One of the main challenges in anesthesia is optimal intra-operative oxygen administration to the patient and maintaining adequate organ perfusion under different anesthesia types. Otherwise, peri-operative hypotension is required for better surgical field visualization. Patients are at risk of cerebral hypoperfusion and desaturation with probable ischemic events and neurological sequels during shoulder arthroscopy in the sitting position.

Peculiarity of shoulder arthroscopy Most of shoulder procedures are performed as arthroscopic surgery: less invasive and painful with lower risk of postoperative complications and faster discharge. There are two patient’s positions possible: lateral or sitting with advantages and risks of each. In lateral position the incidence of post-traction neuropathies is higher.¹ In the sitting position patient’s chest is elevated from 30 to 90 degrees and the head is fixed in a special attachment. Setup and intra-articular visualization are perfect, lower blood loss, easier switch to open surgery.^{2 3} However latter approach can cause cardiovascular instability with hypotensive/bradycardic events leading to cerebral desaturation and ischemia.^{2 4}Cerebral perfusion pressure diminishes proportionally to the lift of the head above the heart level. Surgeon’s requirement for deliberate peri-operative hypotension evokes summarized danger for the patient in the upright position.³ Transient position- or anesthetic-related complications can be observed in 10 – 30% of patients, but serious irreversible organ damage is rare.¹

Visualization of intra-operative field The arthroscopic pump pressure (∼50 mmHg) helps to expand the subacromial space and ameliorate the visibility. the direct correlation was found between systolic blood pressure and subacromial space pressure. Thus, the use of controlled hypotension permits lower irrigation pressures and significantly reduces the risk of fluid extravasation into the subcutaneous tissues of the shoulder. Norepinephrine diluted in irrigation fluid to control intra-articular bleeding was recognized superior to epinephrine regarding the incidence of hypotensive/bradycardic event.⁵

Deliberate hypotension In sitting position, mean arterial pressure (MAP), central venous pressure, pulmonary artery occlusion pressure, cardiac output and index decrease. Cerebral perfusion pressure (CPP) decreases about 15% and is falling more under general anesthesia because of vasodilatation and other circulatory changes like impaired stroke volume and venous return. Cerebral autoregulation limits varies widely in population. Individual predicted value of MAP should not decrease more than 20 – 30% below patient’s usual MAP with a minimum of 50 mmHg in ASA class I and > 65 mmHg in elderly. Measurements should be performed at the level of external meatus in sitting position. Partial mechanical obstruction of major vessels can occur with head and neck movements and influences the cerebral blood flow (CBF).^{3 6}

Deliberate hypotension can be achieved by deepening the level of anesthesia, or with additional antihypertensive medications.⁷ Preoperative cardiac conditions and preload might be important for the development of hypotension after positioning the patient under general anesthesia.⁸ the patients with diabetes mellitus, anemia, renal, hepatic or ischemic cerebrovascular disease, respiratory insufficiency are at higher risk of cerebral hypoperfusion and desaturation.

Cerebral desaturation This is usually is defined when it falls >20% from baseline or absolute number shows < 50. a few portentous neurological ischemic episodes were described.⁹ CPP depends on MAP and intracranial pressure (ICP). CPP is also affected by cerebral metabolic demands and increases with increasing PaCO₂. Autoregulation of cerebral blood flow is maintained through mechanisms related to the intrinsic elasticity of the vascular smooth muscle and vasodilator substances produced in the metabolically active tissues when MAP is 50 – 150 mmHg.¹⁰ Hypotensive anesthesia in the sitting position can aggravate the decrease in CPP. Arterial cannulation and invasive monitoring is highly recommended if deliberate hypotension is used.⁶ Experimental studies showed that cerebral desaturation is associated with declination time from baseline as well as with co-existing cerebrovascular disease.¹¹

Vasovagal syncope This is defined as sudden peri-operative hypotension or/and bradycardia requiring treatment, was reported in 17 – 31% of procedures in sitting position under interscalene brachial plexus block (ISBPB).¹² Classical syncope may be induced by fear and pain due to inhibition of the sympathetic and to activation of the parasympathetic system.² Bezold–Jarisch reflex occurs since venous return is reduced; hypercontraction of empty ventricle leads to stimulation of intramyocardial mechanoreceptors and vasovagal tone is increased.¹³ Large volume of local anesthetics for ISBPB, neck swelling, head turning and arm traction can mechanically stimulate the carotid sinus. Ability to maintain hemodynamic homeostasis during position changes with age becomes less effective and may lead to orthostatic syncope which might be aggravated by vasodilators, diuretics and anti-depressives.²

Myocardial infarction and kidney injury In recent study, important hypotension caused 6% of myocardial infarction, 1.2% was lethal in 30 days from surgery.¹⁴ Reduction of MAP from relative individual baseline 20% (or 50% for at least 1 min) can cause myocardial ischemia and reduction of 30% (or 50% for at least 5 min) – kidney injury.¹⁵

Neurocognitive disorders Mini-mental state examination (MMSE) is widely used for neurological evaluation because of simplicity to perform it and to interpret the result. It can help to detect dysfunction of different cortical regions which may be damaged by hypoperfusion. MMSE is based on questions used to assess main cognitive functions: concentration, memory, reaction, analysis, verbal fluency and motor capacity.⁷ Overall, 0,004% incidence of serious postoperative neurocognitive complications after shoulder surgery in the beach-chair position is described, although the episodes of desaturation are much more frequent.^{3 9} In elderly patients, the cognitive reserve is scarce because of atherosclerotic alteration of vessels reactivity and neuroendocrinologic changes.

Monitoring of cerebral oxygenation The incidence of cerebral desaturation event varies from 28.8 to 41.1%.^{9 16} Cerebral oxygenation should be monitored and optimized in all patients during surgery in sitting position. a few different methods are proposed for the detection of cerebral ischemia. Real-time electroencephalogram has a good sensitivity and reproducibility, however it can be slowed-down by anesthesia and only attending-level neurophysiologist can interpret it.⁷ Evoked potentials monitoring can detect regional ischemic brain damage, but anesthetics depress the responses and make analysis imprecise.¹⁰ Transcranial doppler ultrasound assess the blood velocity of middle cerebral artery, but it could not be performed if temporal bone window is absent, it doesn’t reflect total cerebral perfusion and ischemia.¹⁷ Near-infrared spectroscopy allows measurement of cerebral oxygenation in non-invasive, inexpensive and continuous manner and may detect desaturation events before irreversible ischemic damage occurs.¹⁸

Anesthesia Shoulder arthroscopy can be performed under regional blockade with sedation, general anesthesia or a combination of these two techniques. Each of them has specific advantages but also specific negative effects on blood pressure and oxygenation control.¹⁹ Verbal contact with lightly sedated patients allows recognizing inadvertent events timely. Presumptive larger decrease of MAP can be achieved by applying general anesthesia; however higher incidence of cerebral desaturation events was found in this group.^{20 21} Preoperative ISBPB combined with general anesthesia for arthroscopic shoulder procedures could be beneficial in maintaining haemodynamic stability.²² Preoperative assessment by anesthesiologist is crucial for detection of the coexisting diseases and potential difficulties related to intra-operative positioning. During anesthesia in sitting position, the task is to respect lower limits of MAP, to take correct measurements and to adjust the parameters to maintain cerebral perfusion.

Summary During shoulder arthroscopy, patient’s position and arterial blood pressure, as well as the type of anesthesia and ventilation management has to be considered. Continuous monitoring of cerebral oxygenation can prevent devastating outcomes and ensure safety for the patient.

References

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