The interaction of inhalational anesthetics with the central nervous system and the cardiovascular system constitutes the clinical core of intraoperative anesthetic management. These two organ systems are simultaneously the targets of the desired anesthetic effect, namely unconsciousness and immobility, and the principal sites of dose-limiting toxicity. The CNS effects of volatile agents extend well beyond the production of unconsciousness to encompass changes in cerebral blood flow, cerebral metabolic rate, intracranial pressure, and seizure threshold, each of which carries direct implications for neurosurgical practice. The cardiovascular effects are equally multidimensional: blood pressure, heart rate, myocardial contractility, cardiac output, systemic and pulmonary vascular resistance, coronary perfusion, and arrhythmia susceptibility are all modified by inhalational agents in ways that are both agent-specific and dose-dependent. A thorough understanding of these interactions allows the clinician to anticipate, prevent, and manage the physiological perturbations that accompany anesthetic administration.12
All inhalational anesthetics produce their primary effect, reversible loss of consciousness, through suppression of neuronal activity in the central nervous system. The molecular mechanisms underlying this effect are incompletely understood but involve potentiation of inhibitory neurotransmission (primarily via gamma-aminobutyric acid type A (GABA-A) receptor enhancement and glycine receptor potentiation) and inhibition of excitatory neurotransmission (primarily via N-methyl-D-aspartate (NMDA) receptor antagonism and inhibition of voltage-gated sodium and calcium channels).3 The net effect is a global reduction in cortical and subcortical neuronal excitability that is reflected in characteristic dose-dependent changes on the electroencephalogram (EEG): progressive slowing of background frequency, increasing amplitude, development of burst suppression, and ultimately an isoelectric tracing at supratherapeutic concentrations. The specific EEG trajectory varies among agents (isoflurane readily produces burst suppression at 1.5–2 minimum alveolar concentration (MAC), while halothane and sevoflurane produce more gradual slowing without burst suppression at clinical doses), but the general direction of EEG suppression is shared across all agents.1
Beyond the EEG, inhalational agents profoundly affect the cerebral vasculature and cerebral metabolic rate, creating a physiological tension that is central to neurosurgical anesthetic management: all volatile agents reduce cerebral metabolic rate for oxygen (CMRO2), a desirable effect that reduces the brain's vulnerability to ischemia, but simultaneously cause cerebral vasodilation and increase cerebral blood flow (CBF), which increases intracranial volume and can elevate intracranial pressure (ICP) in patients with reduced intracranial compliance. Managing this tension, that of maximizing metabolic protection while minimizing ICP, is a defining challenge of neuroanesthesia.12
The relationship between inhalational anesthetics and seizure activity is agent-specific and clinically important. The clear outlier among currently discussed agents is enflurane, which is the only volatile anesthetic with a clinically documented epileptogenic potential. At inspired concentrations above approximately 2 minimum alveolar concentration (MAC), particularly when combined with hypocapnia, which lowers seizure threshold by causing cerebral vasoconstriction and neuronal alkalosis, enflurane produces high-amplitude EEG spike-and-wave complexes and can induce generalized tonic-clonic seizure activity even in patients without a pre-existing seizure disorder.4 This property is concentration-dependent and largely reversible upon reducing the inspired concentration, but it renders enflurane contraindicated in patients with epilepsy and unsuitable for neurosurgical procedures requiring cortical monitoring.
Isoflurane, sevoflurane, and desflurane do not possess meaningful epileptogenic potential at clinical doses and are generally considered safe in patients with seizure disorders. Isoflurane at high doses produces burst suppression, which is an antiepileptiform pattern. There are isolated reports of EEG spike activity with sevoflurane during induction at high concentrations, particularly in pediatric patients, but clinically overt seizures attributable to sevoflurane in humans are exceedingly rare, and sevoflurane is not classified as an epileptogenic agent for clinical purposes.4 Halothane does not have epileptogenic potential. Nitrous oxide has no epileptogenic activity and may in fact provide mild anticonvulsant effect through its N-methyl-D-aspartate (NMDA) receptor antagonism.
All volatile halogenated anesthetics cause dose-dependent cerebral vasodilation, increasing CBF above baseline at concentrations above approximately 0.5 minimum alveolar concentration (MAC). This effect is mediated through direct relaxation of cerebrovascular smooth muscle, independent of changes in CMRO2. The magnitude of CBF increase varies substantially among agents: halothane produces the greatest increase in CBF at equivalent MAC fractions, with increases of 200% or more at 2 MAC reported in some studies. Isoflurane and sevoflurane produce more modest CBF increases (typically 20–40% above baseline at 1 MAC in normocapnic patients), while desflurane's effects on CBF are intermediate and similar to isoflurane.12
A key clinical concept is that the cerebral vasodilation caused by volatile agents can be substantially attenuated by hypocapnia. Reducing PaCO2 through controlled hyperventilation causes cerebral vasoconstriction that largely counteracts the vasodilatory effect of the anesthetic, maintaining or even reducing CBF relative to the pre-anesthetic state. This is the pharmacological basis for the clinical practice of moderate hyperventilation (target PaCO2 30–35 mmHg) during induction and maintenance of anesthesia in neurosurgical patients, buying time by blunting the anesthetic-induced rise in CBF and intracranial pressure (ICP) until the cranium is opened and intracranial compliance is restored. The interaction between PaCO2 and volatile-agent-induced vasodilation is largely additive: the CO2 vasomotor reactivity of the cerebral vasculature is preserved during inhalational anesthesia.1
Nitrous oxide causes a modest increase in CBF and CMRO2 when used alone, which is paradoxical given its overall CNS depressant effect but is thought to reflect sympathetic activation. When used as part of a combined technique, the net effect on CBF of adding nitrous oxide to a volatile agent depends on the balance between its mild vasodilatory effect and the MAC-sparing reduction of the volatile agent concentration. In general, the combination has little net effect on CBF compared to either agent used alone at comparable MAC fractions.
Cerebral Autoregulation. Cerebral autoregulation is the intrinsic capacity of the cerebral vasculature to maintain relatively constant CBF across a range of cerebral perfusion pressures, approximately 50 to 150 mmHg in healthy adults. Within this range, cerebrovascular resistance adjusts automatically to compensate for changes in MAP, so that CBF remains stable. Below the lower limit (approximately 50 mmHg CPP), vasodilation is maximal and CBF falls passively with further reductions in MAP, risking ischemia. Above the upper limit (approximately 150 mmHg), forced vasodilation occurs with blood-brain barrier disruption and the risk of hypertensive encephalopathy.5
Volatile anesthetic agents impair cerebral autoregulation in a dose-dependent fashion. At concentrations of 1 MAC or above, all volatile agents substantially attenuate autoregulatory capacity, shifting the relationship between CPP and CBF toward a more pressure-passive state. In practical terms, under deep inhalational anesthesia CBF becomes more directly dependent on MAP: hypotension causes proportional reductions in CBF and hypoperfusion risk, while hypertension causes proportional CBF increases and potentially elevated ICP. This has direct implications for hemodynamic management in neurosurgical patients, where maintaining MAP within a range that supports adequate CPP (target generally 60 to 70 mmHg or higher in patients with chronically elevated ICP) is a primary anesthetic objective. Propofol, at clinical infusion rates, preserves cerebral autoregulation better than volatile agents, which is one rationale for preferring TIVA in patients with compromised intracranial compliance.5
All volatile halogenated anesthetics reduce CMRO2 in a dose-dependent manner. This metabolic suppression is tightly coupled to EEG suppression: as anesthetic depth increases and EEG activity slows, CMRO2 falls progressively. At burst suppression, CMRO2 is reduced by approximately 50–60% from awake baseline, the maximum suppression achievable through electrical (neuronal) inactivation, since approximately 40% of basal cerebral oxygen consumption is devoted to housekeeping metabolic functions (ion pump activity, membrane maintenance) that are not suppressible by anesthetic agents.1 This ceiling on metabolic protection is clinically relevant: increasing anesthetic dose beyond the burst-suppression threshold does not provide additional metabolic protection.
Among the agents, isoflurane has been most studied for its metabolic suppression properties in the context of cerebral ischemia protection. It reduces CMRO2 to a greater degree than halothane at equivalent minimum alveolar concentration (MAC) fractions and can produce complete electrical silence (isoelectric EEG) at clinically achievable concentrations, a property exploited during deliberate hypotension or temporary vessel occlusion in aneurysm surgery to maximize the ischemia-tolerable interval.1 Sevoflurane provides comparable CMRO2 suppression. Desflurane is broadly similar to isoflurane in its effects on CMRO2. Halothane reduces CMRO2 less effectively than isoflurane at equivalent MAC multiples. Nitrous oxide, as noted, may slightly increase CMRO2 when used alone, making it a theoretically less desirable component of neuroanesthetic techniques in patients at risk for ischemia.
The Monro-Kellie doctrine establishes that the cranial vault is a rigid compartment of fixed total volume, the contents of which, brain parenchyma, cerebrospinal fluid (CSF), and blood, must sum to a constant. Any increase in one component must be compensated by a decrease in another or intracranial pressure will rise. The increase in CBF produced by volatile anesthetics translates directly into an increase in intracranial blood volume, which, in a patient with normal intracranial compliance, causes only a modest and transient rise in intracranial pressure (ICP). However, in patients with reduced intracranial compliance, such as those with intracranial mass lesions, traumatic brain injury, cerebral edema, hydrocephalus, or any condition that has exhausted the normal compensatory mechanisms, even a small increase in intracranial blood volume may cause a disproportionate rise in ICP.5
The clinical implications are substantial. Volatile anesthetics should be used with caution (or avoided in favor of total IV anesthesia with propofol) in patients with elevated ICP or severely reduced intracranial compliance. When volatile agents are used in this setting, the following principles apply: concentrations should be kept at or below 1 minimum alveolar concentration (MAC); the agent should be introduced after induction with propofol and after establishing hyperventilation to counteract cerebral vasodilation; and agents with the least CBF effect (isoflurane or sevoflurane) should be preferred over halothane. Nitrous oxide, which increases CBF and CMRO2 and can expand intracranial gas collections, should generally be avoided in neurosurgical patients.
Management of Intracranial Volume. The management of intracranial volume during neurosurgical procedures is multimodal, addressing each of the three Monro-Kellie compartments. Reduction of cerebral blood volume is achieved primarily through controlled hyperventilation and limitation of volatile anesthetic concentration. Reduction of CSF volume is achieved through lumbar or ventricular CSF drainage where appropriate. Reduction of edematous brain tissue volume is achieved through osmotic therapy (mannitol), loop diuretics (furosemide), corticosteroids (in vasogenic edema from tumors or abscesses), and surgical decompression.5
Mannitol (Osmitrol) Mannitol is an osmotic diuretic that reduces intracranial pressure through two distinct mechanisms. The primary mechanism, the osmotic effect, draws free water from the brain parenchyma into the intravascular compartment along the osmotic gradient established by elevating serum osmolality, reducing brain water content and volume. This effect is maximal within 15–30 minutes of administration, sustained for 90–120 minutes, and requires an intact blood-brain barrier for full efficacy; in areas where the blood-brain barrier is disrupted (as by tumor, contusion, or infarction), mannitol may enter the tissue and paradoxically worsen edema over time. The secondary mechanism, an acute rheological effect, reduces blood viscosity and improves microcirculatory flow, which may transiently improve cerebral oxygen delivery and reduce reactive vasodilation.5 Standard dosing is 0.5–1.5 g/kg IV administered over 15–20 minutes. Serum osmolality should be monitored with repeat dosing; osmolality exceeding 320 mOsm/kg is associated with increased risk of acute kidney injury, and the osmolar gap (measured minus calculated osmolality) should be tracked to avoid accumulation. Mannitol also produces an acute volume load followed by vigorous diuresis; hemodynamic monitoring and intravascular volume replacement are important in patients who are already hemodynamically fragile.
Furosemide (Lasix) Furosemide reduces intracranial pressure (ICP) through inhibition of the Na-K-2Cl cotransporter in the loop of Henle, producing diuresis and reducing total body water and, consequently, brain water content. Its onset of ICP reduction is slower than mannitol and its magnitude of effect is more modest when used alone, but it is frequently combined with mannitol in the management of acute intracranial hypertension, a combination that produces synergistic diuresis and more sustained ICP reduction than either agent alone. Furosemide also inhibits CSF production through inhibition of carbonic anhydrase activity in the choroid plexus, contributing a second mechanism of ICP reduction. The usual dose in the neurosurgical setting is 0.5–1 mg/kg IV. Electrolyte monitoring (potassium, sodium) is essential, particularly with repeated dosing.5
Corticosteroids. Corticosteroids, principally dexamethasone, are highly effective in reducing vasogenic cerebral edema associated with primary and metastatic brain tumors and cerebral abscesses, where the blood-brain barrier disruption by the lesion allows protein-rich fluid extravasation into the surrounding parenchyma. Dexamethasone reduces this edema by decreasing blood-brain barrier permeability and reducing tumor-associated inflammatory mediator production. The onset of effect is relatively slow (hours to days) compared to osmotic agents, making corticosteroids unsuitable as acute ICP rescue therapy but important in preoperative optimization for elective tumor resection. Standard dosing for perioperative brain tumor edema management is dexamethasone 4 mg IV every 6 hours. Corticosteroids are not effective for cytotoxic cerebral edema (as in ischemic stroke or traumatic brain injury) and are not recommended in those settings.5
Hyperventilation. Controlled hyperventilation reduces PaCO2, producing cerebral vasoconstriction via CO2-mediated pH changes in perivascular fluid. A reduction in PaCO2 of 10 mmHg below baseline reduces CBF by approximately 20–30% and produces a corresponding reduction in intracranial blood volume and ICP. This effect is rapid (onset within seconds to minutes) and highly reliable, making hyperventilation the fastest available maneuver for acute ICP reduction in the operating room. However, several important limitations apply: the vasoconstrictive effect wanes over 4–6 hours as CSF bicarbonate adapts to the new pH; aggressive hypocapnia (PaCO2 below 30 mmHg) risks cerebral ischemia from excessive vasoconstriction, particularly in injured brain regions with impaired autoregulation; and rebound ICP elevation may occur upon normalization of ventilation if adaptation has occurred.5 The target PaCO2 for intraoperative ICP management is generally 30–35 mmHg, sufficient to attenuate anesthetic-induced vasodilation without approaching ischemic thresholds.
Therapeutic Goals. The integrated therapeutic goal in neurosurgical anesthetic management is to maintain cerebral perfusion pressure (CPP = MAP − ICP) at or above 60–70 mmHg while simultaneously limiting ICP, reducing brain bulk to facilitate surgical exposure, and minimizing additional neurological injury from anesthetic agents themselves. This requires dynamic adjustment of multiple variables simultaneously: ventilation parameters, volatile agent concentration, osmotic therapy, head positioning, and hemodynamic management.5 The preferred anesthetic technique for high-ICP neurosurgical cases is often total IV anesthesia with propofol and remifentanil infusions, supplemented by low-concentration volatile agent (≤0.5 minimum alveolar concentration (MAC)) or avoided entirely, as propofol reliably reduces CMRO2 and CBF without the vasodilatory liability of volatile agents.
ICP and Skeletal Muscle Relaxants: Succinylcholine (Anectine). Succinylcholine, the only depolarizing neuromuscular blocking agent in clinical use, causes a transient but significant increase in ICP following administration. The mechanism involves muscle fasciculation-induced afferent neural input that transiently increases cerebral activity and CBF, as well as direct central effects. The ICP rise following succinylcholine is modest in absolute terms (typically 5–10 mmHg) and brief (approximately 2–5 minutes), but in a patient with severely elevated ICP and reduced compliance, this transient increase may be clinically significant and could theoretically precipitate herniation. For elective neurosurgical procedures with known elevated ICP, succinylcholine is therefore best avoided in favor of a rapid-onset nondepolarizing agent (rocuronium at 1.2 mg/kg approaches the intubating conditions of succinylcholine within 60–90 seconds). However, in a true airway emergency where the risk of aspiration or failed intubation outweighs the ICP concern (as in a trauma patient requiring emergency intubation), succinylcholine remains appropriate, and the incremental ICP risk must be accepted as necessary.6
Nondepolarizing Agents. Nondepolarizing neuromuscular blocking agents do not cause fasciculations and do not increase ICP through the mechanism described for succinylcholine. They are the preferred agents for neuromuscular blockade maintenance during neurosurgical procedures. Vecuronium and cisatracurium are commonly used because of their hemodynamic neutrality. Rocuronium at standard intubating doses (0.6 mg/kg) is intermediate in onset and suitable for most neurosurgical cases; at high doses (1.2 mg/kg) it provides rapid intubating conditions with reversal availability via sugammadex. Atracurium is associated with laudanosine accumulation (a potential CNS stimulant) at high doses with prolonged administration, though this is rarely clinically significant.6
Steroidal Agents. Among the steroidal neuromuscular blocking agents (vecuronium, rocuronium, pancuronium), pancuronium is the least desirable in neurosurgical patients because of its vagolytic properties, which produce tachycardia and hypertension, hemodynamic changes that increase CBF and can worsen ICP. Vecuronium and rocuronium are hemodynamically neutral at standard doses and are the preferred steroidal agents for neurosurgical use.6
Induction Sequence in Patients with Elevated ICP. The induction sequence for patients with known or suspected elevated ICP requires careful attention to the pharmacological profile of each agent at each step. The goal is to achieve rapid, smooth induction with secure airway control while minimizing any drug-related increase in ICP or reduction in CPP. A standard approach proceeds as follows: pre-oxygenation with 100% oxygen, followed by administration of an induction agent that reduces CMRO2 and CBF; propofol (1.5–2.5 mg/kg) is the agent of choice, though etomidate (0.3 mg/kg) is preferred when cardiovascular compromise limits propofol use. Lidocaine (1.5 mg/kg IV) administered 60–90 seconds before laryngoscopy attenuates the ICP response to intubation by blunting airway reflexes. Fentanyl (2–3 mcg/kg) is administered for analgesia and to blunt the hemodynamic response to laryngoscopy. A high-dose nondepolarizing neuromuscular blocker (rocuronium 1.2 mg/kg, or succinylcholine if rapid reversal capability is unavailable and aspiration risk is high) provides intubating conditions. Hyperventilation is initiated immediately upon intubation. The volatile anesthetic, if used, is introduced at low concentration only after the ICP has been reduced by the above measures and, ideally, only after the cranium has been opened.56
Anesthesia Maintenance: Supratentorial Tumor Cases. For elective supratentorial tumor resections, anesthetic maintenance is most commonly achieved with a low concentration of a volatile agent (isoflurane or sevoflurane at 0.5–1.0 MAC) combined with an opioid infusion (remifentanil or fentanyl) and, in many centers, supplemented with propofol infusion to minimize volatile agent requirements. Nitrous oxide is generally avoided because of its effects on CBF, CMRO2, and risk of expanding any intracranial gas collection inadvertently introduced during surgery. Total IV anesthesia with propofol and remifentanil is an equally valid and increasingly preferred approach, particularly for cases requiring intraoperative neurophysiological monitoring (where volatile agents interfere with motor evoked potential monitoring) or awake craniotomy (where the rapid and smooth titratability of propofol-remifentanil is advantageous).5 Mannitol (1 g/kg IV) is administered at the beginning of most supratentorial tumor cases to reduce brain bulk and facilitate surgical exposure.
Intraoperative Neurophysiological Monitoring: Pharmacological Constraints. Intraoperative neurophysiological monitoring (IONM) of motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs) is used during spinal surgery, cerebrovascular procedures, and tumor resections to detect impending neural injury in real time. All volatile anesthetic agents suppress these signals in a dose-dependent fashion, and this suppression is the primary pharmacological constraint on anesthetic technique in monitored cases.6 MEPs are generated by transcranial electrical stimulation and recorded from peripheral muscles; they are exquisitely sensitive to volatile agents. At concentrations of 0.5 MAC, MEP amplitude is reduced by 50% or more; at 1.0 MAC, MEPs are frequently unrecordable. This makes reliable MEP monitoring impossible during standard volatile agent maintenance, and total intravenous anesthesia with propofol and remifentanil is therefore the required technique for cases in which MEP monitoring is planned. SSEPs are somewhat less sensitive to volatile agents than MEPs and can often be monitored at volatile concentrations of 0.5 MAC or below, but the combination of low volatile concentration with propofol supplementation is the preferred approach when both MEP and SSEP monitoring are required simultaneously.
Neuromuscular blockade must be carefully managed during IONM cases. Complete neuromuscular blockade abolishes both MEPs (which require intact motor units for peripheral recording) and the ability to detect motor responses. Train-of-four monitoring is used to confirm partial or absent neuromuscular blockade at the time of MEP recording; typically, one or two twitches of the train-of-four are permitted to ensure adequate surgical conditions without abolishing motor responses. Nitrous oxide also suppresses MEPs and is generally avoided in cases requiring MEP monitoring. The neurophysiology team should be consulted before the anesthetic plan is finalized to ensure that the chosen technique is compatible with the monitoring modalities required.6
Awake Craniotomy: Pharmacological Principles. Awake craniotomy is performed for resection of tumors or epileptogenic foci involving eloquent cortex, where intraoperative language mapping, motor testing, or seizure focus identification requires a cooperative and communicative patient during at least part of the procedure. The pharmacological approach must provide adequate sedation and analgesia for the skin incision, bone flap, and dural opening while allowing reliable awakening and full cooperation during the cortical mapping phase, followed by return to deeper sedation for tumor resection and closure if needed. The asleep-awake-asleep technique is the most widely used approach.5
Propofol and remifentanil, administered by target-controlled or weight-based infusion, are the agents of choice for awake craniotomy. Remifentanil provides reliable analgesia during stimulating phases (pin fixation, incision) and clears within minutes of discontinuation, allowing rapid emergence without residual opioid sedation. Propofol at sedating infusion rates (1 to 2 mcg/mL target plasma concentration via target-controlled infusion (TCI), or 50 to 100 mcg/kg/min weight-based) is readily titratable and allows smooth, predictable awakening when the infusion is reduced. Dexmedetomidine is increasingly used as an adjunct or sole sedative in awake craniotomy, providing cooperative sedation without respiratory depression; its ability to produce a calm, arousable patient who responds to commands makes it well suited to the mapping phase. Local anesthetic scalp block (greater occipital, supraorbital, auriculotemporal, and other nerves) is essential for pain control during pin placement and skin incision. The principal intraoperative hazards are airway obstruction during sedation, patient movement during mapping, intraoperative seizures (managed with iced saline irrigation of the cortex and, if necessary, small doses of propofol or midazolam), and conversion to general anesthesia for patient distress or complications.5
Dose-Dependent and Drug-Specific Effects. Inhalational anesthetics exert complex, multidimensional effects on the cardiovascular system that are simultaneously dose-dependent and agent-specific. No single cardiovascular variable (blood pressure, heart rate, cardiac output, or vascular resistance) can be considered in isolation, because changes in any one variable trigger compensatory or reinforcing responses in the others through baroreceptor reflexes, direct myocardial effects, and peripheral vascular mechanisms. The net clinical effect represents the integral of these interacting forces.12
Mean Arterial Pressure. All inhalational anesthetics reduce mean arterial pressure (MAP) in a dose-dependent fashion, though the mechanism differs among agents. Halothane reduces MAP primarily through myocardial depression, reducing cardiac output while maintaining or increasing systemic vascular resistance. Isoflurane, sevoflurane, and desflurane (at stable concentrations) reduce MAP primarily through peripheral vasodilation, reducing systemic vascular resistance while cardiac output is relatively preserved through baroreceptor-mediated heart rate compensation. This mechanistic distinction has clinical implications: patients with impaired myocardial function tolerate isoflurane- and sevoflurane-induced MAP reduction (which preserves cardiac output) better than halothane-induced MAP reduction (which does not). Nitrous oxide contributes minimal MAP reduction when added to a volatile agent at concentrations that allow reduction of the volatile agent dose.1
Heart Rate. Halothane typically produces bradycardia as a result of direct depression of sinoatrial node automaticity and sensitization to vagal tone. Isoflurane, sevoflurane, and desflurane (at stable concentrations) tend to maintain or mildly increase heart rate through baroreceptor-mediated reflexes to vasodilation. Desflurane at rapidly increasing concentrations causes marked, abrupt tachycardia through sympathetic activation, as previously described.7 Enflurane's heart rate effects are intermediate. Nitrous oxide tends to maintain or slightly increase heart rate through sympathomimetic mechanisms. In clinical practice, heart rate during inhalational anesthesia is also significantly influenced by surgical stimulation, premedication (opioids and beta-blockers), and patient comorbidities.
Cardiac Output and Stroke Volume. Halothane reduces cardiac output primarily by reducing stroke volume (myocardial depression with reduced contractility and ejection fraction). Isoflurane, sevoflurane, and desflurane maintain or modestly reduce cardiac output at clinical concentrations through the combination of reduced afterload (allowing preserved stroke volume) and compensatory heart rate increase. At concentrations above 1.5–2 minimum alveolar concentration (MAC), all agents produce progressively more significant cardiac depression. Patients with severely impaired baseline ventricular function have less cardiac reserve to compensate and therefore experience more pronounced reductions in cardiac output at a given anesthetic concentration.1
Systemic Vascular Resistance. Isoflurane is the most potent peripheral vasodilator among the volatile agents, producing the greatest reduction in systemic vascular resistance (SVR) at equivalent MAC fractions. Sevoflurane and desflurane produce intermediate reductions in SVR. Halothane produces less peripheral vasodilation and more direct myocardial depression. Reduced SVR generally reduces cardiac afterload and may preserve cardiac output in patients with normal or increased baseline afterload, but in patients who are already vasodilated (as in septic shock or hepatic failure), further reduction in SVR precipitates profound hypotension.12
Duration of Anesthetic Administration and Cardiac Effects. The cardiovascular depression produced by volatile anesthetics tends to attenuate somewhat with time, a phenomenon attributed to progressive baroreceptor adaptation, sympathetic activation, and hormonal responses to anesthesia. Heart rate in particular tends to increase over the course of a long anesthetic as these compensatory mechanisms engage. This temporal adaptation means that hemodynamic parameters measured at 30 minutes into an anesthetic may be somewhat different from those at two hours, even at a constant inspired concentration. The clinical implication is that hemodynamic instability in a long case should prompt consideration of both surgical stimulus changes and the evolving sympathetic response, in addition to the anesthetic concentration.
Pulmonary Vascular Resistance. Volatile anesthetics have modest and clinically variable effects on pulmonary vascular resistance (PVR). All inhibit hypoxic pulmonary vasoconstriction (HPV), the physiological mechanism by which pulmonary blood flow is diverted away from poorly ventilated lung regions, to varying degrees. Inhibition of HPV worsens intrapulmonary shunting and ventilation-perfusion mismatch, contributing to intraoperative hypoxemia, particularly during one-lung ventilation for thoracic surgery. Isoflurane and sevoflurane inhibit HPV to a similar degree at equivalent MAC fractions; desflurane's effect is comparable. Propofol (used in TIVA) does not inhibit HPV and is therefore preferred for maintenance during one-lung ventilation in thoracic surgery where oxygenation is marginal.
Cardiac Arrhythmias. Halothane sensitizes the myocardium to catecholamine-induced arrhythmias through mechanisms involving calcium overload and altered automaticity of Purkinje fibers. Ventricular ectopy, bigeminy, and in severe cases ventricular fibrillation may result from epinephrine doses that would be innocuous under isoflurane or sevoflurane anesthesia. The threshold for epinephrine-induced arrhythmias under halothane is approximately 1.5–2 mcg/kg, compared to 7–10 mcg/kg under isoflurane.8 Other arrhythmias encountered during inhalational anesthesia, including junctional rhythms, first-degree heart block, and atrial fibrillation, reflect a combination of anesthetic effects on cardiac conduction, surgical stimulation, electrolyte disturbances, and preexisting cardiac disease rather than specific pharmacological arrhythmogenesis. Desflurane-induced sympathetic surges may provoke ventricular ectopy in susceptible patients.
Coronary Blood Flow. Volatile anesthetics are coronary vasodilators, increasing coronary blood flow in proportion to myocardial oxygen demand, or in the case of marked vasodilation, potentially in excess of demand. The theoretical concern about coronary steal syndrome with isoflurane arises from the possibility that coronary vasodilation in normal vessels diverts flow away from territories supplied by fixed stenotic vessels that cannot vasodilate further. This concern is most relevant in patients with multivessel coronary artery disease and collateral-dependent myocardium, but multiple clinical studies have failed to demonstrate a meaningful increase in myocardial ischemia attributable to isoflurane at clinical doses in patients undergoing coronary artery bypass grafting.8 Conversely, there is evidence from experimental and clinical studies that volatile anesthetics, particularly isoflurane and sevoflurane, produce myocardial preconditioning, reducing ischemia-reperfusion injury through mechanisms involving mitochondrial ATP-sensitive potassium channel (KATP) channel activation and adenosine receptor signaling. This phenomenon of anesthetic-induced preconditioning has been demonstrated to reduce cardiac troponin release and improve cardiac outcomes in some trials of cardiac surgical patients.
Neurocirculatory Responses: Desflurane. Desflurane deserves specific attention for its neurocirculatory effects during rapid concentration increases, as described in Part 2. At stable maintenance concentrations desflurane does not differ substantially from isoflurane in its cardiovascular profile. However, when the inspired concentration is abruptly increased, particularly from concentrations below 1 MAC to above 1 MAC, a marked sympathetic discharge occurs, with tachycardia and hypertension that can be severe and is poorly tolerated by patients with coronary artery disease, aortic stenosis, or hypertensive heart disease. In these populations, desflurane concentration should be increased in small increments and supplemented with opioids or alpha-2 agonists (dexmedetomidine, clonidine) to blunt the sympathetic response.7
Preexisting Myocardial Disease and Inhaled Anesthetic Effects. Patients with preexisting left ventricular dysfunction are particularly sensitive to the negative inotropic effects of volatile anesthetics. In this population, the myocardial depression of halothane is most hazardous; isoflurane and sevoflurane are better tolerated because their predominant mechanism of blood pressure reduction (vasodilation with preserved cardiac output) is more forgiving of impaired contractility. Desflurane at stable concentrations is also acceptable, but its sympathetic surge property during concentration adjustments may impose unacceptable hemodynamic stress. In patients with severely reduced ejection fraction (EF <30%), TIVA with propofol may be hemodynamically superior if kept at low infusion rates, and ketamine may be useful for induction because of its sympathomimetic properties. The choice of agent must always be individualized to the specific cardiac substrate and the degree of hemodynamic compromise.1
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