Håkan Stale

Compliance and diameter in the human abdominal aorta--the influence of age and sex.

OBJECTIVES Females develop cardiovascular diseases and abdominal aortic aneurysms later in life than males. In this study we investigated the diameter and compliance of the distal abdominal aorta in healthy females of varying ages. The results were compared with those obtained previously from healthy males in order to assess potential sex-related differences in the aging process of the abdominal aorta. METHODS An ultrasound phase-locked echo-tracking system was used to determine differences in diameter and pulsatile diameter change of the distal abdominal aorta in 69 Caucasian females from 4 to 74 years. Pressure strain elastic modulus (Ep) and stiffness (beta) were calculated from diameter, pulsatile diameter change and blood pressure obtained by the auscultatory method. Compliance was defined as the inverse of Ep and beta. The pressure dependence of Ep and beta was evaluated in 10 females with intraarterial blood pressure measurement at rest and during isometric exercise. RESULTS The diameter of the distal abdominal aorta increased not only in the period between the ages of 5 and 25 years, but also by about 24% between 25 and 70 years (p < 0.0001). From about the age of 25 years the diameter was smaller in females than in males (p < 0.01). Ep and beta increased nearly linearly with advancing age in females from 0.18 x 10(5) to 1.17 x 10(5) N/m2 (Ep) and from 1.85 to 8.51 (beta). In males the increase in Ep and beta was greater and exponential in nature (p < 0.001). Ep but not beta increased significantly during blood pressure increase (p < 0.05). Stiffness (beta) may therefore be a more useful index of arterial compliance than Ep. CONCLUSIONS This investigation demonstrates age and sex-related differences in diameter and compliance in the normal human abdominal aorta and implies that degenerative changes appear later in females than in males.

NONINVASIVE MEASUREMENT OF DIAMETER CHANGES IN THE DISTAL ABDOMINAL AORTA IN MAN

An ultrasound phase-locked, echo-tracking system was used to determine the dynamic properties of the distal abdominal aorta in 10 Caucasian male subjects (mean age, 25 years). Recordings were made at rest and during the blood pressure increase resulting from isometric exercise. The pressure diameter curve was nonlinear with an inflection at about 90-110 mmHg. Above this pressure range, the vessel was stiffer (less compliant), but the pressure diameter relationship was roughly linear above as well as below the inflection. Individual pressure diameter curves showed hysteresis, i.e., the aorta had a smaller diameter during expansion than during retraction at corresponding pressures. The pressure strain elastic modulus (Ep) and stiffness (beta) were at rest [Mean Arterial Pressure (MAP), 81 mmHg] 0.70 10(5) N/m2 and 6.0, respectively. During isometric exercise (MAP, 122 mmHg), Ep increased significantly by 91% and stiffness (beta) nonsignificantly by 27%. The variability of the compliance determinations was 5% when the ultrasonic system was combined with intra-arterial blood pressure measurements and less than 7% when combined with auscultatory blood pressure measurements. It is concluded that the phase-locked, echo-tracking system fulfills clinical requirements for routine measurements of vascular compliance.

Aortic diameter pulse waves and blood flow velocity in the small, for gestational age, fetus

An ultrasound phase-locked, echo-tracking system was used for noninvasive measurements of pulsatile diameter changes in the descending aorta of 60 small, for gestational age (SGA), fetuses and of 60 fetuses appropriate for gestational age (AGA). Pulsed Doppler ultrasound was used for the recording of blood flow velocity in the aorta and in the umbilical artery of the SGA fetuses. In the SGA fetuses, a weight-related higher end-diastolic diameter and a lower relative pulse amplitude suggest that diastolic blood pressure was increased; a less steep rise of the initial ascending part of the pulse wave and a lower relative pulse amplitude suggest that the absolute stroke volume was decreased. Except for a positive correlation between relative pulse amplitude and mean velocity in the aorta, no correlation was found between diameter pulse waves and blood flow velocity. Aortic diameter pulse waves apparently yield no unequivocal information as to peripheral resistance, for which purpose blood flow velocity waveform analysis would seem, at least at present, to be the only available method.

Derivation of haemodynamic information from ultrasonic recordings of aortic diameter changes

An ultrasound phase-locked echo-tracking system was used for noninvasive measurements of diameter changes in the upper abdominal aorta of the anaesthetised cat. Comparisons were made between the noninvasively recorded diameter changes and central haemodynamic variables measured with inserted catheters and transducers. It was found that noninvasive observations of aortic diameter changes give reliable information on the direction and relative magnitude of the blood pressure change both in systole and in diastole. Indications of the direction of change of stroke volume, cardiac output, and aortic flow acceleration (a measure of cardiac inotropy) could also be gained. The information, taken together, comprises a pattern of response reflecting cardiovascular adjustments likely to have occurred. It is suggested that the technique is suitable for interpretation of (patho-) physiological changes in the foetus, as well as for determinations of great vessel compliance in man, i.e., in atherosclerosis research.

Ventriculo-vascular interaction in the normal development of the fetal circulation

OBJECTIVES To examine cardiovascular physiology in the healthy fetus during normal development. DESIGN Twenty normal fetuses were studied longitudinally from 20 weeks to term. Serial echocardiography was performed, and arterial and venous diameter pulse wave characteristics and aortic pulse wave propagation velocity (PWV) were examined in the thoracic descending aorta (AoD) and inferior caval vein (IVC) using an ultrasonic phase-locked echo-tracking system. Statistical analyses included ANOVA, paired t-test and logistic regression where appropriate. RESULTS Aortic PWV, maximum incremental and late decremental velocities increased with gestation while the relative pulse amplitude decreased, reflecting falling distal impedance. There was a linear increase in cardiac preload and relative pulse amplitude in the IVC with gestation that correlated significantly with the presence of end-diastolic flow in the pulmonary artery and improvement in right ventricular diastolic function. CONCLUSIONS Non-invasive concurrent assessment of preload, ventricular function and impedance are possible in the fetus and may prove useful in the longitudinal study of fetal adaptation to pathophysiological changes.

Blood flow velocity and pulsatile diameter changes in the fetal descending aorta: A longitudinal study

Blood flow velocity and pulsatile diameter changes in the fetal descending aorta were measured subsequently with pulsed Doppler ultrasonography and phase-locked echo-tracking technique, respectively. Blood flow velocity and pulse-wave curves with equal beat-to-beat intervals were synchronized by external fetal electrocardiogram. The systolic increase in blood flow velocity and in vessel diameter started simultaneously. The velocity peak preceded the diameter peak by a significant margin (p in the range 0.0001 to 0.0014). The time interval was more pronounced in the thoracic than in the abdominal part of the aorta and was slightly smaller near term. The effective diameter, calculated from the synchronized integrals of the blood flow velocity and the diameter curves, was significantly greater (p in the range 0.0001 to 0.0016) than the mean diameter. The difference tended to decrease with advancing gestational age and was more pronounced in the thoracic than in the abdominal aorta. The maximum error incurred by the use of nonsimultaneous measurements of blood flow velocity and vessel diameter when calculating volume blood flow was found to be less than 8% and thus probably of little practical importance.

Altered vascular function in healthy normotensive pregnant women with bilateral uterine artery notches

Objectives To assess endothelial function and vascular mechanical properties in normotensive pregnant women with high resistance in the uteroplacental circulation.

Cerebral Doppler blood flow velocimetry and central hemodynamics in the ovine fetus during hypoxemia-acidemia.

The purpose of this study was to assess the effects of hypocapnic hypoxia, acidemia and the combination of hypoxia/acidemia on blood flow velocity variables in the fetal cerebral circulation. Chronically instrumented fetal sheep were used and the ewes were induced to breathe a hypoxic gas mixture for about 90 min. This caused an initial period of hypoxemia followed by a period of mixed hypoxemia/acidemia. When the ewe was reoxygenated, the fetus experienced a period of normoxic acidemia. The fetal cerebral circulation was assessed by recording Doppler blood flow velocity waveform variables in a cerebral vessel and the umbilical artery, using standard ultrasound equipment. External carotid artery blood flow was maintained during hypoxic and hypoxic/acidotic periods despite a fall in cardiac output. In the cerebral vessel, mean maximum velocity (time-averaged maximum velocity), minimum diastolic velocity and maximum systolic velocity manifested increases during hypoxic and hypoxic-acidotic periods, but pulsatility index did not change due to the effect of reduced heart rate on pulsatility index. Umbilical artery pulsatility index increased in the hypoxic and hypoxic-acidotic periods, despite unchanged mean maximum velocity, minimum diastolic velocity and maximum systolic velocity. With acute hemodynamic changes, the measurement of pulsatility index can yield misleading results. For clinical and experimental research on the fetal cerebral circulation, more attention should be paid to the individual Doppler variables, especially to the mean maximum velocity, than to the pulsatility index alone. Changes in mean maximum velocity recorded from the cerebral artery seem to reflect changes in the cerebral arterial flow.