Benefits of carbon dioxide and breath holding
二氧化碳和屏气潜水引起的心脏变化
为了评估在潜水过程中对身体浸泡和环境压力增加的单独心血管反应，12名健康男性受试者(平均年龄35.2±6.5岁)在5种不同条件下接受了二维多普勒超声心动图检查:离水(基础);呼吸时头向外浸泡(条件A);呼吸时完全沉浸于水面(条件B)和屏气(条件C);通过水下超声心动图测量心率、左心室容积、卒中体积和心输出量。通过脉冲波多普勒测量得到左室舒张功能的早期(E)和晚期(A)传导速度、比值(E/A)和减速时间(E
(DTE))。实验方案显著降低了左心室容积、左心室卒中体积(P < 0.05)、心输出量(P < 0.001)和心率(P < 0.05)。E峰显著升高(P <
0.01)， E/A显著升高(P < 0.01)， DTE显著降低(P <
0.01)。潜水过程中发生的变化(条件D)是实验系列中观察到的大部分变化。特别是在D条件显著降低心输出量与每个其他实验条件相比,E
/在D条件显著高于在条件A和c .最后,终端设备明显短在条件D比基底和条件c
.这项研究证实了一个减少心输出量在潜水人类。由于大多数变化是在潜水时观察到的，环境压力的增加似乎是造成这种血流动力学重排的原因。左室舒张功能改变提示心脏收缩作用，可能是心排血量减少的原因。
以前对天然潜水员(主要是海洋哺乳动物)的研究表明，屏气潜水与节能的心血管变化有关，即，心输出量的显著减少[由于心输出量和心率(HR)的减少]，以及从皮肤和富含肌红蛋白的肌肉流出的血液的重新分布有利于大脑和心脏(7,28,30)。然而，多年来，技术上的困难阻碍了对人类屏气潜水时心血管变化的全面评估。大多数关于人类潜水生理学的知识都是通过研究头朝外浸入式的研究对象(9,27)获得的，或者从屏住呼吸的研究对象(无论是否面部浸入式)的结果中推断出来的(2,8,10)。最近广泛传播的娱乐性和竞争性屏气潜水(深度逐渐增加)突出了严重潜水相关疾病的存在，如晕厥(上升障碍)、减压病、咯血和肺水肿，其病理生理学尚未完全了解(13,31)。因此，有必要对人类屏气潜水生理学进行更深入的研究。
利用水下超声心动图仪，我们最近发现，人类在10米深的短屏气潜泳中，其血流动力学模式与海洋哺乳动物相似(22)，其心率、量以及心输出量均显著降低。
屏气潜水包括氧气的逐渐减少和二氧化碳血液含量的增加、温度和导热系数的变化、神经反射的唤起(面部浸泡引起)、环境压力的变化(随深度线性增加)，而环境压力又反过来调节心脏的静脉回流(21)。这些因素在决定潜水时心血管反应的相对作用仍有待阐明。
本研究的目的是单独评估人类对身体浸泡、是否面部浸泡、屏气和潜水的心血管反应
讨论
本研究证实并扩展了先前的观察结果，即在人类屏气跳水时，明显可感知的跳水反应会导致心排血量的减少(22)。浸泡的顺序实际上导致了心输出量的显著减少(由于HR和卒中体积的减少)，以及左室舒张和收缩体积的减少。这种血流动力学模式与前负荷降低一致(因为后负荷的增加和/或心肌收缩力的降低都意味着左心室容量的增加)。
这项研究的设计目的在于区分潜水心血管反应的可能决定因素，因为屏气潜水使机体暴露于一系列刺激(身体沉浸、屏气、潜水反射激发、环境压力效应)，这些刺激相互重叠。在我们的研究中，最显著的心脏变化是在深度潜水时观察到的，而表面浸泡(无论头部进或出，呼吸或屏气)对心脏功能的影响微乎其微。这些数据可以用几个原因来解释。逐步应用沉浸性刺激(从头向外到深度潜水)可能会减弱每一步的心血管变化。此外,减少刺激面部受体在淹没(受试者戴着潜水镜),空气和水的温度之间的细微差别,和相对舒适的水温,热力中性,也可能有助于解释这个观察,因为diving-induced心动过缓和周边血管收缩标记在冷水中(1,23岁,26)。另一方面，全身浸泡在较冷的水中可以引起交感神经的激活，潜在地影响心血管对浸泡和潜水的反应(18)。因此，与深度潜水的效果相比，身体沉浸、屏气和潜水反射的激发在人类身上似乎起着相对次要的作用。因此，在屏气潜水过程中，静水压力的增加似乎是诱发心血管变化的关键因素。之前一项针对人类的研究(在压力舱中评估浸泡、浸没和深度模拟潜水)获得了不同的结果，与1
ATA时的干测量和表面屏气相比，潜水时的心输出量明显更高。心输出量测量方法上的差异，以及由于不熟悉的体验而产生的交感神经激活可能造成的影响，这些都可以解释这种差异(25)。
在深潜水时，经多普勒血流评估发现，E/A随着早期充盈峰减速时间的减少而增加。这种变化已经在屏气跳水运动员中观察到(22)，在临床环境中是典型的限制性/限制性左心室舒张功能障碍(24)。可能是假设减少胸部体积(由于增加了环境压力),结合胸廓内的血液增加内容(16),可能对心脏产生约束,能够诱导损伤的左心室充盈,反过来,一个相对减少预加载(透壁的充填压力)、心输出量(22)。推测这些变化可能与潜水引起的急性肺水肿的病理生理机制有关。在深度和长时间的潜水中，肺血管床充血(由于静脉回流增加和左心室充盈受损)，加上缺氧引起的不均匀的低氧性肺血管收缩(4,15)，可能导致肺毛细血管应力衰竭(33)和肺水肿(34)。大量的胸腔内血液再分配，加上潜水时胸腔的挤压，这些不利的后果得到了两个观察结果的支持。一方面，Lindholm等人(20)最近的一份报告显示，屏气潜水者在浅层潜水(6米)后，在剩余容积下可能会出现咯血和仪器上的低气道水肿体征。另一方面，高度适应潜水的动物(如鳍足类)有专门的解剖结构，用于减少浸泡时胸腔内静脉回流(腔括约肌、肝窦)(11,29)。
值得注意的是，在本方案的任何阶段都没有观察到右心室舒张压的变化，而浸泡引起的胸腔内血液移位(3,16)理论上应该与右心室容量超载有关(17)。我们的阴性发现可能是由于在不同的实验条件下，在心脏成像之前的一段时间内，循环调节迅速发生。或者，由于其复杂的三维解剖结构，它可能反映了超声心动图在准确检测右心室尺寸的微小变化方面固有的低效率(32)。
总之，我们的研究证明，人类在浅层(5米)进行憋气潜水时，心血管反应在性质上与海洋哺乳动物相似。表面的身体浸泡、潜水反射和屏气似乎对在深度观察到的心脏变化影响不大，在深度观察时，胸部的静水压力变得足够高，从而使心脏收缩，阻碍心脏舒张充盈，减少心收缩的工作量。
https://www.physiology.org/doi/full/10.1152/japplphysiol.00126.2008
The Diving Reflex - Breatheology
https://www.breatheology.com/mammalian-dive-response/
The Diving Reflex
Your inner dolphin Humans and other mammals have a Diving Response (also known as The Mammalian Dive Response/Reflex) consisting of a set of reflexes that are activated when our face is cooled (such as by the water during a dive) or if we hold our breath. The diving reflex is a clever physiological mechanism enabling the body to manage and tolerate a lower level of oxygen.
Three main changes occur in the body: Bradycardia, a slowing of the heart rate. The human heart rate slows down 10 – 30% and up to 50% or more in trained individuals. Peripheral vasoconstriction (a narrowing of blood vessels to reduce blood flow by muscle contraction in the blood vessel’s wall), causes reduced blood flow to the limbs ensuring that oxygen-sensitive organs like the brain and heart receive oxygen. During deep dives, a blood shift occurs allowing blood plasma and water to pass through organs and circulatory walls to the chest cavity to protect the organs from the increase in pressure.
The lungs gradually fill up with blood plasma, which is reabsorbed when pressure drops. A slowing of the heart rate occurs relatively quickly upon facial contact with cold water. The trigeminal facial nerves (5th cranial nerve) transmit the information to the brain which innervates the vagus nerve (10th cranial nerve) causing bradycardia and peripheral vasoconstriction. The colder the water, the faster the reaction. Temperatures above 21°C (70°F) do not elicit a response. Reduced blood flow to the limbs occurs more gradually. The reflex is preventive because it is initiated before the level of oxygen becomes critically low. In addition, the large amount of blood that accumulates in the blood vessels of the lungs acts as a protective measure, because fluids – as opposed to tissue and bones – cannot be compressed. The blood thus prevents the lungs from collapsing under the high pressure of the deep. When vasoconstriction shunts blood away from arms and legs, the amount of blood we have available is concentrated in a “small” circulatory system between the lungs, heart, and brain. These are the most oxygen sensitive organs of the body and the blood shunting is thus a perfect survival mechanism to a low oxygen level. In addition, the decrease in heart rate also helps lower oxygen consumption, since the heart muscle is working at a lower intensity. Other responses in humans Another action of the diving response can be observed in infants when they are under water. The windpipe by the vocal chords spontaneously closes to prevent water from entering the lungs. This reflex is initiated as soon as there is contact with water. However, it disappears when the child reaches the age of roughly six months. Recent investigations have shown that the spleen, which contains red blood cells, also plays a significant role during dives and breath holds. Following a number of dives, the spleen contracts and releases a lot of red blood cells to the circulatory system. Spleen contraction occurs much slower than the other diving reflexes. The release of more red blood cells allows more oxygen to be stored in the blood. Finally, the additional amount of blood cells allows the body to regain its normal balance faster after a prolonged breath hold. Popularly speaking, the spleen acts as a kind of “turbo” – during and after a long dive. Diving mammals with strong responses
Diving mammals with strong responses Diving mammals such as whales and seals naturally have a well-developed diving reflex to allow them to forage below the surface for extended periods of time. There are several reasons why the sperm whale, seals and elephant seals are excellent breath holders and can dive for more than an hour. Firstly, these animals have quite a lot of blood and a high concentration of blood cells which bind oxygen in the so-called hemoglobin protein. In addition, they have a higher concentration of an oxygen-binding molecule called myoglobin in their muscles. Have you ever seen a whale or seal meat, and wondered why it is so dark? Myoglobin is the answer. A high content of iron in myoglobin colors the meat brown. The diving mammals are also able to cool their brain down, which helps them during prolonged dives. Several studies show that seals can lower their body and brain temperature with up to 3 degrees Celsius, and thereby lower their metabolism and oxygen consumption dramatically. As opposed to humans, seals may completely shut off blood supply to the limbs and thus direct oxygenated blood to the lungs, heart and brain. How can I use the Diving Response? If you are going to perform e.g. at a competition or at a business event, activating the diving reflex can help you relax. The diving reflex is activated by breath holds and by facial contact with cold water. If you cover your face, especially the forehead and the area around the nose (area of the trigeminal nerve) with a cold wet towel, the diving reflex will be activated. Because the diving reflex innervates the vagus nerve your pulse will drop and your body will relax. In this way, you can quickly calm your nerves before a performance. Demonstration of the Mammalian Dive Response In the Discovery Series Ultimate Superhuman, Stig demonstrates the mammalian diving response.
The Diving Reflex - Breatheology https://www.breatheology.com/mammalian-dive-response/