole in early ventricular diastole is relatively ineffective with regard to the former effect but still aids in valve closure and limits regurgitation. When atria1 action is completely absent, however, both decreased filling and delayed valve closure could result in a further lessening of the useful work accomplished. Simultaneous measurements of left atria1 pressure would be necessary to verify this. One practical point arises from the

657

variations in stroke volume found at different P-R intervals. If the relationship between the P-P and P-R intervals is such that beats both with and without an atria1 effect occur within short intervals of time, repeated measurements of mean cardiac output at the same heart rate should be similar. However, if the relationship between the intervals is such that a long series of beats with effective atria1 contraction is followed by another series without an atria1 effect, determinations of mean flow would be highly dependent on the time of measurement. In judging the response of the heart to changes in idioventricular pacing rates, this phenomenon would have to be taken into account. The extent of the effect of atria1 contraction is dependent on the heart rate, as shown by the preceding results, as well as by those of other investigators.2z10 This dependence presumably reflects the diminution in diastolic filling time at higher rates and the resulting increase in the importance of atria1 contraction to ventricular filling. Summary Beat-to-beat variations of as much as 100 per cent in the external left ventricular stroke work have been found in 3 patients with acquired, chronic, complete heart block studied during idioventricular pacing. These changes in work output reflect the presence or absence of a preceding

500 -

MO-

O

0.25 TIME (SEC)

0.50

Fig. 5. Instantaneous power output of the left ventricle in one subject during two consecutive beats at a pacing rate of 60 beats per minute. Zero time on the abscissa denotes the onset of the QRS deflection on the:ECG. The P-R interval preceding one beat (x curve) was 0.15 sec.; that preceding the other (solid dot curve) was 0.48 sec.

atria1 contraction, its placement and the cardiac rate.

in time,

The assistance of Dr. James M. Clements ill these studies is gratefully acknowledged. Digital computation was dune under the supervision of Dr. -Hubert Pipberger. KEFERENCES 1. Samet, P., Jacobs, n’., Bernstein, W. H., and Shane, R.: Hemcdynamic sequelae of idioventricular pacemaking in complete heart block, Am. J. Cardiol. 11:594, 1963. 2. Carleton, R. A., and Graettinger, J. S.: Atria1 contribution to ventricular performance, J. Clin Invest. 44:1033, 1965. 3. Snell, R. E., Clements, J. M., Pate], D. J., Fry, D. I,., and Luchsinger, P. C.: Instantaneous blood tlow in the human aorta, J. Appl. Physiol. 20:691, 1965. 4. Snel!, R. E., and Luchsinger, P. C.: Determination of the external work and power of the left ventricle in intact man, AX. HEART J. 69:529, 1965. 5. Luchsinger. I’. C., Sachs, M., and Pate], D. J.: Pressure-radius relationship in large blood vessels of man, Circulation Res. 11:885, 1962. 6. Gesell, R. .4 : Auricular systole and its relation

to verrlricular output, ;\III. J. I’hysiol. 29:3?, 1911. of the auricles 7. Jochim, K.: The contribution to ventricular filling in complete heart block, Am. J. Physiol. 122:639, 1938. E.. and Frahm. C. 1.: Studies on 8. Braunwald. Starling’s law of the heart. IV. Observations on the hemodynamic functions of the left atrium in man, Circulation 24:633, 1961. 9. Lee, G. dej., Gillespie, IV. J., and Greene, I). G.: The effect of atria1 systole on right ventricular stroke outout. Fed. Proc. 24:704. 1965. 10. Mitchell, J. H., ‘G&a, 1). N., and Payne, Ii. M.: Influence of atrial systole on effective ventricular stroke volume, Circulation Res. 17:11, 1965. Il. Palermo, H. .4. : Complete heart block and the role of atrial activity, AM. HEART J. 70:449, 1965. 12. Skinner, N. S., Xlitchell, J. H., \4’allace, A. G., and Sarnoff, S. J.: Hemodynamic effects of altering the timing of atria1 systole, Am. J. Phvsiol. 205:493. 1963. 13. Sarnoff, S. J., Gilmore, J. P.! and Mitchell, J. H.: Intluence of atria! contraction and relaxation on closure of the mitral valve; observations on effects of autonomic nerve activity, Circulation Res. 11:26, 1962. ,

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