The Relationship Between Pulmonary System Impedance and Right Ventricular Function in Normal Sheep

Development of a servo pump system for in vivo loading of pathological pulmonary artery impedance on the right ventricle of normal rats

Pulmonary artery (PA) impedance provides detailed information on right ventricular (RV) afterload in pulmonary hypertension (PH). This study aimed to examine PA impedance in a rat model of monocrotaline-induced PH (MCT-PH) and to develop an experimental system for in vivo loading of pathological PA impedance on the RV of normal rats. PA impedance was quantified in normal ( n = 10) and MCT-PH rats ( n = 10) using a three-element Windkessel (3-WK) model. Compared with normal rats, MCT-PH rats had higher characteristic impedance ( ZC) and peripheral pulmonary resistance ( RP) ( ZC: 0.121 ± 0.039 vs. 0.053 ± 0.017 mmHg·min·ml−1, P < 0.001; RP: 0.581 ± 0.334 vs. 0.252 ± 0.105 mmHg·min·ml−1, P = 0.013) and lower pulmonary artery compliance ( CP) (0.242 ± 0.131 vs. 0.700 ± 0.186 ml/mmHg, P < 0.001). In another group of 10 normal rats, a computer-controlled servo pump was connected to the left PA for loading PA impedance with parameters in pathological ranges designed by the 3-WK model. Activation of the servo pump decreased the error of measured vs. target PA impedance (modulus: from 0.047 ± 0.020 without pump activation to 0.019 ± 0.007 with pump activation, P < 0.001; phase: 0.085 ± 0.028 to 0.043 ± 0.012 radians, P < 0.001). In conclusion, MCT-PH increases ZCand RPand decreases CP. Our servo pump system, which is capable of imposing arbitrary PA impedance with pathological parameters, may offer a unique opportunity to delineate the pathological significance of PA impedance in PH.

Determinants of right ventricular afterload (2013 Grover Conference series)

Right ventricular (RV) afterload consists of both resistive and capacitive (pulsatile) components. Total afterload can be measured directly with pulmonary artery input impedance spectra or estimated, either with lumped-parameter modeling or by pressure-volume analysis. However, the inverse, hyperbolic relationship between resistance and compliance in the lung would suggest that the pulsatile components are a predictable and constant proportion of the resistive load in most situations, meaning that total RV load can be estimated from mean resistive load alone. Exceptions include elevations in left atrial pressures and, to a lesser extent, chronic thromboembolic disease. The pulsatile components may also play a more significant role at normal or near-normal pulmonary artery pressures. Measures of coupling between RV afterload and RV contractility may provide important information not apparent by other clinical and hemodynamic measures. Future research should be aimed at development of noninvasive measures of coupling.

Hemodynamic design requirements for in-series thoracic artificial lung attachment in a model of pulmonary hypertension

Recent thoracic artificial lung (TAL) prototypes have impedances lower than the natural lung. With these devices, proximal pulmonary artery (PA) to distal PA TAL attachment may be possible in patients without right ventricular dysfunction. This study examined the relationship between pulmonary system impedance and cardiac output (CO) to create TAL design constraints. A circuit with adjustable resistance and compliance (C) was attached in a PA-PA fashion with the pulmonary circulation of seven sheep with chronic pulmonary hypertension. The pulmonary system zeroth harmonic impedance modulus (Z(0)) was increased by 1, 2.5, and 4 mmHg/(L/min) above baseline. At each Z(0), C was set to 0, 0.34, and 2.1 ml/mmHg. The change in pulmonary system zeroth and first harmonic impedance moduli (ΔZ(0) and ΔZ(1)), the percent change in CO (%ΔCO), and the inlet and outlet anastomoses resistances were calculated for each situation. Results indicate that ΔZ(0) (p < 0.001) but not ΔZ(1) (p = 0.5) had a significant effect on %ΔCO and that %ΔCO = -7.45*ΔZ(0) (R(2) = 0.57). Inlet and outlet anastomoses resistances averaged 0.77 ± 0.16 and 0.10 ± 0.19 mmHg/(L/min), respectively, and the relationship between %ΔCO and TAL resistance, R(T), in mmHg/(L/min) was determined to be %ΔCO = -(7.45f)×(R(T) + 0.87), in which f = the fraction of CO through the TAL. Thus, newer TAL designs can limit %ΔCO to less than 10% if f < 0.75.

In-parallel attachment of a low-resistance compliant thoracic artificial lung under rest and simulated exercise

BACKGROUND

Previous thoracic artificial lungs (TALs) had blood flow impedance greater than that of the natural lungs, which could cause abnormal pulmonary hemodynamics. New compliant TALs (cTALs), however, have an impedance lower than that of the natural lung.

METHODS

In this study, a cTAL of new design was attached between the pulmonary artery (PA) and the left atrium (LA) in 5 sheep (60.2 ± 1.9 kg). A distal PA band was placed to control the percentage of cardiac output (CO) routed to the cTAL. Rest and exercise conditions were simulated using a continuous dobutamine infusion of 0 and 5 μg/kg/min, respectively. At each dose, a hemodynamic data set was acquired at baseline (no flow to the cTAL), and 60%, 75%, and 90% of CO was shunted to the cTAL.

RESULTS

Device resistance did not vary with blood flow rate, averaging 0.51 ± 0.03 mm Hg/(L/min). Under all conditions, CO was not significantly different from baseline. Pulmonary system impedance increased above baseline only with 5 μg/kg/min of dobutamine and 90% of CO diverted to the cTAL.

CONCLUSIONS

Results indicated minimal changes in pulmonary hemodynamics during PA-LA cTAL attachment for high device flows under rest and exercise conditions.

Thoracic artificial lung impedance studies using computational fluid dynamics and in vitro models

Current thoracic artificial lungs (TALs) possess blood flow impedances greater than the natural lungs, resulting in abnormal pulmonary hemodynamics when implanted. This study sought to reduce TAL impedance using computational fluid dynamics (CFD). CFD was performed on TAL models with inlet and outlet expansion and contraction angles, θ, of 15°, 45°, and 90°. Pulsatile blood flow was simulated for flow rates of 2-6 L/min, heart rates of 80 and 100 beats/min, and inlet pulsatilities of 3.75 and 2. Pressure and flow data were used to calculate the zeroth and first harmonic impedance moduli, Z(0) and Z(1), respectively. The 45° and 90° models were also tested in vitro under similar conditions. CFD results indicate Z(0) increases as stroke volume and θ increase. At 4 L/min, 100 beats/min, and a pulsatility of 3.75, Z(0) was 0.47, 0.61, and 0.79 mmHg/(L/min) for the 15°, 45°, and 90° devices, respectively. Velocity band and vector plots also indicate better flow patterns in the 45° device. At the same conditions, in vitro Z (0) were 0.69 ± 0.13 and 0.79 ± 0.10 mmHg/(L/min), respectively, for the 45° and 90° models. These Z(0) are 65% smaller than previous TAL designs. In vitro, Z(1) increased with flow rate but was small and unlikely to significantly affect hemodynamics. The optimal design for flow patterns and low impedance was the 45° model.

Longitudinal myocardial contribution to peak systolic flow and stroke volume in the neonatal heart

Systolic longitudinal myocardial function is important for cardiac ejection. Data describing hemodynamic determinants and the time course of myocardial longitudinal contraction as measured by tissue Doppler are lacking. Ten newborn pigs were used for invasive hemodynamic investigation. Tissue Doppler assessment of the lateral part of the mitral valve annulus during systole was performed during pharmacological modulation of inotropy, cardiac pacing, and modulations of loading conditions. The strongest association was found between peak systolic velocity (S') and peak systolic flow (PSF) and end-systolic pressure (ESP), respectively (β = 0.09 cm/mL, p < 0.001 and β = -0.07 cm/mL, p = 0.003). Displacement (D) was mostly influenced by stroke volume (SV) (β = 0.05 cm/mL, p < 0.001). Ejection time, SV, ESP, maximum first derivative of pressure (dP/dtmax), and PSF were all associated with S' and D under different states of hemodynamic modulation; however, the ratio between PSF and S', SV, and D were stable during hemodynamic modulations. Normalized cross correlations indicate that S' and D follow the same trajectory as flow and SV, respectively. In conclusion, this study provides validity of accounting systolic D in the long axis as the longitudinal contribution to SV and peak systolic tissue velocity as the longitudinal contribution to PSF.

In-parallel artificial lung attachment at high flows in normal and pulmonary hypertension models

BACKGROUND

End-stage lung disease patients who require a thoracic artificial lung (TAL) must be extubated and rehabilitated prior to lung transplantation. The purpose of this study is to evaluate hemodynamics and TAL function under simulated rest and exercise conditions in normal and pulmonary hypertension sheep models.

METHODS

The TAL, the MC3 Biolung (MC3, Inc, Ann Arbor, MI), was attached between the pulmonary artery and left atrium in nine normal sheep and eight sheep with chronic pulmonary hypertension. An adjustable band was placed around the distal pulmonary artery to control the percentage of cardiac output (CO) diverted to the TAL. Pulmonary system hemodynamics and TAL function were assessed at baseline (no flow to the TAL) and with approximately 60%, 75%, and 90% of CO diverted to the TAL. Intravenous dobutamine (0, 2, and 5 mcg . kg(-1). min(-1)) was used to simulate rest and exercise conditions.

RESULTS

At 0 and 2 mcg . kg(-1). min(-1), CO did not change significantly with flow diversion to the TAL for both models. At 5 mcg . kg(-1). min(-1), CO decreased with increasing TAL flow up to 28% +/- 5% in normal sheep and 23% +/- 5% in pulmonary hypertension sheep at 90% flow diversion to the artificial lung. In normal sheep, the pulmonary system zeroth harmonic impedance modulus, Z(0), increased with increasing flow diversion. In hypertensive sheep, Z(0) decreased at 60% and 75% flow diversion and returned to baseline levels at 90%. The TAL outlet blood oxygen saturation was 95% or greater under all conditions.

CONCLUSIONS

Pulmonary artery to left atrial TAL use will not decrease CO during rest or mild exercise but may not allow more vigorous exercise.