Diastolic shape of the right ventricle of the heart.

BACKGROUND: Knowledge of right ventricular (RV) shape is important to the understanding of RV mechanical function and for the improvement of clinically important RV volume estimation techniques. Refinements to the simplest conceptions of RV shape are presented statistically here, based on a quantitative analysis of three-dimensional magnetic resonance (MR) images of excised lamb hearts. METHODS: The passive shape of the heart in six freshly excised lamb hearts was studied with MR imaging with independent passive pressurization of both ventricles. Global features of shape were assessed, including measurement of short-axis, cross-sectional shape parameters associated with the pinched-arc model. RESULTS: The slice-area x apex-base length was found to be highly correlated with the volume of the RV, with little sensitivity to the degree of filling of the ventricle or to the exact slice chosen (r = 0.987; n = 22 from five hearts). The RV was shown to follow a clockwise helical path around the left ventricle of 47 +/- 17 degrees, below the outflow tract, as seen from the apical view, progressing from the apex to the base. Based on the pinched-arc model, the anterior arc is shallower than the posterior arc, with a larger radius of curvature and a smaller angle between the arc and the septal axis. As the RV is passively filled, opposite changes in shape occur between the anterior and posterior regions tending to equalize their shapes. CONCLUSIONS: A high degree of regularity of shape does exist in the RV and, thus, can be characterized effectively in terms of a representative cross-sectional shape and in terms of the changes in that shape proceeding from the base to the apex.

Pulmonary blood flow profiles with reduced right ventricular function in lambs.

The determinants of right ventricular (RV) performance with damaged RV free wall, such as occurs with RV infarction, are still unclear. Using 20-MHz Doppler ultrasound equipment, we investigated the changes in pulmonary blood flow velocity profiles before and after ligation of the right coronary artery. RV dp/dt, stroke volume, RV stroke work, aortic pressure and cardiac output decreased and central venous pressure rose after the ligation. The RV stroke work-end-diastolic pressure relationship indicated impaired RV function following ligation. We observed shortened acceleration time (65.0 +/- 15.1 vs 54.4 +/- 6.2 ms, P < 0.05) and reduced maximum velocity of forward flow (59.0 +/- 5.9 vs 52.5 +/- 7.6 cm/s, P < 0.05) after the ligation. Acceleration was interrupted earlier after ligation than before ligation. These alterations in flow are thought to be a consequence of the altered movement of the RV free wall and ventricular septum induced by RV infarction.

Blood flow velocity profiles in pulmonary branch arteries in lambs.

We studied the detailed profiles of blood flow in the right and left pulmonary arteries using 20 MHz pulsed Doppler ultrasound equipment in a lamb model. Fourteen lambs aged four to six weeks were selected. In six lambs, monocrotaline pyrrole was injected parenterally to create pulmonary hypertension (PH group). Eight other lambs served as unaltered controls (control group). The blood flow velocities were sampled in 1mm increments along the anterior-posterior axis of the branch arteries. The maximum velocity of the forward flow in the left pulmonary artery was higher than that in the right pulmonary artery in the control group (71.7 +/- 15.9 cm/s vs 60.2 +/- 13.5; p < 0.05). The fastest backward flow was located at the posterior position of the vessel in the right pulmonary artery in the control group (71.7 +/- 15.9 cm/s vs 60.2 +/- 13.5; p < 0.05). The fastest backward flow was located at the posterior position of the vessel in the right pulmonary artery in the control group. No significant bias in location was shown in the left pulmonary artery. Using indices of P90, acceleration time, P90*AcT, the velocity waveforms in the PH group were compared with those in the control group. In the left pulmonary artery, every index in the control group showed a significantly greater value that in the PH group. On the other hand, no significant differences were found between either group in the right pulmonary artery.

Effects of chronically elevated pulmonary arterial pressure and flow on right ventricular afterload.

The effects of pulsatile hemodynamics on right ventricle-pulmonary circulation interactions were studied in control lambs and in two lamb models of altered pulmonary hemodynamics induced at infancy: elevated pulmonary arterial pressure (PAP) was created by the infusion of monocrotaline pyrrole (MCTP), and elevated pulmonary arterial blood flow was obtained by the creation of an arteriovenous fistula (Shunt). High-fidelity PAP, midvessel Doppler blood velocity (PAV), and cardiac output (CO) were measured in open-chest, anesthetized lambs. PAV waveforms were normalized to match the measured CO. Measured pressure and flow signals were separated in the time domain into forward and backward components. Pulmonary input impedance and indexes quantifying the timing of the reflected wave pulse (beginning of reflected pulse, duration of reflected pulse in systole, and duration of reflected wave in diastole) were calculated for each group. Results indicate that in control animals the reflected wave returned late in systole and extended through much of diastole, thereby increasing diastolic pressure like a counterpulsation balloon. No significant differences in the timing indexes were found between Shunt and control animals. In the MCTP group, the reflected wave returned significantly earlier than normal with the peak reflected pulse occurring before valve closure. The resulting augmentation of systolic pressure and, therefore, large pulse pressure is consistent with pressure waveforms observed in clinical pulmonary hypertension. We conclude that early wave reflection exerts a detrimental effect in pulmonary hypertension by unfavorably loading the still-ejecting right ventricle.

Induction of right ventricular hypertrophy with obstructing balloon catheter. Nonsurgical ventricular preparation for the arterial switch operation in simple transposition.

BACKGROUND: Recently, a successful result with a rapid two-stage arterial switch operation (ASO) was reported for patients with transposition of the great arteries (TGA) with low left ventricular pressure. In this procedure, the interval between pulmonary arterial banding and ASO was approximately 1 week. This successful result indicates the possibility of a nonsurgical ventricular preparation procedure using an obstructing balloon catheter prior to ASO. METHODS AND RESULTS: A 5F atrioseptostomy catheter was inserted directly into the main pulmonary artery in six lambs aged 20 to 38 days. After the chest was closed, the balloon was inflated twice a day for a period of 2 to 2.5 hours. This procedure was performed for 4 consecutive days. After the final inflation, the ratio of right ventricular weight to total ventricular weight was compared with that in an age-matched control group. After the final inflation, the peak systolic right ventricular pressure and the percentage of peak systolic right ventricular to peak systolic aortic pressure rose to 85.6 +/- 4.7 mm Hg (mean +/- 1 SD) and 79.6 +/- 8.6%, respectively. The percentages of the right ventricular weight to the total ventricular weight were significantly higher after the balloon inflation than those in the control group in terms of wet heart weight (29.5 +/- 1.2% versus 23.0 +/- 1.0%; P < .0001) and dry heart weight (27.0 +/- 2.0% versus 21.0 +/- 1.1%; P < .0001). CONCLUSIONS: The myocardial mass in the right ventricle increased after 4 days of intermittently applied pressure overload. Nonsurgical preparation of the ventricle for ASO in TGA is feasible.

The effects of curvature on fluid flow fields in pulmonary artery models: flow visualization studies.

In vitro pulsatile flow visualization studies were conducted to assess the effects of varying radii of curvature of the right ventricular outflow tract (RVOT) and main pulmonary artery (MPA) on the flow fields in the main, right, and left pulmonary arteries of a one month lamb pulmonary artery model. Three glass flow-through models were studied; one with no curvature, one with the correct anatomic curvature, and one with an overaccentuated curvature on the RVOT and MPA. All other geometric parameters were held constant. Pulsatile flow visualization studies were conducted at nine flow conditions; heart rates of 70, 100, and 140 bpm, and cardiac outputs of 1.5, 2.5 and 3.5 l/min with corresponding mean pulmonary pressures of 10, 20, and 30 mmHg. Changes were observed in the pulmonary flow fields as the curvature of the outflow tract, heart rate and mean pulmonary pressure were varied. An increase in vessel curvature led to an increase in the overall radial nature of the flow field as well as flow separation regions which formed faster, originated further downstream, and occupied more of the vessel area. At higher heart rates, the maximum size of the separation regions decreased, while flow separation regions appeared earlier in the cardiac cycle and grew more quickly. Heart rate also affected the initiation of flow reversal; flow reversal occurred later in the cardiac cycle at lower heart rates. Both heart rate and mean pulmonary pressure influenced the stability of the pulmonary flow field and the appearance of coherent structures. In addition, an increase in mean pulmonary pressure increased the magnitude of reverse flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional visualization of pulmonary blood flow velocity profiles in lambs.

For a better understanding of the characteristics of blood flow in the pulmonary artery, we constructed three-dimensional images of velocity profiles of blood flow in the pulmonary artery from pulsed Doppler ultrasound recordings in 14 lambs aged 28-40 days. In 8 lambs, pulmonary hypertension was created by the central venous injection of monocrotaline pyrrole. Six lambs served as unaltered controls. The velocity data were sampled in 2 mm increments along both an anterior-posterior axis and a right-left orthogonal axis in the main pulmonary artery. Using a computer-generated cross-sectional velocity matrix consisting of 0.25 mm square grids, the velocity of blood flow was estimated at each intersection. The cross-sectional velocity matrices were generated at 5 msec intervals during the entire cardiac cycle. In all animals, significant velocity reversal was detected near the posterior wall. In 7 of 14 animals, the peak forward velocity was located near the posterior wall. Three of 8 hypertensive models showed reacceleration during the mid-systolic phase at the center of the velocity waveform, but one reacceleration disappeared at a point only 2 mm away from the center of the vessel toward the posterior wall. Acceleration time correlated well with the mean pulmonary arterial pressure (PAP) (r = -0.85) and the log10 PAP (r = -0.86). Corrected acceleration time (acceleration time divided by the square root of the cardiac cycle length) also correlated with PAP (r = -0.78) and the log10 PAP (r = -0.81).

Intraluminal pulsed Doppler evaluation of the pulmonary artery velocity time curve in a canine model of acute pulmonary hypertension.

The velocity pattern of the blood flow in the pulmonary artery was investigated in an animal model of acute pulmonary hypertension. Nine anesthetized, open-chest dogs were embolized with polystyrene microspheres, and the velocity pattern of the blood flow in the pulmonary artery was studied with use of an invasive pulsed Doppler technique. Phasic intraluminal velocity was recorded with use of a miniature piezoelectric crystal activated by 20-MHz Doppler pulses and mounted on the tip of a needle probe introduced into the pulmonary artery. The recorded Doppler quadrature signals were processed by spectral analysis. Significant increases occurred in mean, systolic, and diastolic pulmonary arterial pressures (p less than 0.0002), in pulmonary vascular resistance (p less than 0.005), and in negative velocity time (duration in milliseconds that the mean velocity was directed toward the pulmonic valve) (p less than 0.002). Significant decreases occurred in right ventricular ejection time (p less than 0.006) and in positive velocity time (duration in milliseconds that the mean velocity was directed away from the pulmonic valve) (p less than 0.005). A significant shortening in the time to peak velocity (acceleration time) was found (p less than 0.005). Second-order regression analyses demonstrated an inverse correlation between the ratio of positive velocity time to negative velocity time and the mean pulmonary artery pressure in all animals (r = 0.71). These findings should be compared with the velocity patterns of the blood flow in the pulmonary artery obtained under pulmonary hypertensive conditions due to various causes to facilitate interpretation and understanding of clinical investigations.

Pulmonary blood velocity profile variability in open-chest dogs: influence of acutely altered hemodynamic states on profiles, and influence of profiles on the accuracy of techniques for cardiac output determination.

Clinical investigations focused on finding characteristics of noninvasively obtained measurements of pulmonary blood velocity that can be used to quantitate pulmonary blood flow and/or pulmonary pressure have often yielded results whose imprecision has been attributed to flow pattern variability. To determine flow pattern variability in an in vivo animal model in varying hemodynamic states, main pulmonary artery blood velocity waveforms were recorded in 17 dogs at 2-mm intervals along an anterior to posterior wall-oriented axis using a 20-MHz pulsed Doppler needle probe. Control data were obtained before the animals were subjected to altered flow (atrial level shunts) and pressure (10% O2 inhalation) states. Instantaneous velocity profiles were computed throughout the cardiac cycle. Estimates of pulmonary blood flow were obtained assuming an elliptical model of the pulmonary artery which allowed computation of velocity at all points in the cross section, based on the measured values along the axis. Model-based estimates were compared to measured values and estimates obtained in the traditional fashion, i.e., the product of centerline velocity and cross-sectional area. Results clearly showed marked interanimal variability, even in control states. Reverse flow in the posterior half of the vessel, which tended to become more pronounced with increased pulmonary artery pressure, was observed during late systole and early diastole. Elevated pulmonary blood flow tended to increase the maximum velocities along the anterior wall relative to midline velocities. Neither estimate of cardiac output yielded consistently accurate results (r = 0.77 for model-based method, r = 0.80 for area times central velocity method). Findings of this study, which highlight the dependency of waveform characteristics on sampling site, the large degree of intersubject variability, and the need for large or multiple sample volumes for pulmonary blood flow determination, help clarify inconsistencies observed by clinicians and suggest that future work with animal models will facilitate a greater understanding of the determinants of human pulmonary velocity waveforms.

Continuous measurement of pulmonary blood flow using a retractable pulsed Doppler probe.

The feasibility of measuring pulmonary blood flow (PAQ) continuously using a removable, extraluminal 20 MHz pulsed Doppler probe, which has been used successfully to measure aortic blood flow, was assessed in seven anesthetized mongrel dogs. Simultaneous recordings were made from the Doppler probe (range-gated 5-6 mm from the anterior wall of the main pulmonary artery) and an electromagnetic flow probe (encircling the aorta) over cardiac outputs (CO) ranging from 0.2 to 5.5 L/min. Assuming a flat velocity profile and a fixed cross-sectional area, PAQ was initially calculated as the product of area and mean velocity. Regression analyses (PAQ = a + b X CO) indicated good intraanimal linear correlations in six animals (r greater than or equal to 0.84) and no correlation in one animal (r = 0.003); however, PAQ was consistently higher than CO and interanimal variability was marked, as suggested by large deviations in mean intercept and slope values (a = 1.67 +/- 1.09 L/min and b = 0.70 +/- 0.33). Results improved (r greater than or equal to 0.79 in all animals, a = 0.47 +/- 0.52 L/min, and b = 0.77 +/- 0.21) when the method to estimate PAQ was altered to assume that the starting cross-sectional area was the area that would make baseline PAQ and CO agree, and that the area during each subsequent CO level changed as a function of pulmonary artery pressure and an estimate of pulmonary artery compliance. Results of this study imply that it will be more difficult to use this Doppler probe to monitor CO from the pulmonary artery than it was from the aorta due to the elliptical, more compliant pulmonary vessel walls and the irregular pulmonary artery velocity profile.

Characteristics of blood flow velocity in the hypertensive canine pulmonary artery.

Pulmonary artery blood flow velocity was measured in 15 dogs by a recently developed direct intraluminal pulsed Doppler technique. Changes in velocity characteristics under conditions of experimentally induced hypoxic pulmonary hypertension were observed. Experimental conditions (fractional inspired oxygen concentration = 0.10) produced significant increases in mean pulmonary artery pressure and pulmonary vascular resistance. Overall and maximal negative velocity increased with pulmonary hypertension. Negative velocity occurred predominantly in the posterior half of the pulmonary artery during both control and experimental conditions. With pulmonary hypertension, diastolic negative velocity increased only in the posterior half of the pulmonary artery and systolic negative velocity decreased only in the anterior half. More basic knowledge of pulmonary artery blood flow characteristics may facilitate an informed approach to noninvasive detection of pulmonary hypertension. Direct measurements by this recently developed intraluminal technique will be useful in studying various conditions with altered pulmonary blood flow.

Two-dimensional in vivo pressure/diameter relationships in the canine main pulmonary artery.

Ultrasonic measurement of blood flow within the main pulmonary artery (MPA) requires a precise knowledge of the mean blood velocity within this vessel and the cross-sectional area (CSA). Small conformational changes in the elliptical shape of the MPA have substantial effects on the calculation of CSA and, subsequently, flow. We examined the extent of these changes by measuring the pulsatile and mean elliptical dimensions of the MPA in nine anaesthetised, open-chested, mechanically ventilated mongrel dogs using two pairs of 10 MHz ultrasonic, piezoelectric crystals. These custom-made devices were sutured to the PA adventitia along the long and short cross-sectional axes 2 cm distal to the pulmonary valve. Axial dimensions were collected during normal, elevated (via noradrenaline and fluid additions) and reduced (via exsanguination) PA pressures. We confirmed the linear pressure/diameter response in 15/18 axial data sets (r greater than 0.80). Further, the linear axial responses of the long and short diameters were parallel (7/9, p less than 0.05) and have different zero-pressure intercepts (7/9, p less than 0.0001). A mathematical consequence of this parallelism is predictable, although non-constant, eccentricity. Finally, error analysis of multi-axial measurement techniques were shown to improve CSA accuracy by as much as 50% when compared with uni-axial determinations.

Ultrasound detection and quantitation of left to right aortopulmonary shunt flow in a canine model.

To test the sensitivity and accuracy of pulsed Doppler ultrasonography to detect and quantitate left to right aortopulmonary shunt flow, an arterial allograft aortopulmonary anastomosis was constructed in nine adult mongrel dogs. Cardiac output and allograft flow were measured as the diameter of the allograft was varied. Piezoelectric crystals attached to the carotid artery and proximal descending aorta were energized with 20 MHz pulsed Doppler signals. Negative Doppler shift and negative Doppler shift/positive Doppler shift were calculated for seven dogs. All dogs exhibited negative Doppler shift in the carotid artery at zero allograft flow; five of the seven dogs exhibited a similar pattern in the descending aorta. Increasing negative Doppler shift was measured in all dogs from both sites as the allograft flow increased. Excellent linear correlation existed between allograft flow and negative Doppler shift and negative Doppler shift/positive Doppler shift for each dog from both sampling sites. However, marked interanimal variation in the slopes of the linear regression lines existed, making the composite linear correlation very poor. Detection of small left to right aortopulmonary shunting and single measurements to quantitate accurately left to right aortopulmonary shunting introduce errors due to intersubject variation. However, these results suggest that serial ultrasound measurements made over a short time can accurately predict changes in left to right aortopulmonary shunting.

Velocity profile in the main pulmonary artery in a canine model.

The velocity profile of the main pulmonary artery was determined in nine adult, open-chested, mechanically ventilated mongrel dogs using an intraluminal, needle-mounted, range-gated, pulsed Doppler technique. Mean phasic point velocities were determined at 2 mm intervals across the lumen of the vessel, 2 cm above the pulmonary valve, by recording the Doppler shift of an activated 20 MHz piezoelectric crystal, range-gated 3.5 mm in the direction of the pulmonary valve. Mean Reynolds numbers from the main pulmonary artery ranged from 275 to 1140. Radially normalised intraluminal distance versus mean phasic point velocity plots were constructed which demonstrated a curved profile in all 9 dogs. First order regression analysis demonstrated a poor fit (r: 0.05-0.68). Second order (r:0.61-0.97) and third order (r:0.72-0.99) regression analyses markedly improved the fit, confirming the non-linear nature of the velocity profile. Step-wise third order regression analysis to determine the importance of the entry sequence demonstrated that the most important term for determining the regression coefficient was the X2 term in six dogs. In addition, the velocity profile was noted to be shifted, with the highest velocities recorded between the centre of the vessel and the anterior wall in eight of nine dogs (location of highest velocity: +0.26 radius +/- 0.25 (mean +/- SD).