Technique and Technology of Whole-Body Cryotherapy ...
Technique and Technology of Whole-Body Cryotherapy
Whole-body cryotherapy (WBC) has garnered attention as a potent therapeutic approach for various severe health conditions. This technique relies on the activation of cold receptors on the skin to induce a cooling of the skin surface down to approximately -2°C. Achieving this level of cooling necessitates a heat removal intensity of at least W/m², with outgoing gas temperatures not exceeding -130°C. While shorter sessions under 2 minutes fail to produce the desired therapeutic outcomes, prolonged exposures beyond 3 minutes pose risks to patient health. WBC is performed in both single-seat and multi-seat devices, where close patient positioning allows for up to 70% of effective thermal load on the cryogenic system in single-seat setups. In contrast, this utility drops to around 50% in multi-seat units. Each WBC session consumes approximately 3.77 kg of liquid nitrogen per individual, underscoring the need for thorough requirements for cooling system capacities and operational temperatures.
1. Introduction
For over four decades, clinics worldwide have utilized equipment designed for whole-body cryotherapy (WBC) [1, 2]. Despite its long history, a universally accepted understanding of the mechanisms behind the healthcare benefits of this modality still eludes the medical community. Critical safety parameters and effective physical conditions for cryogenic cooling of a patient's skin remain undefined [3, 4, 5, 6]. Key technological factors, including gas temperatures and the duration of contact with the skin, vary considerably. Manufacturers have progressively increased the minimum gas temperatures within WBC devices, exemplifying an alarming trend over the last 40 years; initial setups operated at 98 K, while current devices operate at 192 K [4, 5, 6, 7]. Higher operating temperatures significantly reduce manufacturing costs and have led to a 30-fold reduction in device prices. However, this has inadvertently compromised the essential heat power of the cryogenic systems. Many new installations now operate with a maximum heat-removing capacity of merely 400 W/m³ at 170 K, which is close to the physiological heat output of a comfortably situated patient (150 W) [7].
Escalating gas temperatures within these systems have raised concerns about the effectiveness of modern WBC technology compared to its predecessors. A growing number of publications question whether contemporary WBC systems can indeed deliver the healthcare benefits reported in older literature [7, 8]. This discrepancy correlates directly to the increased operational temperatures, contributing to diminished cold exposure for patients, which is critical for the desired therapeutic effect. Statistical analyses indicate an absolute temperature increase of 1.6 times from the earlier 170°C (102 K) to current mean values of 110°C (163 K). In light of this change, it becomes evident that the fundamental operational paradigm of WBC has been altered fundamentally, and not in a favorable direction.
The above circumstances underscore the necessity of exploring the causal relationships between WBC technology parameters and therapeutic effectiveness. Establishing a robust thermophysical theory of WBC will serve as an essential foundation for reviving the production of effective cryotherapeutic units that meet modern technical standards.
2. Historic Reference
The method of WBC is predicated on facilitating complete exposure of the patient's skin to cryogenic gas. With exposure times capped at 3 minutes and gas temperatures below 140 K, WBC displays numerous positive therapeutic effects [11, 12]. One of the most noteworthy outcomes of WBC is a significant analgesic effect that can persist for 6 to 8 hours [7]. Dr. Yamauchi of Japan was the first to recognize and apply this analgesic action, devising a specialized machine known as 'Cryotium' for WBC treatments [2]. The design of Cryotium resembled that of a refrigeration unit for storing perishables, featuring an isolation chamber aimed at minimizing cold air loss.
This sizable chamber allowed for simultaneous WBC treatments of multiple patients (ranging from 5 to 12). Liquid nitrogen replaced conventional refrigerants to achieve cryogenic temperatures in Cryotium's heat exchangers. The innovation underlying Cryotium arose from the belief that maximizing the temperature differential yields optimal therapeutic results. This principle fundamentally informed the chamber's design, with Japanese engineers employing insulation technologies characteristic of existing refrigeration units to reduce production costs. Care was taken to ensure that air condensation did not occur on the heat exchangers throughout the cooling process by maintaining temperatures above the critical condensation threshold.
The Cryotium's design encapsulated the principles of WBC technology, something that Dr. Yamauchi did not justify in subsequent literature. The reciprocal relationship between the boiling points of nitrogen and air combined with the structural features of the device fortuitously created conditions for safe, effective cryotherapy.
3. Thermophysical Theory of WBC
The thermophysical underpinnings of WBC were first articulated at St. Petersburg’s ITMO University to eliminate ambiguities surrounding technological specifications for specialized cryotherapy devices. By integrating real-world applications of WBC and its efficacy [1, 2, 7, 13], researchers can delineate optimization criteria that focus on the duration of the analgesic effects [16].
A formula has emerged to quantify this analgesic effect, associating the skin surface temperature (Ts) with the onset threshold for cryogenic damage (Tcr = 270.5 K) as well as factoring in the area (fs) and duration (τmax) of contact with the cryogenic gas:
The maximum duration of WBC (τmax) hinges on maintaining safe hypothermic limits, ensuring regulated surface temperature changes (Ts) and inner fat boundary temperatures (Tf) (Figure 3).
It is crucial to adhere to the established temperature thresholds for both Ts (≤ 271 K) and Tf (> 309 K) to avert frostbite and hypothermia complications during WBC treatments.
4. Mathematical Model of the Human Body Shell
The skin surface, or body shell (BS), is relatively thin, with an effective average thickness of 16 mm. Understanding heat transfer processes through BS tissues begins with a one-dimensional energy equation:
The above energy equation allows for dynamic simulation of heat exchange, providing insights into how temperature fluctuations impact clinical outcomes. This mathematical model also enables real-time calculations of the enthalpy values during cooling procedures.
5. Thermophysical Foundations of Achieving WBC Healthcare Effects
In comparative studies of WBC versus cold water immersion, the notable findings suggest that the higher the temperature gradient between the cooling source and the skin, the greater the heat removal potential. Numerous studies have suggested that while cold immersion procedures deliver transient benefits, WBC offers longer-lasting therapeutic effects due to the supercooling of the skin surface.
Simulation modeling demonstrated that effective cooling rates are significantly improved with the application of cryogenic gas compared to water immersion, marking a distinct advantage for cryotherapy protocols.
Overall, understanding the thermophysical properties of human body tissues is essential for assessing optimal WBC methodologies and addressing ongoing challenges in the field. Enhanced familiarity with the mechanisms at play will bolster cryotherapy's application in clinical settings.
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