Theory and in vitro validation of a new extracorporeal arteriovenous loop approach for hemodynamic assessment in pediatric and neonatal intensive care unit patients

Abstract

Objectives: No simple method exists for repeatedly measuring cardiac output in intensive care pediatric and neonatal patients. The purpose of this study is to present the theory and examine the in vitro accuracy of a new ultrasound dilution cardiac output measurement technology in which an extracorporeal arteriovenous tubing loop is inserted between existing arterial and venous catheters.

Design: Laboratory experiments.

Setting: Research laboratory.

Subjects: None.

Interventions: None.

Measurements and main results: In vitro validations of cardiac output, central blood volume, total end-diastolic volume, and active circulation volume were performed in a model mimicking pediatric (children 2-10 kg) and neonatal (0.5-3 kg) flows and volumes against flows and volumes measured volumetrically. Reusable sensors were clamped onto the arterial and venous limbs of the arteriovenous loop. A peristaltic pump was used to circulate liquid at 6-12 mL/min from the artery to the vein through the arteriovenous loop. Body temperature injections of isotonic saline (0.3-10 mL) were performed. In the pediatric setting, the absolute difference between cardiac output measured by dilution and cardiac output measured volumetrically was 3.97% +/- 2.97% (range 212-1200 mL/min); for central blood volume the difference was 4.59% +/- 3.14% (range 59-315 mL); for total end-diastolic volume the difference was 4.10% +/- 3.08% (range 24-211 mL); and for active circulation volume the difference was 3.30% +/- 3.07% (range 247-645 mL). In the neonatal setting the difference for cardiac output was 4.40% +/- 4.09% (range 106-370 mL/min); for central blood volume the difference was 4.90% +/- 3.69% (range 50-62 mL); and for active circulation volume the difference was 5.39% +/- 4.42% (range 104-247 mL).

Conclusions: In vitro validation confirmed the ability of the ultrasound dilution technology to accurately measure small flows and volumes required for hemodynamic assessments in small pediatric and neonatal patients. Clinical studies are in progress to assess the reliability of this technology under different clinical situations.

Figures

Figure 1

Schematic of the arteriovenous (AV) loop setup to measure hemodynamic variables by ultrasound dilution technology.

Figure 2

Raw data curves from a 5.6-kg patient. Upper curve represents the increase in the flow recorded by the venous sensor due to injection of 5 mL of isotonic saline. Lower curve represents the decrease of blood ultrasound velocity recorded by the arterial sensor during the saline bolus pass.

Figure 3

HCM 101 screen shot from a 5.6-kg patient. Scale is the percent concentration of saline in blood: mL (saline)/mL (blood). The concentration of injected isotonic saline (5 mL) becomes largely stable within 40-60 secs from the time of injection. MTTa, time during which the indicator travels from the injection site (venous sensor) to the arterial sensor; CBV, central blood volume; ACV, active circulation volume; Clw, cardiac index; cardiac output hormalized on body weight.

Figure 4

Schematic of the in vitro model. 1, transit time clamp-on flow sensor connected to HT109 measuring the total flow; 2, mixing chamber containing one to five bubble traps to simulate the total end-diastolic volume; 3 and 4, arterial and venous flow/dilution clamp-on sensors, respectively; a, venous catheter location for injection; b, arterial catheter location for withdraw. Catheters and the mixing device used in the saline jar are not shown in the figure. AV, active circulation volume.

Figure 5

Bland-Altman analysis comparing volumetric flow rates with in vitro ultrasound dilution cardiac output (UDCO) measurements. Bias for the pediatric setting was 6 mL/min and for the neonatal setting -2 mL/min (not shown on the graph). Every measurement was taken into account without averaging.

Figure 6

Results of the bench flow measurements from different injection volumes. Injection volume varied from 2 mL to 10 mL (five times) while observed changes measured by dilution were within 5%. The venous sensor measures the injection volume, and the HCM 101 automatically accounts for this, making the system operator independent. UD, ultrasound dilution.