Correction of pressure waveforms recorded by fluid-filled catheter recording systems: a new method using a transfer equation

Low cost circulatory pressure acquisition and fluid infusion rate measurement system for clinical research

Acquiring patient physiological waveforms is useful for studying hemodynamic management and developing medical monitoring systems. A low cost, Arduino controlled data acquisition system acquires arterial pressure waveforms (Edwards Lifesciences TruWave compatible) and measures fluid infusion rate using hanging scales. This system can be used at the same time as a clinical monitor, enabling recording of patient arterial pressure and fluid delivery for clinical research. The system is powered via a USB connection, which additionally provides serial output, aiding compatibility and customisation. A simple software user interface, developed in Python, shows outputs. Each data acquisition system, including all necessary connection cables costs ~US$90 and is multiple-use.

High accuracy differential pressure measurements using fluid-filled catheters - A feasibility study in compliant tubes

High accuracy differential pressure measurements are required in various biomedical and medical applications, such as in fluid-dynamic test systems, or in the cath-lab. Differential pressure measurements using fluid-filled catheters are relatively inexpensive, yet may be subjected to common mode pressure errors (CMP), which can significantly reduce the measurement accuracy. Recently, a novel correction method for high accuracy differential pressure measurements was presented, and was shown to effectively remove CMP distortions from measurements acquired in rigid tubes. The purpose of the present study was to test the feasibility of this correction method inside compliant tubes, which effectively simulate arteries. Two tubes with varying compliance were tested under dynamic flow and pressure conditions to cover the physiological range of radial distensibility in coronary arteries. A third, compliant model, with a 70% stenosis severity was additionally tested. Differential pressure measurements were acquired over a 3 cm tube length using a fluid-filled double-lumen catheter, and were corrected using the proposed CMP correction method. Validation of the corrected differential pressure signals was performed by comparison to differential pressure recordings taken via a direct connection to the compliant tubes, and by comparison to predicted differential pressure readings of matching fluid-structure interaction (FSI) computational simulations. The results show excellent agreement between the experimentally acquired and computationally determined differential pressure signals. This validates the application of the CMP correction method in compliant tubes of the physiological range for up to intermediate size stenosis severity of 70%.

Accuracy of invasive arterial pressure monitoring in cardiovascular patients: an observational study

INTRODUCTION

Critically ill patients and patients undergoing high-risk and major surgery, are instrumented with intra-arterial catheters and invasive blood pressure is considered the "gold standard" for arterial pressure monitoring. Nonetheless, artifacts due to inappropriate dynamic response of the fluid-filled monitoring systems may lead to clinically relevant differences between actual and displayed pressure values. We sought to analyze the incidence and causes of resonance/underdamping phenomena in patients undergoing major vascular and cardiac surgery.

METHODS

Arterial pressures were measured invasively and, according to the fast-flush Gardner's test, each patient was attributed to one of two groups depending on the presence (R-group) or absence (NR-group) of resonance/underdamping. Invasive pressure values were then compared with the non-invasive ones.

RESULTS

A total of 11,610 pulses and 1,200 non-invasive blood pressure measurements were analyzed in 300 patients. Ninety-two out of 300 (30.7%) underdamping/resonance arterial signals were found. In these cases (R-group) systolic invasive blood pressure (IBP) average overestimation of non-invasive blood pressure (NIBP) was 28.5 (15.9) mmHg (P <0.0001) while in the NR-group the overestimation was 4.1(5.3) mmHg (P < 0.0001). The mean IBP-NIBP difference in diastolic pressure in the R-group was -2.2 (10.6) mmHg and, in the NR-group -1.1 (5.8) mmHg. The mean arterial pressure difference was 7.4 (11.2) mmHg in the R-group and 2.3 (6.4) mmHg in the NR-group. A multivariate logistic regression identified five parameters independently associated with underdamping/resonance: polydistrectual arteriopathy (P = 0.0023; OR = 2.82), history of arterial hypertension (P = 0.0214; OR = 2.09), chronic obstructive pulmonary disease (P = 0.198; OR = 2.61), arterial catheter diameter (20 vs. 18 gauge) (P < 0.0001; OR = 0.35) and sedation (P = 0.0131; OR = 0.5). The ROC curve for the maximal pressure-time ratio, showed an optimum selected cut-off point of 1.67 mmHg/msec with a specificity of 97% (95% CI: 95.13 to 99.47%) and a sensitivity of 77% (95% CI: 67.25 to 85.28%) and an area under the ROC curve by extended trapezoidal rule of 0.88.

CONCLUSION

Physicians should be aware of the possibility that IBP can be inaccurate in a consistent number of patients due to underdamping/resonance phenomena. NIBP measurement may help to confirm/exclude the presence of this artifact avoiding inappropriate treatments.

Method for high accuracy differential pressure measurements using fluid-filled catheters

The advantage of measuring differential pressure using fluid-filled catheters is that the system is relatively inexpensive, but the readings are not accurate and affected by the common mode pressure (CMP) distortion. High accuracy differential pressure measurements are required in various biomedical applications, such as in fluid-dynamic test rigs, or in the cath-lab, from cardiac valves efficacy to functional assessment of arterial stenoses. We have designed and built a unique system in which the pressure difference was measured along the fluid flow inside a rigid circular tube using a fluid-filled double-lumen catheter. The differential pressure measurements were taken across two side-holes near the catheter distal tip, spaced apart by 3 cm. The goal was to overcome the CMP error, which significantly distorted the output differential pressure signal and to formulate a restoration factor. A restoration formula was developed based on simultaneous gauge pressure measurements, and was tested in several different cases. Several representative cases are presented and show that the common mode artifact was reduced by factors of 12-27. The restored pressure gradient signal was validated using direct pressure drop measurements, and showed very good agreement.

Arterial dP/dtmax accurately reflects left ventricular contractility during shock when adequate vascular filling is achieved

Background

Peak first derivative of femoral artery pressure (arterial dP/dt_max) derived from fluid-filled catheter remains questionable to assess left ventricular (LV) contractility during shock. The aim of this study was to test if arterial dP/dt_max is reliable for assessing LV contractility during various hemodynamic conditions such as endotoxin-induced shock and catecholamine infusion.

Methods

Ventricular pressure-volume data obtained with a conductance catheter and invasive arterial pressure obtained with a fluid-filled catheter were continuously recorded in 6 anaesthetized and mechanically ventilated pigs. After a stabilization period, endotoxin was infused to induce shock. Catecholamines were transiently administrated during shock. Arterial dP/dt_max was compared to end-systolic elastance (Ees), the gold standard method for assessing LV contractility.

Results

Endotoxin-induced shock and catecholamine infusion lead to significant variations in LV contractility. Overall, significant correlation (r = 0.51; p < 0.001) but low agreement between the two methods were observed. However, a far better correlation with a good agreement were observed when positive-pressure ventilation induced an arterial pulse pressure variation (PPV) ≤ 11% (r = 0.77; p < 0.001).

Conclusion

While arterial dP/dt_max and Ees were significantly correlated during various hemodynamic conditions, arterial dP/dt_max was more accurate for assessing LV contractility when adequate vascular filling, defined as PPV ≤ 11%, was achieved.

Wave intensity analysis of right ventricular and pulmonary vascular contributions to higher pulmonary than aortic blood pressure in fetal lambs

Although fetal pulmonary trunk (PT) blood pressure may exceed aortic trunk (AoT) pressure, the specific mechanism(s) underlying this pressure difference remain undefined. To evaluate the potential role of ventricular and vascular factors in the generation of a fetal PT-AoT pressure difference, nine anesthetized late-gestation fetal sheep were instrumented with PT and AoT micromanometer catheters to measure high-fidelity pressure and transit-time flow probes to obtain blood velocity. The PT-AoT instantaneous pressure difference (IPD(PT-AoT)) was calculated from PT and AoT pressure profiles. PT and AoT wave intensity (WI) was derived from the product of the appropriate pressure and velocity rates of change. While diastolic pressures were near identical, systolic PT pressure exceeded AoT pressure (P < 0.001), with a maximal IPD(PT-AoT) of 6.5 +/- 2.5 mmHg. The comparison of IPD(PT-AoT) with wave-related PT and AoT pressure changes indicated that 1) a greater pressure-generating effect of the PT forward-running compression wave arising from impulsive right ventricular contraction in early and midsystole accounted for 2.3 +/- 2.3 mmHg (35%) of the maximal IPD(PT-AoT) and 2) a larger pressure-generating effect of a large midsystolic backward-running compression wave transmitted into the PT from the pulmonary vasculature contributed 4.0 +/- 1.5 mmHg ( approximately 60%) of the maximal IPD(PT-AoT). These results indicate that the higher PT than AoT blood pressure observed in fetal lambs is a systolic phenomenon principally related to the combination of a relatively higher level of right ventricular pump function manifest in early and midsystole and a pressure-increasing energy wave arising from the fetal pulmonary vasculature in midsystole.

Integrated assessment of diastolic and systolic ventricular function using diagnostic cardiac magnetic resonance catheterization: validation in pigs and application in a clinical pilot study

OBJECTIVES

This study sought to develop and validate a method for the integrated analysis of systolic and diastolic ventricular function.

BACKGROUND

An integrated approach to assess ventricular pump function, myocontractility (end-systolic pressure-volume relationship [ESPVR]), and diastolic compliance (end-diastolic pressure-volume relation [EDPVR]) is of high clinical value. Cardiac magnetic resonance (CMR) is well established for measuring global pump function, and catheterization-combined CMR was previously shown to accurately measure ESPVR, but not yet the EDPVR.

METHODS

In 8 pigs, the CMR technique was compared with conductance catheter methods (gold standard) for measuring the EDPVR in the left and right ventricle. Measurements were performed at rest and during dobutamine administration. For CMR, the ESPVR was estimated with a single-beat approach by synchronizing invasive ventricular pressures with cine CMR-derived ventricular volumes. The EDPVR was determined during pre-load reduction from additional volume data that were obtained from real-time velocity-encoded CMR pulmonary/aortic blood flow measurements. Pre-load reduction was achieved by transient balloon occlusion of the inferior vena cava. The stiffness coefficient beta was calculated by an exponential fit from the EDPVR. After validation in the animal experiments, the EDPVR was assessed in a pilot study of 3 patients with a single ventricle using identical CMR and conductance catheter techniques.

RESULTS

Bland-Altman tests showed good agreement between conductance catheter-derived and CMR-derived EDPVR. In both ventricles of the pigs, dobutamine enhanced myocontractility (p < 0.01), increased stroke volume (p < 0.01), and improved diastolic function. The latter was evidenced by shorter early relaxation (p < 0.05), a downward shift of the EDPVR, and a decreased stiffness coefficient beta (p < 0.05). In contrast, in the patients, early relaxation was inconspicuous but the EDPVR shifted left-upward and the stiffness constant remained unchanged. The observed changes in diastolic function were not significantly different when measured with conductance catheter and CMR.

CONCLUSIONS

This novel CMR method provides differential information about diastolic function in conjunction with parameters of systolic contractility and global pump function.