Time-dependent Pressure Distortion in a Catheter–Transducer System: Correction by Fast Flush

We assembled three catheter–transducer systems using identical components for each: (1) a 7.5-French standard pulmonary artery catheter, 110 cm long (CAP 93A-834H-7.5F; Baxter Edwards Critical Care, Irvine, CA); and (2) a standard disposable pressure transducer with an associated flush device and three-way stopcock (33-260; Baxter Edwards Critical Care). An additional three-way stopcock was interposed between the flush inlet of the pressure transducer and the flush line of the pressurized saline bag. A 1-l saline bag was inserted in a cuff pressurized at 300 mmHg (Colson 331,001; Comeda, La Tronche, France), which was selected after checking that two other pressurizing cuffs (Baxter Travenol FRA 4,403; Ethox 4,010, Europe Medical, Le Faguet, France) created a similar pressure in the flush line.

Each catheter–transducer system was slowly primed with saline, and thereafter a slow continuous flush of the transducer and pulmonary artery catheter was maintained by the pressure transducer–associated flush device. The pulmonary artery catheter was maintained at a temperature of 37°C inside a pipe (100 cm long, 10 mm ID), with running water maintained at 37°C. Each test lasted 48 h.

The frequency response of each catheter–transducer system was tested using a sine wave generator (Hewlett-Packard 3,311A Function generator, Palo Alto, CA) from 1–40 Hz. The pressure waves were generated in the chamber of a hydraulic pressure generator (Biotek 601A; Biotek Instruments, Winooski, Vermont). This generator includes a saline-filled clear plastic dome closed by a sealed piston system. The piston volume displacements are small and allow the conversion of electrical signals into calibrated pressures. The plastic dome has two Luer lock ports. One port was connected to a hemostatic valve (Desilet-intro 7-8F valve 2,825.27; Plastimed, Le Plessis-Bouchard, France). The second port was connected to a three-way stopcock, which was in turn connected to the diaphragm valve of a pulmonary artery catheter introducer sheath. The latter valve allows a watertight introduction of catheters of various sizes into the dome. The intravascular tip of the 7.5-French pulmonary artery catheter, protruding from the thermostatized jacket, was introduced 10 mm into the plastic dome through the hemostatic valve. A high-frequency response catheter-tipped pressure transducer (Millar MPC-500; Millar Instruments, Houston, TX) was introduced 15 mm into the plastic dome through the other valve. The flush solution was allowed to leak from the chamber to the atmosphere through the three-way stopcock connecting the dome to the catheter-tipped pressure transducer, except during the generation of pressure waves inside the dome.

The disposable pressure transducer and the Millar catheter-tipped pressure transducer were calibrated simultaneously using the same water column. They were preamplified, respectively, by a Bridge amplifier (MacLab; AD Instruments, Castle Hill, Australia) and by a Millar TCB-500 Transducer Control Unit (Millar Instruments). Both signals were sampled in parallel at a frequency of 200 Hz by a multichannel analog-to-digital converter (MacLab/8; AD Instruments) and recorded on a Macintosh computer (Apple Computer, Cupertino, CA). The Millar catheter-tipped pressure transducer measured the reference pressure.

The pressure signal measured by the tested catheter–transducer system was compared with the reference pressure signal of the catheter-tipped pressure transducer for sine waves generated from 1–40 Hz by the hydraulic pressure generator (figs. 1A–C). The frequency response was measured in two steps. First, the response to a spectrum of frequencies from 1–40 Hz was scanned to detect the resonant frequency (F^r), i.e. , the frequency at which the amplitude of the pressure signal is maximal. Second, F^rwas refined using 0.5-Hz increments in the 5-Hz range lower and higher than F^r. The whole procedure for measuring the frequency response lasted ≈2 min. The generation of pressure waves inside the dome of the hydraulic pressure generator during the measurement of the frequency response involved the interruption of the slow catheter–transducer system flushing and the closure of the chamber. The chamber was reopened to the atmosphere after the measurement of the pressure waves and the slow flushing resumed.

The harmonic analysis of the frequency response of the catheter–transducer system permitted the determination of the transfer function. The transfer function of the catheter–transducer system allows the calculation of the output pressure signal from any arbitrary pressure signal. The amplitude of the transfer function (fig. 2) was obtained from the amplitude ratio of the catheter–transducer system pressure signal to the reference pressure signal for all measured frequencies. The maximal amplitude of the transfer function at F^ris the gain, G. The phase of the transfer function (fig. 2) was obtained from the phase lag of the catheter–transducer system signal versus  the reference signal of the catheter-tipped pressure transducer. We assumed that the frequency response of the catheter–transducer system was a second-order transfer function characterized by two parameters, the natural frequency (Fⁿ) and the damping coefficient (ξ) ( Appendix 1).

The transfer function of the catheter–transducer system was measured 0, 1, 6, 24, and 48 h after the initial priming with saline. At each time, the measurement was repeated five times in series.

Because trapped air bubbles modify the frequency response of the catheter–transducer system, 10–12a different catheter–transducer system fast flush protocol was performed initially in each of the three systems being tested. The first protocol (P1 standard catheter–transducer system rapid flush) used the rapid flush device built into the pressure transducer. Pushing the button allowed a rapid flush by the pressurized saline bag. The P1 standard catheter–transducer system rapid flush was performed twice, for 5 s each. The second protocol (P2 whole catheter–transducer system rocket flush) involved a manual fast flush of the whole system by the fast injection of 5 ml saline using a syringe through the additional three-way stopcock interposed between the flush inlet of the pressure transducer and the flush line of the pressurized saline bag. To evaluate the necessity to flush the transducer separately from the catheter, the third protocol (P3 separate catheter–transducer system rocket flush) involved a manual fast flush of the catheter and of the transducer separately by the fast injection of 5 ml saline through the three-way stopcocks described previously.

After the last measurement at 48 h, two additional measurements after a fast flush were performed in all experiments. The first was aimed at observing the ability of the P1 standard catheter–transducer system rapid flush to reverse the time dependence of the frequency response of the system (test 1). The second was aimed at verifying that the three catheter–transducer systems retained similar behaviors at the end of all experiments after a P3 separate catheter–transducer system rocket flush (test 2).

To extend the clinical pertinence of the study, we measured the transfer function of the catheter–transducer system after a P3 separate catheter–transducer system rocket flush not only at 37°C but also at ambient temperature.

A pulmonary arterial occlusion pressure transient was measured during apnea in an anesthetized dog using a pressure microtransducer-tipped catheter (Millar SPC320) introduced in the lumen of the distal pressure channel of a 7.5-French pulmonary artery catheter (CAP 93A-631H-7.5F, Baxter Edwards Critical Care). The signal was sampled at a frequency of 100 Hz using the described apparatus. The data were stored in IBM PC format and reconverted into an analog electrical signal form by a PCL-812PG (Advantech, Taipei, Taiwan) using a PC-compatible AT 486DX computer (ASC Inc., West Allis, WI). This analog electrical signal was then used to generate a pressure signal using the in vitro  pressure generator.

The in vitro –generated pulmonary arterial occlusion pressure transient was measured in duplicate after the sine pressure waves in the three catheter–transducer systems at two temperatures (room temperature and 37°C) after our best version of the fast flush procedure, i.e. , a P3 separate rocket flush. The catheter–transducer system-induced distortion of the pulmonary arterial occlusion pressure tracing was illustrated by comparing the systolic, mean, diastolic, and wedge pressure values taken from the catheter–transducer system pressure with those taken from the reference catheter-tipped pressure transducer. A pulmonary capillary pressure value was obtained from the biexponential adjustment of the arterial occlusion pressure profile using a model of the pulmonary circulation including three compartments separated by two resistances. 13As the early segment of the pulmonary arterial occlusion pressure profile decreases rapidly, it is sensitive to the catheter–transducer system-induced distortion. Therefore, the early exponential time constant value is also given.

The parameters of the transfer function of the catheter–transducer system changed with time. Natural frequency decreased with a steeper slope during the first hours (fig. 3). In contrast, the damping coefficient increased with time. The relations of natural frequency and damping coefficient with time clearly differ depending on the initial catheter–transducer system fast flush protocol; the means and SDs of the five replicate measurements obtained at 0, 1, 6, 24, and 48 h do not overlap ( Appendix 2).

After the last measurement at 48 h, the efficacy of two catheter–transducer system fast flush protocols in restoring the transfer function of the catheter–transducer system back to baseline was tested in the three catheters (fig. 4and  Appendix 2). The final P1 standard catheter–transducer system rapid flush performed in test 1 had essentially no effect. The final P3 separate catheter–transducer system rocket flush performed in test 2 enabled the natural frequency and the damping coefficient to reach a similar value for the three catheters regardless of their initial fast flush protocol, as observed from their means ± SDs and low coefficients of variation (17.07 ± 0.45, 0.6%; 0.234 ± 0.002, 0.9%, respectively). Two conclusions may be drawn from the high reproducibility of the transfer function parameter values of the three catheter–transducer systems. First, the final P3 separate catheter–transducer system rocket flush not only reversed the time-dependent changes of the transfer function of the catheter–transducer system but also the effects of the initial fast flush on the transfer function of the system. Second, the measurements of the behavior of the catheter–transducer system in the current study are highly reproducible from one catheter–transducer system to the other.

After a P3 catheter–transducer system separate rocket flush in the three catheters, natural frequency was 16.50 ± 0.33 Hz (n = 9) at ambient temperature versus  17.06 ± 0.47 Hz (n = 14) at 37°C, and the damping coefficient was 0.245 ± 0.015 (n = 9) at ambient temperature versus  0.234 ± 0.002 (n = 14) at 37°C. These differences are small regarding transfer function of the catheter–transducer system.

A pulmonary arterial occlusion pressure transient signal previously obtained in an anesthetized dog using a high-fidelity microtransducer-tipped pressure catheter was used to pilot the hydraulic pressure generator. To illustrate the effects of catheter–transducer system-induced distortion on the values taken from a pulmonary arterial occlusion pressure tracing, the generated pressure signal was measured (1) using the reference catheter-tipped pressure transducer; and (2) using the pulmonary artery catheter after a P3 separate catheter–transducer system rocket flush. The obtained values are displayed in table 1. Using the most efficient fast flush protocol, the estimate of pulmonary capillary pressure using the catheter–transducer system was not significantly modified compared with the estimate using the reference transducer. Overestimation of the pulmonary arterial systolic pressure by the catheter–transducer system, however, increased up to almost 5 mmHg.

The current in vitro  test of a pulmonary artery catheter in physical conditions approximating some of those met in a clinical environment (37°C, slow continuous flushing of the catheter, 48 h of monitoring) shows that the physical characteristics of a liquid-filled catheter–transducer system appear to change gradually in the hours after its assembly. The consequences of these changes are illustrated in an example in figures 5A and 5B, in which a baseline overestimation of the pulmonary arterial systolic pressure is amplified within 48 h. Another finding is the inability of the standard rapid flush device of the disposable transducer to prevent time-dependent changes of the transfer function of the catheter–transducer system. The most important finding of this study, however, is that a specific manual catheter–transducer system syringe fast flush, or “rocket flush,” can prevent these time-dependent changes of the transfer function of the catheter–transducer system, enabling a correction based on constant catheter–transducer system transfer function parameters.

A catheter–transducer system induces a well-known distortion of the pressure signal. 1–7It is most often an overestimation, which may reach 30% even after a careful initial standard rapid flush. 5,6The current data confirm that the physical characteristics of a simple catheter–transducer system as described in the current study (one disposable pressure transducer, one pulmonary artery catheter) induce a distortion of the pressure signal. When the frequency components of the pressure signal are close to the resonant frequency, they become more and more amplified (figs. 1B and 2). When the frequency components of the pressure signal reach high frequencies, they become more and more attenuated (figs. 1C and 2). The transfer function of the catheter–transducer system, therefore, is one source of distortion of the pressure signal. The distortion of the pressure signal, however, also depends on the frequency components of the pressure signal, which vary with the hemodynamic status (e.g. , tachycardia, increase in systolic volume). 4The pressure distortion may be corrected by in-line mechanical damping devices 5,8,14or by electronic and computerized filters.

The changes of the transfer function parameters of the catheter–transducer system within 48 h observed in the current study (fig. 3and  Appendix 2) modulate the importance of the pressure distortion (figs. 5A and 5B). The corrective mechanical or electronic filters assume that the transfer function of the catheter–transducer system remains constant. The current data question this assumption. If the time-dependent changes of the transfer function of the catheter–transducer system are not also reversed by the filters, unpredictable changes of a given pressure waveform may appear. Unless the changes of the transfer function of the catheter–transducer system with time are periodically accounted for or prevented, the pressure distortion is almost impossible to correct reliably. TABLE 2 

The transfer function of the catheter–transducer system differs depending on the initial catheter–transducer system fast flush protocol. This difference also changes with time, as evidenced by the differences in natural frequency and damping coefficient at 0, 1, 6, 24, and 48 h (fig. 3and  Appendix 2). Another finding is the inability of the standard rapid flush device built into the pressure transducer to prevent time-dependent changes of the transfer function of the system (fig. 4and  Appendix 2) and thus a change in the pressure distortion at 48 h. Three different cuffs around the saline bag pressurized at 300 mmHg could barely generate a pressure in the flush line > 150 mmHg. This may be a contributing factor in the inability of the standard rapid flush device built into the pressure transducer to prevent the time-dependent changes of the transfer function and pressure distortion.

The most important finding of this study, however, is that a separate manual fast flush by a syringe or rocket flush, not only of the catheter but also of the transducer itself (P3 separate catheter–transducer system rocket flush), can prevent the time-dependent changes in the transfer function of the catheter–transducer system. The final P3 separate catheter–transducer system rocket flush performed at 48 h allows the natural frequency and the damping coefficient of the three catheters to reach a similar value, regardless of the initial fast flush protocol (fig. 4). Not only did the final P3 separate catheter–transducer system rocket flush reestablish a transfer function of the catheter–transducer system at 48 h similar to that observed at time 0 but it also reversed the effects of the two other initial fast flush protocols on the transfer functions of the other catheter–transducer systems. This observation confirms the reproducibility of the transfer function of the catheter–transducer system from one catheter to the other as long as a P3 separate catheter–transducer system rocket flush is performed periodically.

The catheter–transducer system mechanical parameters were obtained ( Appendix 315–17). The increases in inertance (see  Appendix 3) within 48 h were low (15–60%) compared with increases in compliance (175–270%;fig. 6). All changes in inertance and compliance were reversed by the P3 separate catheter–transducer system rocket flush, demonstrating that the three catheter–transducer systems were manufactured with reproducible mechanical characteristics and that our experimental data were highly reproducible; further, the observations supported that the physical environment of the catheter did not notably alter the catheter compliance within 48 h. This study finds no direct evidence for the cause of the initial changes in inertance. We suggest, however, that the presence of microscopic air bubbles in the liquid-filled catheter–transducer system may explain three specific characteristics of the catheter–transducer system compliance behavior: (1) an initial level related to the initial catheter–transducer system fast flush protocol; (2) a time-dependent increase; and (3) the reversal of this increase back to a common value for the three catheter–transducer systems by the P3 separate catheter–transducer system rocket flush. Previous studies have suggested that an increase in compliance may be related to the presence of bubbles in the catheter–transducer system liquid. 7,10–12Moreover, Taylor et al.  11demonstrated a linear relation between pulmonary artery catheter compliance and the volume of an air bubble introduced in the catheter. We made the hypothesis that the vaporization of air dissolved in saline inside the catheter–transducer system may generate bubbles. We suggest that the volume of these bubbles depends on the time-dependent growth of bubbles and on the efficacy of the flush.

In clinical conditions, part of the catheter is in the air at ambient temperature while the remaining part is intravascular at body temperature. The current data show, confirming previous data, 6that the temperature of the catheter environment (37°C vs.  ambient) does not have a critical effect on the transfer function of the catheter–transducer system. From a practical point of view, the current data also emphasize the need for manufacturers to improve the efficacy of the rapid flush devices built into the disposable pressure transducers. A specific manual catheter–transducer system syringe fast flush or rocket flush can prevent time-dependent changes of the transfer function of the catheter–transducer system, enabling a correction by a filter with a fixed set of parameters. No interpretation of repeated snap tests is needed, only a rocket flush at regular time intervals. To avoid any damage in the pulmonary circulation, however, the rocket flush should not be applied on a routine basis without checking that the pulmonary artery catheter is not in a wedge position. Finally, the conclusions of this study are not limited to the clinical measurement of pulmonary arterial pressure. They also apply to the invasive measurement of systolic arterial blood pressure in the systemic circulation, in which the possibility of catheter–transducer system-induced distortions have been demonstrated repeatedly. 7,8,10Some attention should be given to these distortions, especially when variations in systolic arterial blood pressure are analyzed to assess preload during mechanical ventilation. 18,19 

The authors thank Michèle Delaire and Frédéric Thouvenin for technical assistance and the C.A.M.S. (Disposable medical equipment service) of the Centre Hospitalier Universitaire de Grenoble for supplying the catheters.

We assumed that the frequency response of the catheter–transducer system was a second-order transfer function characterized by two parameters, the natural frequency (Fⁿ), which is the oscillation frequency resulting from a step pressure change, and the damping coefficient (ξ). Using this model, the ratio P²(f)/P¹(f) reads:

where j is the imaginary unit, P¹is the pressure signal output of the transducer, P²is the driving pressure at the intravascular tip of the catheter, and f is the frequency.

The damping coefficient ξ is calculated from 5,9:

where

where F^ris resonant frequency (see Materials and Methods).

The natural frequency Fⁿis calculated from 9:

The transfer function of the catheter–transducer system also may be related to the mechanical parameters of the catheter–transducer system (resistance, inertance, and compliance; Appendix 3).

The changes of natural frequency and damping coefficient at 0, 1, 6, 24, and 48 h are given in detail in table 2. These changes clearly differ depending on the initial CT fast flush protocol. The results of two CT fast flush protocols in restoring the CT transfer function back to baseline at 48 h are also given in detail (final test 1 and final test 2) in table 2.

To relate the frequency response of the catheter–transducer system to the mechanical parameters of the catheter–transducer system (resistance [R], inertance [I], and compliance [C]), the frequency response of the catheter–transducer system is generally described by a second-order differential equation equivalent to the transfer function in 15,16:

where I is the liquid inertance of the catheter, i.e. , the coefficient relating the pressure increment to the flow acceleration. Inertance is proportional to the volumetric mass of the liquid inside the catheter and to the catheter length. It is inversely proportional to the cross-sectional area. C is the compliance of the catheter–transducer system, R is the liquid resistance of the catheter, P¹is the pressure signal output of the transducer, P²is the driving pressure at the intravascular tip of the catheter, and t is time.

This transfer function is characterized by a natural frequency (Fⁿ) and by a damping coefficient (ξ). These two parameters are obtained by identification from equations 1 and 59:

The parameters Fⁿand ξ of the transfer function of the catheter–transducer system allow the calculation of I and C if R is measured independently. The measurement of the pressure difference δP and of the saline flow (Q) through the catheter allowed the calculation of the resistance of the catheter, 17which is the major component of the catheter–transducer system resistance. 11The external port of the pulmonary artery catheter was connected through a three-way stopcock to the infusion line of the saline bag and to the disposable pressure transducer while the intravascular extremity of the catheter was open to the atmospheric pressure into a glass reservoir. The filling time of the reservoir allowed measurement of the constant flow through the catheter.

Resistance is 373 ± 12 mmHg · s · l⁻¹(mean ± SD, n = 3). Catheter–transducer system inertance increases from 0 to 48 h, ranging from 15–60% with time with a marked initial increment (fig. 6). Catheter–transducer system compliance presents a linear increase with time, with an increase ranging from 175–270%. The relations between catheter–transducer system compliance and time clearly differ according to the protocol of initial catheter–transducer system fast flush (fig. 6). Compliance during test 2 (P3 separate catheter–transducer system rocket flush) reached similar values of 0.115, 0.122, and 0.115 10⁻⁴l/mmHg for initial P1 standard catheter–transducer system rapid flush and P2 whole and P3 separate catheter–transducer system rocket flushes, respectively. The inertance and compliance of the three catheter–transducer systems in test 2 reflect the high reproducibility of the catheter–transducer system mechanical characteristics from one system to the other, as observed from their means ± SDs and low coefficients of variation (7.45 ± 0.15 s²· mmHg · l⁻¹, 1.99%; 0.118 ± 0.004 10⁻⁴l/mmHg, 3.52%).