Doppler Cardiac Output and Left

Doppler Cardiac Output and Left VentricularPerformance after Cardiac Surgery* Pulsed Doppler Ascending Aorta and Pulmonary Blood Flows and Left Ventricular Function Using

ImplantedMicroprobes Marie-Dominique Fratacci, M. D. ;t Didier Pawn, M. D. , Ph. D.;t Sadek Beloucif M.D.;t and François Laborde, M.D4

We have developed novel implantable Doppler microprobes to monitor beat-by-beat stroke volume and cardiac output (CO) after cardiac surgery. In 11 adults undergoing either coronary

(n

artery

bypass

grafting

(n

6) or valve replacement

5), Doppler microprobes were implanted on the as

cending aorta or the main pulmonary artery to measure aortic

blood

flow (ABF)

(PBF). The diameters

or pulmonary

artery

blood

flow

of both vessels were determined

before surgery using two-dimensional echocardiography. Stroke

volume

was obtained

from velocity

tracings

velocity and maximum acceleration of blood in the ascend ing aorta

were

measured

after

inotropic

stimulation

with

dobutamine; both values increased significantly from con trol values (25.2±6.1 percent and 44.6±8.6 percent,

respectively, at 7.5 @g/kg/min). We conclude that implanted aortic Doppler microprobes provide a sensitive and reliable method to measure aortic blood flow velocity after surgery

and then allow monitoring of stroke volume and CO and analysis of left ventricular function after cardiac surgery@

meas

(Chest

1992; 102:380-86)

ured by a 4-MHz zero-crossing pulsed Doppler flowmeter. Simultaneous measurements of Doppler and thermodilu

ABFpulsed

tion CO (TDCO) were compared. We found the following: ABF = 1.03 TDCO—O.22 L/min (r 0.89); while PBF 0.69

flow; PDSV Doppler stroke volume; PFV peak flow velocity; TDCO = thermodilution cardiac output; TDSV thermo

TDCO— 1.24

H

L/min

(r

0.75).

Furthermore,

peak

flow

emodynamic monitoring during and after cardiac surgery is essential to guide therapeutic man

agement.

Among commonly

monitored

parameters, frequent measurements

hemodynamic

of cardiac output

(CO)andstrokevolumearefundamental to assess circulatory status and ventricular function.' The inter mittent thermodilution method of determining CO with

a Swan-Ganz

technique,

catheter

remains

the

standard

although this technique does not measure

rapid or transient hemodynamic variations nor allow continuous monitoring of stroke volume or CO. More

over, thermodilution calculations may be erroneous during arrhythmias or in low CO states.2 For these reasons,

noninvasive

Doppler

techniques

have been developed and validated in critically ill patients.@ Additional information can be gained from

celeration

Doppler of blood;

aortic blood flow; MA PBFpulsed

Doppler

maximum

pulmonary

ac

blood

dilution stroke volume

beat-by-beat analysis, such as peak flow velocity (PFV) and the maximum

acceleration

rate of blood (MA) as

indices of global left ventricular performance.7.8 Pre vious Doppler measurements were performed using external probes, linked to a pulsed or continuous velocimeter; however, transcutaneous Doppler echo cardiography

is often technically

difficult after cardiac

surgery, since interposition of mediastinal air, lung, or blood hampers the transmission of ultrasound. These technical limitations led us to develop novel Doppler microprobes and implant them on the ascending aorta or the main pulmonary artery. This allowed us to measure beat-by-beat aortic and pulmonary blood velocities after surgery and thus derive stroke volume and CO. The aims of our study were (1) to compare pulsed

Doppler ascending aortic and pulmonary blood flows *Fmm the Department of Anesthesiology and Intensive Care and the Cardiothoracic Surgical Unit, Lariboisiere University Hospital, Paris, France.

Presented in part at the annual meeting, American Heart Associ ation, Washington, DC, November 1988. Supported by the Institutional Grant Program ofParis VII Univer sity, Credit C, the Fédération Françaisede Cardiologie (Bourse Squibb) and the Institut National de Ia Sante et de la Recherche

(ABF and PBF) with values obtained using the ther modilution technique and (2) to determine the effect

of inotropic stimulation on PFV and the maximum acceleration ofaortic blood as measured by an implan table pulsed

Médicale (INSERM),86-3-32-E. tDepartment

ofAnesthesiology

lCardiothoracic Surgical Unit.

380

probe.

MATERIALS

and Intensive Care.

Manuscript received July 17; revision accepted January 2. Reprint requests: Dr. Thyen, Department of Anesthesiology, boisiere University Hospital, 75010 Fbi-is, France

Doppler

AND METHODS

Patients Lan

This study was performed

in accordance

tions of the Human Study Committee

with the rec()mmenda

of the French Society of

Cardiac Output and LV Performanceafter Cardiac Surgery (Fratacci et a!)

Table

1 —Clinical Characteristks

Implantation ofthe Probe

of Patients (No. of

Grafts)*157CABG PatientAge,

Coronary

yrOperation

(2)254CABG(1)355CABG

At the end ofeach surgical procedure, the microprobe was affixed to the adventitia of the aorta or the main pulmonary artery 1 cm above the aortic or pulmonary annulus. Four 7.0 sutures were placed on either side of the probe passing through its siliconized

envelope. When the best audio and visual signals were obtained, the probes were secured. The connecting tube was externalized through a drain. Six days after surgery, the probe was removed by gentle traction without causing injury to any patient.

(2)464CABG (2)547CABG (3)651CABG(2)744AS;MS863AS943MS1029Al1144Al

The Ranged Gated Doppler Flowmeter

*CABG, Coronary artery bypass graft; MS. mitral stenosis; AS, aortic stenosis; and Al, aortic insufficiency.

A zero-crossing pulsed Doppler flowmeter was used for the aortic and pulmonary arterial measurements. The device has been previ ously described and validated.@-'°@° This apparatus operates at an ultrasound frequency of 4 M Hz with an emission duration ranging from 0.5-p.s to 2-p.s and a repetition rate of 10 kHz, allowing velocity measurements up to 1.9 m/s at an incidence angle of 60°. In addition to pulsed emission, this apparatus has an adjustable range gating system which permits selecting the time delay from emission

Cardiology

Written

informed

consent

was

obtained

from

each

patient. Clinical characteristics and surgical procedures performed on the patients are summarized in Table 1. Eleven patients (mean age, 50±10 yr [ ±SD]) were studied. Six had coronary bypass surgery, and a microprobe was implanted on the ascending aorta; five had cardiac valve replacement, and a microprobe was implanted on the main pulmonary artery. We excluded patients with a preoperative left ventricular ejection fraction below 45 percent or postoperative arrhythmias. Description

ofthe

Flow Probe

The principle and design of the pulsed Doppler microprobe are similar to the one previously reported for coronary flow measure ments (Fig 1).@For flow measurements, we used a 4-MHz piezoelec tric crystal (Sonox P4; Rosenthal, Paris) furnished as a disk 5 mm in diameter and 0.25 mm thick. Two bared copper electrodes were

soldered onto one side of the crystal with insulated copper leads 80 cm long. A silicone prism supported the crystal and was cut so the angle between the ultrasonic beam and the vessel axis was 60°. The crystal and its connections were protected by silicone to dampen ultrasonic back radiation. The dimensions of the probe were 6 mm in width and 7 mm in length, and its shape was oblong to facilitate removal. The thickness of silicone rubber between the

crystal and the vessel wall averaged 2 mm. For each probe the linearity

of maximum

detectable

velocity was tested in vitro, and

we verified that 1 kHz of Doppler shift corresponded to a velocity of38 cm/s at the incidence

angle of 60°.

(depth) and the duration

ofemission

These times are converted echographically

(sample

volume

size).

into depth and width of

Doppler sample volume, according to the following equation: d = (C/2) x t, where d is the distance between the red blood cells of the blood column and the transducer, C is the celerity of ultrasound at 37°Cin biologic tissues (1,550 mIs), and t is the reception time delay. A pedal control near the apparatus enabled the investigator to sequentially vary the depth and width of the sample volume.

Blood Flow Measurement To compute blood flow, the following parameters are needed (Fig 2): (1) cross-sectional area of the artery (CSA); (2) mean blood flow velocity (V) during one cardiac cycle; and (3) heart rate (HR). Stroke volume (SV) was calculated as follows: SV CSA x V; and ABF or PBF was calculated as SV x HR.

Arterial Diameter Measurements. Vessel diameters were meas ured in the preoperativeperiod using two-dimensional (2D) and M mode echocardiograms because postoperative determinations are difficult. Ascending aortic diameter was measured at the level of the aortic leaflets;'@ the echo transducer was placed along the left sternal border in the third or fourth intercostal space and directed at the base of the heart, ie, a parasternal long-axis view. Tilting the transducer superiorly and medially permitted the beam to traverse the root of the aorta. Aortic diameter corresponded with the midsystolic distance between the leaflets on the M-mode echocar diogram. A mechanical sector scanner was used to image the main

pulmonary artery by placing the transducer in the third or fourth intercostal space and orienting it so the scanned plane was perpen

dicular to the long axis of the left ventricle (ie, a parasternalshort @

axis The transducer was then angulated medially or laterally until the main pulmonary artery was imaged in cross section. We obtained pulmonary artery diameter measurements during early to midsystole using frame-by-frame videoanalysis of the M-mode echocardiogram.

Pulmonary artery diameter was measured at midsystole between leading edges of the anterior and posterior vessel walls. Cross sectional area was computed as follows: CSA = (11d2)/4.

Blood Flow %4@locity Measurements. The mean Doppler frequency shift (SF) was obtained by a zero-crossing system. Thus, the frequency shift of the reflected ultrasonic signal was converted into velocity as follows: V °(@F x C)/(2 Fe X cos (1)), where V is the mean blood velocity in centimeters per second, @Fis the Doppler shift frequency in kilohertz, 41 is the incidence angle between the ultrasonic beam and flow direction, and Fe is the emitted ultrasonic FRONTAL

VIEW

FIGURE 1. Essential

LA TERAL VIEW features

of Doppler

microprobe.

frequency.Localizationofthe measuredblood flowvelocity required precise position of the sample volume. Since the thickness of silicone at the crystal was 2 mm and the arterial wall thickness 1 CHEST I 102 I 2 I AUGUST, 1992

381

@ @

0

@

e

3

:-@:: ‘ -@-@--:@-@

@

@@±LLL11•1±L\± . __ f@@\

@

PFV .

@

38

..@

I-

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..

:./.@.

•

0@LL• A fH•@ • • u->::T:;:@W\J@J@' . . . : V..: .

@

/.v.

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:\

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1'@)

.

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A

/:

J

Ficua@ 2. Tracingsof ascending aortic flow velocity and indices. MA was calculated as slope of velocity dV/dT ET, Ejection time; SV,stroke volume. 15 and 18 mm from the

at the point of maximum systolic blood flow velocity, while the MA

crystal, near the center of the vessel. Therefore multiple positions were attempted untiloptimal audioandvisualsignalswere obtained.

mm, we set the sample volume between

was calculated as the first derivative of the flow velocity during the

The output velocity signal was recorded (Gould ES 1000 recorder) using a 100-mm/s paper speed.

The mean velocity of blood flow during one cardiac cycle was calculated manually on a digitizing tablet linked to a microcomputer

(Apple lIE). We report the average of six cardiac cycles.―We measured

@

PFV on paper tracings, using a 100-mm/s paper speed,

Y.-O.234

1.03'

P •o.e@.

.001

first 40 ms of ejection. The reported represent the average of six consecutive flow variations due to respiration.

values of PF\' and MA beats, in order to reduce

Protocol Measurements

were performed in the intensive care unit within

the first 24 h after surgery and were made when the patient was

II • 79

.

C S

E

S S

LI@

•S

TDCO(I/mm) 2 C

@

E

meen 2 SD

S SSS

@

5

‘1 c:3

S

0

C

I f. S

0

5

S

a@

0

555

55

S

S

S@@s_ •

55

•@5

S5

S55

—I

S55SS S555

S

0

mean

sS

IM•fl

2 SD

—2 FIGURE 3A (top). Linear regression

Average CO by 2 methods (@Ã-'min) 382

between

TDCO

and ABE B (bottom). Difference against mean for ABF data. CardiacOutputand LVPerformanceafter CardiacSurgery(Fratacciat a!)

3000

Table 2—Data Obtained with and without Dobutamine Infusion*

@

,_ I —0.61.

DataControlDobutamine5jxgfkg/min7.5@ag/k@minHR.

< 0.005

U S

beatspermin90±596±6106±8ttABF, ljmin5.35±0.746.18±[email protected]±0.7311PDSV,ml60.2±8.666.0±11.164.1±9.8MA,

ni/s/s14.5±1.316.7±1.320.6±1.6thPFV, cm/s91.2±10.397.7±7.5111.2±7.9th!SAP,mmHg112±4119±4142±13t@

500

*SAP Systolic arterialpressure.

60

80

60

20

40

tp