Temperature monitoring for high frequency welding of soft biological tissues: A prospective study

Abstract

BACKGROUND:

Monitoring of temperature changes and accurately determining the moment of electrode removal during open heart operations is not well recognized.

OBJECTIVE:

We investigated the temperature fields distribution in the biological tissues affected by electrosurgery upon use of an infrared thermograph.

METHODS:

The dynamics of temperature distribution in the tissue was registered by the thermal imaging camera FLIR i7. Measurement of the temperature between electrode couples was carried out for two operation modes: coagulation (100% power) and coagulation (50% power).

RESULTS:

The most important result of the applied method of temperature monitoring is a determination of the moment for electrodes removal that ensures the avoidance of carbonization of the cardiac tissues during their ablation.

CONCLUSIONS:

Temperature monitoring for connection of soft live biological tissues by welding allows the power to be fed in the amount sufficient for a formation of continuous ablation of the myocardium tissue.

Keywords

IR-imaging myocardium electrosurgery HF welding ablation control

1. Introduction

High frequency (HF) current is widely applied in many sectors of surgery [1, 2]. It is also used for the connection of soft live biological tissues by welding [3, 4, 5]. In particular, the use of equipment for HF welding of live tissues for diathermocoagulation of tissues and haemostasis during open heart surgery is considered as an advanced trend [6, 7]. Depending on the operation mode, the maximum initial power of a welding unit, e.g., ASU (Atricure Ablation and Sensing Unit; AtriCure, Mason, OH, USA), varies between 12 W and 30 W. The welding temperature is measured by the thermocouple element fixed to the immovable electrode of the bipolar terminal. The closest analogue in Ukraine is the unit for welding live tissues EKB3-300 produced by E.O. Paton Electric Welding Institute of the National Academy of Sciences of Ukraine.

It is crucial to monitor temperature changes and accurately determine the moment of electrode removal during open heart operations. This would ensure that the carbonation of the heart tissue during their thermal ablation is avoided. Thermographic imaging is a safe, non-invasive, easy to apply method, and technically well-engineered due to its wide range use in medicine (e.g., to detect inflammation by irregular cutaneous blood flow, for diagnosing rheumatoid arthritis patients, etc.) [8, 9, 10]. It uses radiation within the long-infrared range of the electromagnetic spectrum (9–14 $\mu$ m) that is emitted by all objects. This technique may potentially investigate the distribution of temperature fields in heart tissues affected by the electrosurgery upon use of an infrared thermograph.

2. Materials and methods

The study was performed at the E.O. Paton Electric Welding Institute of the National Academy of Sciences of Ukraine. The measurements were taken for two specimens of the myocardium of different thicknesses (2 mm and 4 mm). For implementation of the non-invasive method for temperature monitoring in tissues of the isolated heart, the ablation of the tissues by HF current was performed at the following parameters of the unit EKB3-300: the initial amplitude of voltage 45 V; modulation 20 kHz; frequency 440 kHz.

The non-invasive measurement of the temperature between couples of electrodes was carried out by the thermographic system based on thermograph FLIR i7 (FLIR, Wilsonville, OR, USA) for several chosen optimum modes of operation of the bipolar terminal in case of resection of remote cardiac tissue. The thermograph FLIR i7 registered thermograms with a resolution of 320 $\times$ 240 pixels and a thermal sensitivity of 0.1 ${}^{\circ}$ C. The camera was calibrated by the manufacturer for reproducibility and accuracy of readings. The camera was mounted on a standard camera tripod located 0.5 meter from the heart. The ambient temperature during measurements was 22 ${}^{\circ}$ C and the relative humidity 55%. The documented thermographic images were evaluated concerning the temperature profiles using the manufacturer’s professional infrared reporting software (FLIR Tools Plus software).

Measurement of the temperature between electrode couples (Fig. 1) was carried out for two operation modes: coagulation (100% power) and coagulation (50% power).

Figure 1.

Ablation of cardiac tissues by high frequency current.

The initial conditions of the investigation were as follows: an isolated heart of a pig cooled down to 10 ${}^{\circ}$ C, i.e. it conforms to the status of deep hypothermia upon the conditions of an artificial blood circulation (Fig. 2).

Figure 2.

An isolated heart of a pig cooled down to 10 ${}^{\circ}$ C. (a) a thermogram of the cooled heart, (b) an isolated heart.

3. Results and discussion

The dependence of the output power $P$ , $W$ of the electrocoagulator on the resistance of load $R$ , $\Omega$ for all the operation modes are shown in Fig. 3.

Figure 3.

The dependences of the output power on the resistance of the electrocoagulator’s load for all the operation modes: 1 – coagulation (100% power), 2 – coagulation (75% power), 3 – coagulation (50% power), 4 – cutting (100% power), 5 – cutting (75% power), 6 – cutting (50% power), 7 – welding (100% power), 8 – welding (75% power), 9 – welding (50% power).

Figure 4.

The thermograms of a 2 mm thick fragment of the myocardium that was affected by HF welding in the mode of a continuous voltage: (a) at the beginning of the HF welding process, (b) at the end (after 3 sec) of the HF welding process.

Figure 5.

The thermograms of a 4 mm thick fragment of the myocardium that was affected by HF welding in the mode of a continuous voltage: (a) at the beginning of the HF welding process, (b) at the end (after 5 sec) of the HF welding process.

Figure 6.

The thermograms of a 2 mm thick fragment of the myocardium that is cooling: (a) 1 sec after the HF welding, (b) 1 min after the HF welding.

Figure 7.

The thermograms of a 4 mm thick fragment of the myocardium that is cooling: (a) 1 sec after the HF welding, (b) 1 min after the HF welding.

The mode of coagulation – 1 (100% power) was applied for 4 mm thick fragment of the myocardium and the mode of coagulation – 3 (50% power) was applied for 4 mm thick fragment of the myocardium. The thermograms of the pig’s heart at the beginning of HF welding (Figs 4 and 5) as well as at the end of HF welding and during the process of cooling (Figs 6 and 7) for two specimens of the myocardium of different thicknesses (2 mm and 4 mm) show different dynamics of heating and cooling after the removal of the terminal, respectively.

The changes of the 2 mm thick fragment of the myocardium and the 4 mm thick fragment of the myocardium in the process of HF welding and sequential cooling of the cardiac tissues down to the initial are shown in Fig. 8.

Figure 8.

The changes of temperature of 2 mm and 4 mm thick fragments of the myocardium during the process of HF welding and the process of cooling down to the initial temperature.

The obtained dependences of the temperature of the myocardium on the time of HF welding process describes the process of cooling of 2 mm and 4 mm thick fragments of the myocardium; in addition, they show a possibility of applying the method of non-invasive temperature monitoring for choosing an optimum operation mode for the welding unit in case of coagulation of the cardiac tissue. In such a case, the most important result of application of the method of temperature monitoring is a determination of the moment for the removal of electrodes that ensures the avoidance of carbonization of the cardiac tissues during their ablation.

As the dependence of the process of heart cooling after the HF welding is exponential, the estimated time of a repeated ablation of a fragment of the myocardium equals:

$\displaystyle t=3,14\cdot\tau,$ (1)

where $\tau$ is the duration of HF welding before removal of the terminal.

The above-described process enables the avoidance of destruction of the tissue in the welding zone. For example, if during HF welding of the myocardium, the temperature of its tissues increased up to 60 ${}^{\circ}$ C for 3–4 seconds, the repeated coagulation (in order to avoid a destruction of the cardiac tissues) should be carried out after 9–12 seconds.

The typical IR-image of a heart at the end of HF welding of myocardium tissues is provided in Fig. 9.

Figure 9.

A fragment of a thermal image of a heart at the end of HF welding of myocardium tissues.

The minimum temperature of the myocardium (28.2 ${}^{\circ}$ C) was registered in the point Sp1 and a relatively high temperature (32.6 ${}^{\circ}$ C) was fixed in the point Sp2. Therefore, the difference of temperatures on fragments of an open surface of the myocardium may achieve 6–7 ${}^{\circ}$ C. Such a temperature difference in the zone of the myocardium affected by the HF attests an effective destruction of pathological conduction tracts in the heart.

4. Conclusions

The use of the unit EKB3-300 for HF welding of live tissues and the thermograph FLIR i7 combined with a modern surgical equipment that ensures a protection of the myocardium upon the conditions of an artificial blood circulation shows that application of the said complex may be effective for making ablation (destruction of pathological conduction tracts in the heart).

Using the contactless temperature monitoring method based on a developed thermographic system together with the unit EKB3-300 enables the monitoring of the maximum permissible temperature between electrode couples during HF welding of tissues of the myocardium. Due to the dynamic monitoring and an adjustment of the power dependent on the conductivity of the tissue, the energy is fed in the amount required for the formation of continuous ablation of the tissue of the myocardium. The limitation of this study is the small number of samples. Further research in this field is advised.

Footnotes

Conflict of interest

None to report.

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