兽研简要总结
犬在全身麻醉期间血压可能下降。用于升压的药物也可能触发减慢心率的反射，从而部分限制血压上升。随后给予阿托品可减轻这种减慢效应，并可能导致心率突然升高。如果血管已经收缩，心率的升高反过来可能促使血压突然升高。本文探讨在增加升压药物之前先减弱迷走神经对心脏的减慢效应，是否能使后续血压调整以更小的幅度逐步进行。这一设想称为心脏迷走解耦，其前提是根据观察到的血压反应来调整这些药物。已发表的研究支持该设想的各个组成部分，但其是否改善血压调整尚未经过验证。因此，本文提出的是一个待研究的假说，而非治疗方案。对器官血流和患者结局的影响需要另行评估。
关键词：阿托品、压力感受器反射、心脏迷走解耦、犬、全身麻醉、术中低血压、反射性心动过缓、升压药滴定
1. 引言
犬术中低血压通常根据推测的病因，通过调整麻醉药物给药、液体治疗和血管活性支持来管理。在许多情况下，平均动脉压（MAP）是主要的床旁目标。本文所讨论的问题出现在升压药相关的MAP升高伴随心率（HR；更准确地说，在存在窦性心律时为窦性心率）减慢时。相似的MAP升高可发生于窦性心率减慢或窦性心率维持的情况下，而由此产生的心输出量取决于伴随的每搏输出量反应。
心脏迷走解耦是指窦房结处迷走神经介导的毒蕈碱能约束减弱。该术语指的是心脏效应器作用，而非压力感受、中枢反射整合或压力感受器反射交感支的中断。我们先前关于人类麻醉的论文引入了这一概念，并假设在升压药大幅升级之前减弱迷走约束，与较晚给予抗毒蕈碱药相比，可能使升压药反应更具比例性并减少高血压性超射[1]。本文借鉴犬的生理学和临床证据，进一步发展了这一假说：预先减弱窦房结毒蕈碱能约束可能有助于在MAP指导的升压药滴定过程中实现分级血压控制。
本文聚焦于升压药反应中这一序列依赖性成分，而非整体心血管功能。
2. 升压药滴定期间的两种血流动力学模式
图1对比了两种假想的反应模式。在非解耦模式中，升压药滴定在功能性窦房结毒蕈碱能约束仍具影响时开始。在解耦模式中，升压药滴定在该约束已被减弱后开始。在图1A中，随着MAP升高，反射相关的迷走影响可减慢窦性心率，因此预期的MAP升高可能部分被反射性窦性心率减慢所抵消。
[图：犬在无心脏迷走解耦与有心脏迷走解耦情况下的假想血流动力学轨迹（示意图；非实验性）]
图1A的后半部分展示了晚期抗毒蕈碱效应可能导致MAP突然升高的一种可能机制。阿托品或其他抗毒蕈碱药可能导致窦性心率突然升高。如果血管张力已经很高，心率的增加反过来可能促使MAP突然或过度升高。压力变化的幅度还取决于静脉回流、心室充盈、每搏输出量和血管阻力，而非与心率变化成比例。增量给药不一定产生成比例的窦性心率增量，如第5节所述。
图1B展示了解耦模式。窦房结处的功能性毒蕈碱能约束在升压药大幅升级之前被减弱。随后的升压药滴定预期较少受到反射性窦性心率减慢的约束。该假说是，根据观察到的MAP反应指导的小幅升压药剂量调整可能实现更分级的血压控制。
在操作层面，相关的生理变化是在相当的升压药所致压力升高下窦性心率减慢减少，并与MAP控制的成功与否分开评估。给予阿托品后达到的HR及其相对基线的绝对或百分比增加本身均不能量化心脏迷走解耦的程度。它既不需要目标HR，也不意味着完全迷走神经阻滞。
3. 犬为何是相关的研究场景
犬是该概念在临床上重要的研究场景，因为现有证据表明窦房结处存在显著的迷走神经介导的毒蕈碱能影响[2,3]。
在清醒犬中，Little 等人发现累积给予阿托品可缩短平均 R–R 间期，并使由这些间期得出的心脏迷走神经活动实时指数降至接近零。O’Leary 等人报道，在清醒犬跑台运动期间，与生理盐水处理相比，阿托品显著增加 HR，即使在高负荷下对 MAP 也几乎没有影响。这些研究并未再现麻醉临床环境，但它们证实犬窦房结可在显著且动态可调的迷走神经约束下运作。
Schuessler 等人在氯醛糖-吗啡麻醉的开胸犬中，用去氧肾上腺素和硝普钠改变动脉压，并记录 HR 变化及启动心房激动的部位。使用阿托品和普萘洛尔的实验支持这些压力反射性 HR 反应由自主神经介导。Tanaka 和 Nishikawa 在戊巴比妥麻醉犬中，通过测量去氧肾上腺素诱导升压和硝普钠诱导降压期间 MAP 与 R–R 间期的关系来评估压力反射敏感性。急性等容血液稀释后压力反射敏感性无显著变化。Frankel 等人根据清醒犬脉搏间期逐搏变化对收缩压变化的回归计算自发压力反射敏感性。该指标可被阿托品、六甲铵神经节阻滞和双侧颈动脉窦去神经术消除。
Fahim 和 Arndt 比较了麻醉、机械通气、开胸犬和猫的压力反射性 HR 反应，发现犬的敏感性更高。变时反应涉及交感和副交感通路，因此这些发现并不能确立物种差异具有特异性迷走神经基础。
Seagard 等人检查了不同异氟烷浓度下的压力反射性 HR 反应。当防止麻醉诱导的血压变化时，压力反射性 HR 反应的显著抑制仅发生在较高测试浓度下。Bagshaw 和 Cox 在改变颈动脉窦压力和异氟烷浓度时测量了中枢和区域血流动力学。异氟烷降低了反射设定点处颈动脉窦刺激引起的平均主动脉压反应。Szilagyi 使用去氧肾上腺素和硝普钠评估麻醉犬的压力反射性 HR 反应，发现纳洛酮降低压力反射敏感性。总之，这些研究支持心脏频率的毒蕈碱调节、对压力变化的心脏反射反应，以及自主神经和麻醉条件对这些反应的修饰。它们确立了犬的生理学基础，而非在反复调整血管升压药剂量期间改善 MAP 控制。
4. 支持该概念的实验与临床证据
Seki 等人的两项研究检查了预先给予或不给予阿托品时对麻黄碱的心血管反应。在一项六只健康比格犬、异氟烷诱导低血压的单盲交叉实验中，每只犬在阿托品或生理盐水预处理后接受固定静脉麻黄碱推注。每搏量和心输出量通过多普勒超声心动图估算，并以指数化值报告。与生理盐水预处理相比，预先给予阿托品可防止麻黄碱相关的 HR 下降，并与更大幅度的早期 MAP 升高和麻黄碱后更高的心脏指数值相关。基线测量在阿托品或生理盐水后 15 分钟、麻黄碱给药前即刻获得。未测量器官血流和组织灌注。
在一项对接受初始麻黄碱推注的麻醉犬和猫的单中心回顾性分析中，麻醉前阿托品与犬中 ≥20% 的 HR 下降发生频率较低以及达到研究反应标准（麻黄碱后 MAP > 70 mmHg）的比例较高相关。在猫中，阿托品术前用药与达到该 MAP 标准无显著相关。这些关联不能确立因果关系。
Alibhai 等人检查了六只犬中肌内注射美托咪定联合或不联合阿托品的心血管效应。当美托咪定后立即给予阿托品时，初始心动过缓之后在注射后 10–15 分钟出现 HR 快速升高和明显高血压。当阿托品先于美托咪定 30 分钟给予时也发生明显高血压；美托咪定后 HR 下降但仍高于单用美托咪定。两种组合均观察到间歇性快速性心律失常。作者将高血压解释为在较高 HR 存在下美托咪定诱导的血管收缩所致。美托咪定以固定剂量给予，而非根据观察到的 MAP 反应调整。
对固定剂量的更大 MAP 反应不同于在连续剂量调整期间更容易控制 MAP。滴定假说关注的是：减少窦房结毒蕈碱约束，随后根据观察到的 MAP 反应调整血管升压药，是否允许以更分级的方式达到目标血压。所引研究未检验该问题。
升压药类别可能改变所提出的反应。麻黄碱具有直接的α-和β-肾上腺素能作用，并通过内源性去甲肾上腺素释放产生间接效应[14]。因此，其与抗毒蕈碱预处理的相互作用不能确立与以血管收缩为主的药物相同的反应。该假说需要针对具体药物进行验证，而不能假设所有升压药具有统一效应。
5. 抗毒蕈碱效应的表达与分级滴定
非解耦模式提出了一个实际问题：如果延迟出现的抗毒蕈碱效应能够产生MAP-HR的突然反应，为什么不能简单地更缓慢地滴定抗毒蕈碱药物？关键区别在于滴定药物暴露量与滴定可观察的窦率反应之间的差异。药物暴露量可以逐步增加，但不能假定窦率反应呈比例性变化。
在七只吗啡和芬太尼诱导的心动过缓犬中，Rishniw等人比较了阿托品0.02 mg/kg经静脉、肌内或皮下给药的效果[15]。当心房率在心室率之前增加时，出现了短暂性房室传导阻滞。
我们先前的人体麻醉论文提出了一个药理学-生理学解释，说明为什么抗毒蕈碱相关的窦率加速在床旁可能表现为阈值样特征[1]。简言之，观察到的窦率反应反映了窦房结毒蕈碱抑制、交感背景、固有窦房结行为、麻醉药物效应以及当前反射背景之间的相互作用。在部分暴露范围内，额外的毒蕈碱阻断可能产生很少的可见变时性变化；在另一范围内，约束的进一步相对小幅降低可能随后出现窦率的陡峭增加。这仍然是一个针对犬的机制性假说；所引用的单剂量观察并未在递增抗毒蕈碱给药过程中检验所提出的关系。
如果该陡峭的窦率反应在升压药暴露已经增加血管张力之后出现，则心率增加可能在该高后负荷背景下促成MAP的突然或过度升高。
6. 该概念的范围与局限
6.1. 可检验的预测与评估
该概念关注窦房结毒蕈碱约束对升压药反应的贡献，而非MAP的所有决定因素。所提出的益处是更分级化的MAP控制：在剂量调整期间以更少的突然或过度血压升高达到并维持指定的MAP范围。仅有反射性窦性减慢减轻的证据并不能证明该益处已经实现。
一种可能的检验是在麻醉期间进行抗毒蕈碱预处理与对照的随机比较，连续记录动脉压和心电图，并对指定升压药采用相同的预设MAP指导给药算法。目标MAP范围、观察期、剂量调整规则和补救标准应在两组中预先指定。记录应在预处理前开始，以纳入任何初始压力或节律紊乱。若要分离时机因素的贡献，还需要前瞻性定义的后期抗毒蕈碱比较。
一个候选的主要终点是超过预定义上限的MAP超额时间积分（mmHg·min），同时评估达到目标的时间和在目标范围内观察时间的百分比。逐次给药的MAP增量、最大超射、低血压负担和调整次数提供补充信息。超射减少必须与目标达成延迟或低血压负担增加一并解读。如果反射性窦性减慢减轻但预设的MAP控制终点未改善，则该结果不支持所提出的滴定益处。
当心律失常或房室传导障碍导致监测HR与窦率不一致时，HR变化不能直接视为窦率反应。剂量调整期间麻醉药物给药或手术刺激的变化需要与升压药本身的反应相区分。
即使观察到解耦模式，仅凭MAP和HR也不能显示心输出量、每搏输出量、器官血流或组织灌注是否改善。心输出量或指数、每搏输出量和适当的灌注指标可在同一实验中作为独立的机制性、安全性和次要结局进行测量。
6.2. 药理学实施与安全性
心脏迷走解耦联所确定的是心脏生理学靶点，而非心脏选择性药物作用。相关顺序为在升压药升级之前减弱窦房结毒蕈碱性约束；其在麻醉方案中的时机可以不同。在吗啡和芬太尼诱导的心动过缓犬中，Rishniw等发现，静脉注射阿托品（0.02 mg/kg）后短暂性房室传导阻滞的发生和消退均早于肌内或皮下给药[15]。在清醒犬中，Hendrix和Robinson发现，静脉注射阿托品（30或60 μg/kg）后的HR在首次测量（1分钟）时显著高于生理盐水组，并分别在30分钟和60分钟内保持较高水平[16]。这些观察描述的是传导和变时反应的时间进程，而非心脏迷走解耦联的程度或持续时间。因此，未来研究应比较抗毒蕈碱药物的种类、剂量、给药途径和方案，包括单次或重复推注及持续输注。这些比较应评估毒蕈碱约束减弱的程度和持续时间如何影响整个滴定期间的MAP控制及伴随的心脏和心外反应。
美托咪定和阿托品所报道的高血压和快速性心律失常[13]值得在前瞻性滴定研究中设定明确的安全性终点。Lemke等发现，在六只异氟烷麻醉犬中，于赛拉嗪后给予静脉注射阿托品（0.1 mg/kg）或格隆溴铵（0.025 mg/kg），可使心脏指数和心率-血压乘积较赛拉嗪后值升高，而每搏量指数仍低于赛拉嗪前基线[17]。心率-血压乘积是与心肌氧耗相关的间接指标；其实验中该指标升高并未证明心肌缺血或损伤。心动过速可能增加心肌氧耗并缩短舒张充盈时间，这些担忧在患有心血管疾病的犬中尤其值得评估[17]。
在一项以六只清醒犬完成的交叉研究中，Hendrix和Robinson发现，静脉注射阿托品在升高HR的剂量下可抑制小肠运动[16]。这证明了与安全性相关的心外作用，但该研究测量的是肠道收缩活动，而非围手术期肠梗阻的发生率。
7. 结论
本文提出犬麻醉中的心脏迷走解耦联作为分级升压药滴定的生理学概念。减弱窦房结毒蕈碱约束可能减轻升压药给药期间的反射性窦性心动过缓。核心假设是，当根据观察到的MAP反应调整后续升压药剂量时，这种减弱可能允许更精细的血压控制。
该滴定假设尚未被直接检验。即使MAP控制改善，对心输出量、组织灌注和临床结局的影响仍需单独评估。
致谢
作者感谢Sanae Shibata（日本岐阜大学应用生物科学学部联合兽医系）和Takayuki Nakagawa（日本东京大学农学生命科学研究科兽医外科实验室）的有益讨论和评论。作者使用生成式AI工具（OpenAI；ChatGPT GPT-6 Pro）协助润色英文措辞、检查逻辑一致性并支持文献探索。
缩写
本手稿中使用以下缩写：
| HR | Heart rate |
| MAP | Mean arterial pressure |
机构审查委员会声明
不适用。本文基于已发表文献提出一种假设，未报告涉及人类或动物的新研究。
知情同意声明
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数据可用性声明
本研究未创建或分析新数据。数据共享不适用于本文。
利益冲突
S.N.受雇于JAVA Incorporated Association。作者声明，该研究是在不存在任何可被解释为潜在利益冲突的商业或财务关系的情况下进行的。
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脚注
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参考文献
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关联数据
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Simple Summary
Blood pressure can fall during general anesthesia in dogs. Drugs used to raise it may also trigger a reflex that slows the heart, partly limiting the rise in pressure. Atropine given afterward can reduce this slowing effect and may cause heart rate to rise abruptly. If blood vessels are already constricted, that rise in heart rate may in turn contribute to an abrupt increase in blood pressure. This article considers whether reducing the vagus nerve’s slowing effect on the heart before increasing drugs used to raise blood pressure could allow subsequent blood pressure adjustment in smaller steps. The proposal is called cardiac vagal decoupling and assumes that these drugs are adjusted according to the observed blood pressure response. Published studies support individual components of this idea, but whether it improves blood pressure adjustment remains untested. The article therefore presents a hypothesis for investigation rather than a treatment protocol. Effects on blood flow to organs and patient outcomes would require separate evaluation.
Keywords: atropine, baroreflex, cardiac vagal decoupling, dog, general anesthesia, intraoperative hypotension, reflex bradycardia, vasopressor titration
1. Introduction
Intraoperative hypotension in dogs is commonly managed by adjusting anesthetic drug administration, fluid therapy, and vasoactive support according to the suspected cause. In many cases, mean arterial pressure (MAP) is the main bedside target. The problem considered here arises when a vasopressor-associated MAP increase is accompanied by slowing of heart rate (HR; more precisely, sinus rate when sinus rhythm is present). A similar MAP increase can occur with sinus-rate slowing or preserved sinus rate, while the resulting cardiac output depends on the accompanying stroke-volume response.
Cardiac vagal decoupling denotes reduced vagally mediated muscarinic restraint at the sinoatrial node. The term identifies a cardiac effector action, not interruption of pressure sensing, central reflex integration, or the sympathetic limb of the baroreflex. Our prior human-anesthesia paper introduced this concept and hypothesized that reducing vagal restraint before substantial vasopressor escalation might allow more proportional vasopressor responses and reduce hypertensive overshoot compared with later antimuscarinic administration [1]. Drawing on canine physiological and clinical evidence, this article further develops the hypothesis that prior reduction of sinoatrial muscarinic restraint may facilitate graded blood pressure control during MAP-guided vasopressor titration.
The article focuses on this sequence-dependent component of the vasopressor response rather than global cardiovascular performance.
2. Two Hemodynamic Patterns During Pressor Titration
Figure 1 contrasts two hypothetical response patterns. In the non-decoupled pattern, pressor titration begins while functional sinoatrial muscarinic restraint remains influential. In the decoupled pattern, pressor titration begins after that restraint has already been reduced. In Figure 1A, as MAP rises, reflex-linked vagal influence can slow sinus rate, so the intended MAP increase may be partly opposed by reflex sinus slowing.
[图：Hypothetical hemodynamic trajectories in dogs without versus with cardiac vagal decoupling (schematic; not experimentall]
The latter part of Figure 1A illustrates one possible mechanism by which a late antimuscarinic effect could contribute to an abrupt MAP rise. Atropine or another antimuscarinic may cause sinus rate to rise abruptly. If vascular tone is already high, that rate increase may in turn contribute to an abrupt or excessive increase in MAP. The magnitude of the pressure change also depends on venous return, ventricular filling, stroke volume, and vascular resistance, rather than following the rate change proportionally. Incremental drug administration does not necessarily produce proportional sinus-rate increments, as discussed in Section 5.
Figure 1B illustrates the decoupled pattern. Functional muscarinic restraint at the sinoatrial node is reduced before substantial vasopressor escalation. Subsequent vasopressor titration is expected to be less constrained by reflex sinus slowing. The hypothesis is that small vasopressor dose adjustments guided by the observed MAP response may permit more graded pressure control.
Operationally, the relevant physiological change is less sinus-rate slowing for a comparable pressor-induced pressure rise, assessed separately from the success of MAP control. Neither the HR attained after atropine nor its absolute or percentage increase from baseline can, by itself, quantify the degree of cardiac vagal decoupling. It neither requires a target HR nor implies complete vagolysis.
3. Why Dogs Are a Relevant Setting
Dogs are a clinically important setting for this concept because available evidence suggests substantial vagally mediated muscarinic influence at the sinoatrial node [2,3].
In conscious dogs, Little et al. found that cumulative atropine administration shortened mean R–R intervals and reduced a real-time index of cardiac vagal activity derived from those intervals to near zero [2]. O’Leary et al. reported that atropine markedly increased HR relative to saline treatment during treadmill exercise in conscious dogs, with little effect on MAP even at high workloads [3]. These studies do not reproduce the anesthetized clinical setting, but they establish that the canine sinus node can operate under substantial and dynamically modifiable vagal restraint.
Schuessler et al. varied arterial pressure with phenylephrine and nitroprusside in chloralose-morphine-anesthetized, open-chest dogs and recorded changes in HR and the sites initiating atrial activation. Experiments with atropine and propranolol supported autonomic mediation of these baroreflex HR responses [4]. Tanaka and Nishikawa assessed baroreflex sensitivity in pentobarbital-anesthetized dogs by measuring the relationship between MAP and R–R intervals during phenylephrine-induced pressure increases and nitroprusside-induced decreases. Baroreflex sensitivity did not change significantly after acute normovolemic hemodilution [5]. Frankel et al. calculated spontaneous baroreflex sensitivity from the regression of beat-to-beat changes in pulse interval on changes in systolic pressure in conscious dogs. This measure was abolished by atropine, ganglionic blockade with hexamethonium, and bilateral carotid sinus denervation [6].
Fahim and Arndt compared baroreflex HR responses in anesthetized, mechanically ventilated, open-chest dogs and cats and found greater sensitivity in dogs [7]. The chronotropic responses involved both sympathetic and parasympathetic pathways, so these findings do not establish a specifically vagal basis for the species difference.
Seagard et al. examined baroreflex HR responses at different isoflurane concentrations. When anesthetic-induced changes in blood pressure were prevented, significant depression of the baroreflex HR response occurred only at the higher tested concentration [8]. Bagshaw and Cox measured central and regional hemodynamics while varying carotid sinus pressure and isoflurane concentration. Isoflurane reduced the mean aortic pressure response to carotid sinus stimulation at the reflex set point [9]. Szilagyi used phenylephrine and nitroprusside to assess baroreflex HR responses in anesthetized dogs and found that naloxone reduced baroreflex sensitivity [10]. Together, these studies support muscarinic modulation of cardiac rate, reflex cardiac responses to pressure changes, and modification of those responses by autonomic and anesthetic conditions. They establish the physiological substrate in dogs, rather than improved MAP control during repeated vasopressor dose adjustment.
4. Experimental and Clinical Evidence Supporting the Concept
The two Seki et al. studies examined cardiovascular responses to ephedrine with and without prior atropine. In a single-blind crossover experiment in six healthy Beagles with isoflurane-induced hypotension, each dog received a fixed intravenous ephedrine bolus after atropine or saline pretreatment [11]. Stroke volume and cardiac output were estimated by Doppler echocardiography and reported as indexed values. Prior atropine prevented the ephedrine-associated HR decrease and was associated with a greater early MAP rise and higher post-ephedrine cardiac index values than saline pretreatment. Baseline measurements were obtained 15 min after atropine or saline, immediately before ephedrine administration. Organ blood flow and tissue perfusion were not measured.
In a single-center retrospective analysis of anesthetized dogs and cats receiving an initial ephedrine bolus, preanesthetic atropine was associated with less frequent HR decreases of ≥20% and a higher rate of reaching the study’s response criterion (MAP > 70 mmHg after ephedrine) in dogs [12]. In cats, atropine premedication was not significantly associated with attainment of this MAP criterion. These associations do not establish causation.
Alibhai et al. examined the cardiovascular effects of intramuscular medetomidine with and without atropine in six dogs [13]. When atropine was administered immediately after medetomidine, initial bradycardia was followed by a rapid HR increase 10–15 min after injection and marked hypertension. Marked hypertension also occurred when atropine preceded medetomidine by 30 min; HR fell after medetomidine but remained higher than with medetomidine alone. Intermittent tachyarrhythmias were observed with both combinations. The authors interpreted the hypertension as resulting from medetomidine-induced vasoconstriction in the presence of higher HR. Medetomidine was administered at a fixed dose rather than adjusted to the observed MAP response.
A greater MAP response to a fixed dose is distinct from easier control of MAP during successive dose adjustments. The titration hypothesis concerns whether reducing sinoatrial muscarinic restraint, followed by vasopressor adjustments guided by the observed MAP response, permits a more graded approach to the intended pressure. This question was not tested by the cited studies.
Vasopressor class may modify the proposed response. Ephedrine has direct α- and β-adrenergic actions and indirect effects through endogenous norepinephrine release [14]. Its interaction with antimuscarinic pretreatment therefore cannot establish an identical response to predominantly vasoconstrictor agents. The hypothesis requires drug-specific testing rather than assuming a uniform effect across vasopressors.
5. Antimuscarinic Effect Expression and Graded Titration
The non-decoupled pattern raises a practical question: if a late antimuscarinic effect can produce an abrupt MAP-HR response, why not simply titrate the antimuscarinic drug more gradually? The key distinction is between titrating drug exposure and titrating the observable sinus-rate response. Drug exposure can be introduced incrementally, but a proportional sinus-rate response cannot be assumed.
In seven dogs with morphine- and fentanyl-induced bradycardia, Rishniw et al. compared atropine 0.02 mg/kg given intravenously, intramuscularly, or subcutaneously [15]. Transient atrioventricular block occurred when atrial rate increased before ventricular rate.
Our prior human-anesthesia paper proposed a pharmacophysiological explanation for why antimuscarinic-associated sinus-rate acceleration can appear threshold-like at the bedside [1]. In brief, the observed sinus-rate response reflects the interaction among muscarinic inhibition at the sinoatrial node, sympathetic background, intrinsic sinus-node behavior, the effects of anesthetic drugs, and the current reflex context. Across part of the exposure range, additional muscarinic blockade may produce little visible chronotropic change; over another range, a relatively small further reduction in restraint may be followed by a steep increase in sinus rate. This remains a mechanistic hypothesis for dogs; the cited single-dose observations did not test the proposed relation during incremental antimuscarinic dosing.
If that steep sinus-rate response is expressed after vasopressor exposure has already increased vascular tone, the rate increase may contribute to an abrupt or excessive MAP rise on that high-afterload background.
6. Scope and Limits of the Concept
6.1. Testable Predictions and Evaluation
The concept concerns the contribution of sinoatrial muscarinic restraint to the vasopressor response, rather than all determinants of MAP. The proposed benefit is more graded MAP control: reaching and maintaining a specified MAP range with fewer abrupt or excessive pressure increases during dose adjustment. Evidence of reduced reflex sinus slowing alone does not establish that this benefit has been achieved.
One possible test is a randomized comparison of antimuscarinic pretreatment and control during anesthesia, with continuous arterial pressure and electrocardiographic recording and the same predefined MAP-guided dosing algorithm for a specified vasopressor. The target MAP range, observation period, dose-adjustment rules, and rescue criteria should be specified in advance for both groups. Recording should begin before pretreatment to include any initial pressure or rhythm disturbance. Isolating the contribution of timing would additionally require a prospectively defined later-antimuscarinic comparison.
A candidate primary endpoint is the time-integrated MAP excess above the predefined upper limit (mmHg·min), assessed alongside time to target and the percentage of observation time within the target range. Dose-to-dose MAP increments, maximum overshoot, hypotension burden, and the number of adjustments provide complementary information. A reduction in overshoot must be interpreted alongside any delay in target attainment or increase in hypotension burden. If reflex sinus slowing is reduced but the prespecified MAP-control endpoint is not improved, the result would not support the proposed titration benefit.
When arrhythmia or atrioventricular conduction disturbance causes monitored HR to differ from sinus rate, changes in HR cannot be treated directly as sinus-rate responses. Changes in anesthetic drug administration or surgical stimulation during dose adjustment need to be distinguished from the response to the vasopressor itself.
Even if the decoupled pattern is observed, MAP and HR alone do not show whether cardiac output, stroke volume, organ blood flow, or tissue perfusion improved. Cardiac output or index, stroke volume, and appropriate perfusion indices can be measured in the same experiment as distinct mechanistic, safety, and secondary outcomes.
6.2. Pharmacological Implementation and Safety
Cardiac vagal decoupling identifies a cardiac physiological target, not a cardiac-selective drug action. The relevant sequence is the reduction of sinoatrial muscarinic restraint before vasopressor escalation; its timing within the anesthetic plan can vary. In dogs with morphine- and fentanyl-induced bradycardia, Rishniw et al. found that transient atrioventricular block developed and resolved earlier after intravenous atropine (0.02 mg/kg) than after intramuscular or subcutaneous administration [15]. In conscious dogs, Hendrix and Robinson found that HR after intravenous atropine (30 or 60 μg/kg) was significantly higher than with saline at the first measurement (1 min) and remained higher through 30 and 60 min, respectively [16]. These observations describe the time courses of conduction and chronotropic responses, not the degree or duration of cardiac vagal decoupling. Future studies should therefore compare antimuscarinic drugs, doses, routes, and regimens, including single or repeated boluses and continuous infusion. These comparisons should assess how the degree and duration of reduced muscarinic restraint affect MAP control and the accompanying cardiac and extracardiac responses throughout the titration period.
The hypertension and tachyarrhythmias reported with medetomidine and atropine [13] warrant explicit safety endpoints in prospective titration studies. Lemke et al. found that intravenous atropine (0.1 mg/kg) or glycopyrrolate (0.025 mg/kg) given after xylazine in six isoflurane-anesthetized dogs increased cardiac index and rate–pressure product from post-xylazine values, while stroke volume index remained below the pre-xylazine baseline [17]. Rate–pressure product is an indirect index related to myocardial oxygen demand; its increase did not demonstrate myocardial ischemia or injury in that experiment. Tachycardia may increase myocardial oxygen demand and shorten diastolic filling time, concerns that merit particular assessment in dogs with cardiovascular disease [17].
In a crossover study completed with six conscious dogs, Hendrix and Robinson found that intravenous atropine suppressed small-intestinal motility at doses that also increased HR [16]. This demonstrates an extracardiac action relevant to safety, but the study measured intestinal contractile activity rather than the incidence of perioperative ileus.
7. Conclusions
Cardiac vagal decoupling in canine anesthesia is proposed here as a physiological concept for graded vasopressor titration. Reducing sinoatrial muscarinic restraint may lessen reflex sinus slowing during pressor administration. The central hypothesis is that, when subsequent vasopressor doses are adjusted according to the observed MAP response, this reduction may permit more graded pressure control.
This titration hypothesis has not yet been directly tested. Even if MAP control improves, effects on cardiac output, tissue perfusion, and clinical outcomes require separate evaluation.
Acknowledgments
The author thanks Sanae Shibata (Joint Department of Veterinary Medicine, Faculty of Applied Biological Sciences, Gifu University, Gifu, Japan) and Takayuki Nakagawa (Laboratory of Veterinary Surgery, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Tokyo, Japan) for helpful discussions and comments. The author used a generative AI tool (OpenAI; ChatGPT GPT-6 Pro) to assist in refining English phrasing, checking logical consistency, and supporting literature exploration.
Abbreviations
The following abbreviations are used in this manuscript:
| HR | Heart rate |
| MAP | Mean arterial pressure |
Institutional Review Board Statement
Not applicable. This article presents a hypothesis based on previously published literature and reports no new studies involving humans or animals.
Informed Consent Statement
Not applicable.
Data Availability Statement
No new data were created or analyzed in this study. Data sharing is not applicable to this article.
Conflicts of Interest
S.N. was employed by JAVA Incorporated Association. The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Funding Statement
This research received no external funding.
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References
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Associated Data
Data Availability Statement
No new data were created or analyzed in this study. Data sharing is not applicable to this article.