Intravascular Temperature Management (IVTM) Techniques in Therapeutic Hypothermia

Harsha V. Ganga, MD, MPH and
Justin B. Lundbye, MD, FACC†
Hospitalist, Henry Low Heart Center, Hartford Hospital, Hartford, CT;
†Chief of Cardiology, Hospital of
Central Connecticut, New Britain, CT

Harsha V. Ganga, MD, MPH and
Justin B. Lundbye, MD, FACC†
Hospitalist, Henry Low Heart Center, Hartford Hospital, Hartford, CT;
†Chief of Cardiology, Hospital of
Central Connecticut, New Britain, CT

Background

Over the last decade, temperature management is being used widely and frequently as a tool to prevent neurological damage after cardiac arrest. Cardiac arrest survivors can have devastating neurologic impairments after recovery. There are nearly 383,000 out-of-hospital cardiac arrests in the United States each year (Heart Disease and Stroke Statistics—2012 Update).1 Therapeutic mild hypothermia has clearly been shown to improve neurologic outcomes after cardiopulmonary resuscitation.2 American Heart Association (AHA) guidelines in 2010 recommend therapeutic hypothermia (TH) as a Class I recommendation for witnessed out-of-hospital ventricular fibrillation arrest and as a Class IIb recommendation for non-ventricular fibrillation cardiac arrest.3

Precise temperature regulation is vital in achieving adequate neurological benefits in post-cardiac arrest survivors. Studies in animal models have shown that mild hypothermia is beneficial, whereas deep hypothermia has no added benefit when compared to mild hypothermia.4 Furthermore, a delay in cooling decreases the benefits achieved with mild hypothermia.5 Optimal TH treatment comprises of rapid cooling, precise temperature management during maintenance phase, and controlled rewarming. However, temperature control is not easy to achieve using conventional methods. Inadequate temperature control is common and is associated with poor outcomes. Hyperthermia is a very common problem in neurocritical care patients, and is a predictor of poor outcome and increased length of stay.6 Similarly, overshooting the target hypothermia is a concern.

Intravascular temperature management systems have been proven to be the best available option and are being employed successfully to achieve target core body temperature within the recommended narrow range in TH. Intravascular cooling methods are superior to extracorporeal cooling methods.7,8 Traditional surface cooling methods are slow and inaccurate in temperature management. In comparison, endovascular cooling methods have the advantage of enabling better temperature regulation when compared to extracorporeal or surface cooling methods. In one study, intravascular cooling achieved target temperature in 100% of patients.9

In the following paragraphs we will discuss the application and advantages of intravascular temperature management (IVTM) over conventional methods of cooling in TH management.

Discussion

Conventional Methods of Cooling

Conventional methods of cooling include cooling blankets, ice bags, and gel pads. These techniques are easy to implement and are available for immediate use. While employment of these traditional methods may be easier and quick, the primary concern of inaccurate temperature control remains. Studies have shown that surface cooling methods could not achieve the targeted temperature in a significant number of patients.9 Additional concerns with conventional cooling methods are the inability to sustain the core body temperature at the recommended level, overshooting to lower body temperatures, and rapid uncontrolled rewarming.10-12

Intravascular Temperature Management System: Procedure

The principle of intravascular temperature management during therapeutic hypothermia involves use of a catheter with a closed circuit for circulating cold saline through the catheter to cool the blood.

The IVTM system has distinct advantages as it manages the temperature from inside out. A cooling catheter is inserted into the central venous system of the patient. The common approach is with the femoral central vein, but the internal jugular or subclavian vein approaches can also be used. Some cooling catheters can have an integrated temperature sensor that helps in precise temperature control.13 In other cases, the temperature regulation system can control the temperature of the saline that is circulating through the catheter balloon by remote sensing of the patient’s temperature.14 There is no exchange or infusion of saline into the patient’s body. The patient’s body is cooled or warmed as the venous blood passes over the catheter balloons, thereby exchanging heat without infusion of saline. The catheters vary in size and length, and can be used as per patient’s needs. The insertion of an intravascular catheter requires a physician or a midlevel practitioner; therefore, IVTM cannot be used to cool the patient outside the hospital setting. Compared to external cooling techniques such as cooling blankets, ice bags, or gel pads, the IVTM system directly cools the patient’s blood and achieves better control of core body temperature, as well as gives unrestricted access to the patient for routine care and management. The placement of an intravascular catheter carries the same overall risks as central venous catheter placement in regards to bleeding, infection and local trauma.

Types of Intravascular Cooling Catheters Used in Therapeutic Hypothermia

  • The Thermogard Family of Endovascular Cooling Catheters and the Thermogard XP® Temperature Management System (Zoll Medical Corporation, Chelmsford, MA): The Thermogard catheters have a control console and a range of 9 French catheters to accommodate patient sizes (Figure 1) for central venous insertion. The catheters are coated with heparin to prevent thrombosis. The catheters have a closed-loop balloon for heat exchange. Additionally, three lumens are present for infusion or phlebotomy purposes. All the catheters are radiopaque and safe for MRI.15 (Figure 2)
  • The InnerCool RTx Cooling Catheter and Endovascular System (Philips Healthcare, Best, The Netherlands): The InnerCool RTx uses either a 10.7 or 14 French catheter inserted into the central vein for cooling. The catheter is coated with heparin to prevent thrombosis. The catheter is inserted using a modified Seldinger technique. The cold saline is circulated through the catheter and exchanges heat with the blood stream. Catheters are available with and without a temperature probe. In catheters that do not have an integrated temperature probe, the console accepts the temperature through a separate temperature probe. The InnerCool RTx catheter is radiopaque, and the placement can be confirmed radiographically. In patients who need head-only MRI, the catheter can safely remain in place.15 (Figures 3 and 4)

When to Place the Cooling Catheter in TH Management

Before considering placement of a cooling catheter, comatose patients are assessed in the emergency department (for out-of-hospital cardiac arrests) or coronary intensive care unit (for in-hospital arrests) for indications and contraindications for TH. If the cardiac arrest survivors have evidence of myocardial ischemia, they are immediately taken to the cardiac catheterization laboratory for an angiogram. An intravascular temperature control catheter can be placed during or after the procedure in the cath lab. If the cardiac arrest survivors do not need emergent cardiac catheterization, then they are taken to the coronary intensive care unit for placement of the intravascular temperature control catheter. Temperature is monitored with the placement of a probe in the bladder, rectum, or esophagus. Since the insertion of the intravascular catheter requires a physician or midlevel practitioner, the IVTM system cannot be used to cool the patient outside of the hospital setting. The goal in all cases is rapid induction of therapeutic hypothermia for optimal neurologic recovery.15

Cooling Using IVTM Methods in Therapeutic Hypothermia

The TH protocol is broadly divided into three phases: 1) the induction phase, which involves initial cooling of the body’s core temperature; 2) the maintenance phase, which involves maintenance of core temperature for 12 to 24 hours; and 3) the rewarming phase, which involves gradual rewarming to core body temperature. An integrated team approach helps in successful implementation of the TH protocol in hospitals. Successful implementation of the therapeutic hypothermia protocol involves rapid, effective temperature control without interfering with other lifesaving therapies.

I. Role of IVTM Methods During the Induction Phase of Therapeutic Hypothermia:

Initial cooling of core temperature (induction phase) requires a large transfer of heat energy from the body and should be implemented as expeditiously as possible. Animal studies have shown that a delay as little as 15 minutes after return of circulation before onset of cooling can attenuate the benefit of hypothermia after an induced VF arrest.5 In an observational study by Wolff and colleagues, the chances of a favorable neurologic outcome decreases by one-third with every hour delay in achieving therapeutic hypothermia.16

Mild hypothermia or a core body temperature of 32–34 ºC was associated with improved neurologic outcomes among survivors of cardiac arrest due to ventricular arrhythmias.17 This target temperature should therefore be used in TH protocol for clinical benefit. This target temperature of 32–34 ºC can be achieved by rapid, large-volume infusion (30 ml/kg) of 4 ºC cold saline.15 However, use of cold saline has its limitations. It may achieve rapid induction, but one cannot continue to infuse rapid, large-volume cold saline without the risk of pulmonary edema. Surface cooling methods such as ice packs or gel pads may be easier to start but interfere with access to the patient for routine care and physical examination. Surface cooling methods or cold saline can also overshoot the target temperature as discussed above. Furthermore, there can be interference to therapeutic and diagnostic procedures due to external cooling methods.

Shivering is our body’s natural defense mechanism to resist cooling. During induction phase, shivering is commonly seen. Shivering increases metabolic activity, increases oxygen demand, and produces more body heat. One of the challenges of the cooling technique is to get below the shivering threshold quickly to avoid resistance to cooling therapy. IVTM techniques extract heat as fast as possible, thereby saving time and decreasing the need for medications to eliminate shivering. The rapid cooling ability of the IVTM techniques enables cooling of awake, non-paralyzed patients.13,18

IVTM methods not only ensure uninterrupted care and access to patients, but also help in standard intensive care treatment like fluid resuscitation in case of hypotension. IVTM methods help achieve precise target temperature by enabling nursing staff to dynamically modulate temperature control through the use of consoles with intuitive user interface. Furthermore, IVTM methods are effective in cooling patients with large body mass index (>30 kg/m2). The consoles are compact and easy to move in the hospital.

II. Role of IVTM During Maintenance Phase of Therapeutic Hypothermia:

Once the target hypothermia temperature (32–34 ºC) is achieved, this temperature should be maintained for 12 to 24 hours. This requires ongoing cooling with precise measurement of core body temperature. Therapeutic hypothermia is maintained by the IVTM system during this maintenance phase. However, infusion of cold saline cannot be continued to maintain this core temperature. IVTM successfully achieves the objective of precise temperature control without the risk of hypervolemia and pulmonary edema; using the console’s interface, temperature variations and adjustments can be done as the information on core body temperature variation is easily noted.

III. Role of IVTM During Rewarming Phase of Therapeutic Hypothermia:

Once the maintenance phase of temperature control (12–24 hours) is completed, gradual rewarming to normal core temperature (36.5–37.5 ºC) can begin.15 Precise temperature changes of 0.3–0.5 ºC are recommended to allow for gradual, steady rewarming.15 When surface cooling methods are used, either the cooling blanket or ice packs are removed or warm air or blankets can be applied. Note that if the temperature of the heating blankets crosses 40 ºC, skin burns can occur. Overshoot rewarming can cause an increase in intracranial pressures, changes in peripheral vascular resistance leading to hemodynamic instability, and worsened neurologic outcomes.15

It is vital to maintain normothermia for 24–48 hours after core body temperature has been achieved. Hyperthermia during this time can worsen neurologic outcomes.20 IVTM methods have been shown to cause the least variation in temperature control and the greatest time in target temperature range when compared to traditional surface cooling methods.21

When to Remove the Cooling Catheter in TH Management

The intravascular cooling catheter is removed as soon as possible once the patient is normothermic. At Hartford Hospital, we have not noticed any increase in infection complications with the use of a cooling catheter. This could be due to the use of prophylactic antibiotics at our institution.

Technical Advantages of IVTM Techniques

The technical advantages of an IVTM system include precise temperature control, reduced nursing time, and immediate availability of triple lumen access. Cooling catheters support the standard central venous catheter functions; therefore, there is no need to place an additional central venous line, saving critical time in intensive care units.

Intravascular Temperature Management Versus Conventional Cooling Methods

Some of the important critical problems seen by intensive care staff during employment of conventional cooling methods are poor maintenance of body temperature, uncontrolled rewarming, and frequent overshoot to lower temperatures.10-12 Rapidly rewarmed patients can develop cerebral edema or increased intracranial pressure.

In a retrospective study, the total time taken to insert the cooling catheter and reach target temperature was shorter than the time taken to use conventional cooling techniques.22 In another retrospective study, patients treated with intravascular cooling had a shorter length of stay and more favorable short-term neurological outcomes when compared to patients treated with extracorporeal cooling methods.9 Furthermore, Hoedemaekers et al compared five different cooling methods (conventional cooling methods, water circulating external cooling methods, air circulating external cooling methods, water circulating external cooling device with gel-coated pads, and intravascular temperature management with cooling catheters) and found that intravascular temperature management was superior to all other cooling methods for maintaining stable body temperature.21

In a recent study that evaluated the role of intravascular temperature management in the control of brain temperature in traumatic brain injury (TBI) patients,6 it was shown that brain temperature could be successfully modulated by strictly preventing fever and maintaining core body temperature through the use of an intravascular cooling device.

Summary

IVTM is an effective, reliable and rapid method of cooling in critical care patients and helps to decrease nursing time and length of stay in the hospital. It is comparatively a safer method as it decreases complications such as neurologic injury by maintaining a stable target temperature when needed. There is ongoing research on the potential application of therapeutic hypothermia in myocardial infarction, TBI, decompression surgery, and cerebral infarction patients. Given the potential for such widespread application of therapeutic hypothermia, knowledge and expertise in IVTM techniques are beneficial for healthcare systems.

Acknowledgements. The authors thank Zoll Medical Corporation and Philips Healthcare for granting permission to use the images of catheters and equipment in the article.

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