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Design Innovation Papers

Improved Ease of Use Designs for Rapid Heart Cooling

[+] Author and Article Information
Thomas L. Merrill

Mechanical Engineering Department,  Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028; FocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062merrill@rowan.eduFocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062merrill@rowan.edu

Denise R. Merrill

Mechanical Engineering Department,  Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028; FocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062deemerrill@focalcool.comFocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062deemerrill@focalcool.com

Jennifer E. Akers

Mechanical Engineering Department,  Rowan University, 201 Mullica Hill Road, Glassboro, NJ 08028; FocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062jakers@focalcool.comFocalCool, LLC, 107 Gilbreth Parkway, Suite 103, Mullica Hill, NJ 08062jakers@focalcool.com

J. Med. Devices 6(3), 035001 (Jul 30, 2012) (10 pages) doi:10.1115/1.4006853 History: Received November 25, 2011; Revised April 04, 2012; Published July 30, 2012; Online July 30, 2012

Mild hypothermia has been shown to reduce heart tissue damage resulting from acute myocardial infarction (AMI). In previous work we developed a trilumen cooling catheter to deliver cooled blood rapidly to the heart during emergency angioplasty. This paper describes two alternative designs that seek to maintain tissue cooling capability and improve “ease of use.” The first design was an autoperfusion design that uses the natural pressure difference between the aorta and the coronary arteries to move blood through the trilumen catheter. The second design used an external cooling system, where blood was cooled externally before being pumped to the heart through a commercially available guide catheter. Heat transfer and pressure drop analyses were performed on each design. Both designs were fabricated and tested in both in vitro and in vivo settings. The autoperfusion design did not meet a cooling capacity target of 20 W. Animal tests, using swine with healthy hearts, showed that the available pressure difference to move blood through the trilumen catheter was approximately 5–10 mmHg. This differential pressure was too low to motivate sufficient blood flow rates and achieve the required cooling capacity. The external cooling system, however, had sufficient cooling capacity and reasonable scalability. Cooling capacity values varied from 14 to 56 W over a flow range of 30–90 ml/min. 20 W and 30 W were achieved at 38 ml/min and 50 ml/min, respectively. Animal testing showed that a cooling capacity of 30 W delivered to the left anterior descending (LAD) and left circumflex arteries (LCX) of a healthy 70 kg swine can reduce heart tissue temperatures rapidly, approximately 3 °C in 5 min in some locations. Core temperatures dropped by less than 0.5 °C during this cooling period. An autoperfusion design was unable to meet the target cooling capacity of 20 W. An external cooling design met the target cooling capacity, providing rapid (1 °C/min) localized heart tissue cooling in a large swine model. Future animal testing work, involving a heart attack model, will investigate if this external cooling design can save heart tissue.

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Copyright © 2012 by American Society of Mechanical Engineers
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Figures

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Figure 3

CoolGuide™ heat transfer processes. Three processes (Q1, Q4, and Q5) exchange heat from the aorta blood to either coolant or internal blood flow and two processes (Q2 and Q3) exchange heat from the internal blood flow to coolant flow. The net cooling effect results from Q2 and Q3 exceeding Q5. This is the heat transfer process in the section of the catheter with internal blood flow. Upstream of this section, where blood is stagnant, Q2 and Q3 are altered to reflect the no-flow physics.

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Figure 4

Autoperfusion catheter cooling capacity as a function of blood cooling length (distance from autoperfusion holes to distal tip) and differential pressure. The optimal cooling capacity location varies between 20 and 45 cm depending upon the differential pressure.

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Figure 5

The blood pathway showing heat exchanger segments (1–6) and temperatures. Note the direction of increasing temperature. Segment 1 involves cooling in the annular flow of the insertion sheath. Segment 5 involves heating inside the catheter as it lies inside the insertion sheath. Blood enters at approximately 37 °C and is cooled to a minimum temperature of approximately 16 °C. The final temperature difference, ΔT, is approximately 9 °C.

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Figure 6

Predicted external cooling design performance with four different sized heat exchangers. Lengths 2.5, 2.0, 1.5, and 1.0 m indicate the length of tubing inside the external heat exchanger shell. Model assumes an 11 °C coolant inlet temperature.

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Figure 7

Schematic of the mock cardiovascular system showing autoperfusion catheter distal tip detail. The autoperfusion catheter was placed in this configuration to ensure that only the fluid traveling through the catheter exited. Pressure differences were measured between the autoperfusion holes in the aorta and the catheter distal tip.

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Figure 8

Autoperfusion blood analog flow rate as a function of differential pressure. Autoperfusion holes are located at 30 cm from the catheter distal tip. Error bars denote standard deviation from three data sets. This linear relationship reveals that entrance effects from the autoperfusion holes and developing boundary layers do not dominate the overall pressure drop behavior. Instead, wall friction along the blood cooling pathway (Lbc ) is the dominant cause of pressure drop.

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Figure 9

Autoperfusion cooling capacity as a function of analog flow rate and coolant flow rate with autoperfusion holes centered at 30 cm. All cooling capacity values fall below the target value of 20 W. The error bars denote standard deviation from three data sets.

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Figure 10

External cooling design cooling capacity and exit temperature performance as a function of analog flow rate using a 7 Fr guide catheter. The EHX tubing length was 2.0 m; the EHX inlet coolant temperature was 11 °C. Error bars denote standard deviation from three data sets.

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Figure 11

Typical tissue cooling versus time using the external cooling design. Catheter is located in the LCX (a) and LAD (b). The heart sketch shows approximate thermocouple locations. The gray area denotes the anticipated area at risk following an occlusion in the respective coronary.

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Figure 1

Schematic of the autoperfusion design. The wing-shaped lumens (1, 2) are for coolant flow. The circular lumen (3) is for blood flow and intervention. Blood enters the shaft autoperfusion holes and exits the distal tip. The distance between these holes and the distal tip is called the blood cooling length, Lbc . A typical guide catheter only has a single central lumen.

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Figure 2

Schematic of the external cooling design concept showing the blood circulation path. Blood is pulled from the body using an insertion sheath and pump. Blood is returned to the body using a commercially available guide catheter. A data acquisition system monitors and controls operation.

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