1. University College of Medical Sciences & GTB Hospital, Delhi
INDUCED HYPOTENSION
Dr. Emeni Ophrii
University College of Medical Sciences & GTB Hospital, Delhi

2. Induced Hypotension Definition and objectives Historical background
Purpose
Indications
Techniques with physiological basis, drugs
Contraindications
Limitations
Post-operative care
Specific applications
Current status

3. Definition
A technique that lowers blood pressure (BP) to reduce blood loss and the need for transfusion during surgery and to improve the quality of surgical field
Also known as deliberate hypotension / controlled hypotension / hypotensive anaesthesia

4. Objectives Fall in systolic BP to 80-90 mmHg
Fall in mean arterial pressure(MAP) to mmHg
Fall in 30% of MAP in hypertensive patients
In majority of cases, optimum operating conditions are the goal, not a specific pressure → good communication between anaesthetist and surgeon is essential

5. Historical background
Harvey Cushing proposed in 1917
Gardner practised first in 1946
Griffith and Gillies used high spinal analgesia
Enderby used ganglionic blockers
Trimetaphan
Sodium nitroprusside(SNP)-1962
Halothane
β-adrenergic blockers
Nitroglycerine and labetalol-1970

6. Purpose Less infection transmission Reduction in blood loss
Haemodynamic and metabolic stability
Less transfusion requirements
Less infection transmission
Improvement of surgical field
Improved visualization → better and faster surgery
Facilitation of vascular surgery
Prevention of progressive stretching of thin walls of vessels → easier suturing and clipping (aortic surgery, AVM, intracranial aneurysm clipping )

7. Indications
Neurosurgery (AVM, intracranial aneurysm, vascular tumours, trans-sphenoidal hypophysectomy)
Major orthopaedic surgery (spinal surgery, shoulder/hip arthroplasty)
ENT surgery (middle ear surgery)
Head and neck (faciomaxillary tumours, block dissection of neck, laryngectomy)

8. Indications…
Ophthalmic surgery (intraocular tumours, vitrectomy, orbital surgery)
Pelvic (Major gynaecological or rectal surgery)
Plastic and reconstructive surgery
Jehovah’s witness

9. Techniques Physical measures
a) Postural manoeuvers - keeping operated area higher than heart
Risk: Air embolism
b) Artificial ventilation - lowers BP secondary to decreased venous return
Pharmacological agents

10. Effects of posture on local blood pressure

11. Pharmacological agents
Characteristics of an ideal agent
Easy to administer
Short onset time
Effect should disappear quickly on discontinuation
Rapid elimination
No toxic metabolites
Negligible effects on vital organs
Predictable effect
Dose dependent effect
No ideal hypotensive pharmacological agent exists

12. Classification Alone or as adjunct Used alone As adjunct only
Calcium channel blockers(nicardipine)
β-blockers (propanolol,esmolol)
Fenoldopam
As adjunct only
ACE inhibitors
Clonidine
Used alone
Inhalational anaesthetics
Sodium nitroprusside
Nitroglycerine
Trimetaphan
Prostaglandin E1 (Alprostadil)
Adenosine
Remifentanil
Spinal/epi anaesthesia

13. Principle of Induced Hypotension
BP = Cardiac Output (CO) x Peripheral Vascular Resistance (PVR)
CO = HR x SV
PVR α 1/r4
Reduction in CO should be avoided as organ perfusion may be compromised
Induce a low pressure, vasodilated circulation

14. Physiological effects of induced hypotension
Marked physiological response designed to restore BP back to within normal range
Central neuronal reflexes
Arterial baroreceptors
Cardiopulmonary receptors
Hormonal changes
Renin-angiotensin system
Catecholamines

15. Physiological effects...
Reflex rise in HR, myocardial contractility, vasomotor tone → Low pressure, hyperdynamic circulation
May result in increasing vasodilator doses
May require use of adjuncts

16. Anaesthetic Agents Drugs Site of action Predominant action Side effect
Bupivacaine (spinal/ epidural)
Medulla
Blockade of sympathetic nervous system
May need vasoconstrictor
Ropivacaine (spinal/epidural)
Opioids: remifentanil
Heart: bradycardia
none
Inhalational anaesthetics (isoflurane, sevoflurane)
Vessels: vasodilatation
Blockade of α-adrenoceptors
Resistance
Need high concentration of adjuvants

17. Vasodilators Drugs Site of action Predominant action
Side effect /Problem
Sodium nitroprusside
Resistance/ capacitance vsls:
Vasodilatation
Direct acting
Cyanide toxicity
Srict monitoring
Nitroglycerine
Capacitance vessels: vasodilatation
Resistance, strict
monitoring
PGE1 (Alprostadil)
Resistance vessels: vasodilatation
Heart -↓chronotropic
cost
Calcium channel blockers (Nicardipine)
Heart -↓ionotropic
none
Fenoldopam
Dopamine DA1 receptor agonist
Adenosine
Cost
Histamine release

18. Autonomic nervous system inhibitors
Drugs
Site of action
Predominant action
Side effect
Trimetaphan
Vessels: vasodilatation
Heart: ↓ contractility
Blockade of the ganglia of ANS
Histamine release
Resistance
Labetalol
α/β adrenoceptor antagonist
Slow onset
Esmolol
Heart: bradycardia
↓ contractility
β-adrenoceptor antagonist
Cardiac failure
Clonidine
CNS
Presynaptic α-adrenoceptor agonist
Unpredictable effect
ACE inhibitors

19. ACE inhibitors Drugs Site of action Predominant action Side effect
Captopril, enalapril
Vessels: vasodilatation
Angiotensin ІІ inhibitors
Long duration of action

20. Sodium nitroprusside (SNP) Nitroglycerine (NTG) Labetalol
Characteristics
Sodium nitroprusside (SNP)
Nitroglycerine (NTG)
Labetalol
Onset of action
Rapid onset,
Rapid recovery
Rapid onset, moderate slow recovery
Gradual onset,
Slower recovery
Duration
Evanescent action
Short acting
Long acting
Method of administration
IV drip (0.01% solution)
IV drip (0.01% in 5%D/W or 0.9% NS)
IV injection repeated in increment
Mode of action
Direct effect: resistance / capacitance vessels
Direct : capacitance vessels predominantly
α and β adrenoceptor antagonist
Tachycardia
Very common
May occur
None
Cardiac output
Depends on many factors (posture, HR, preload, afterload, drugs,volume status)
Slight decrease
Blood brain barrier
Pronounced dysfunction

21. Sodium nitroprusside (SNP) Nitroglycerine (NTG) Labetalol
Characteristics
Sodium nitroprusside (SNP)
Nitroglycerine (NTG)
Labetalol
Metabolism
To cyanide, thiocyanate
Degraded rapidly in liver
Degraded in liver
Stability
Available as powder. Unstable when reconstituted. Protect from light & use within 12 hrs
Stable, colourless,
Absorbed by plastics
Stable
Dose
μg/kg/min
Total dose not to exceed 1.5mg/kg in 4 hrs
Plasma CN conc should not exceed 3 μmol/l
Start μg/min,
1-4 μg/kg/min
mg/kg followed by increment
Histamine release
unknown
Rebound hypertension
Occurs in absence of β blockade
Does not occur
Intracranial pressure
Increases in early stage
Increases
Does not increase

23. Cyanide Toxicity - Mechanism

24. Cyanide toxicity - Manifestations
Resistance to high infusion rates of SNP
Metabolic acidosis, raised lactate levels
Elevated venous oxygen saturation
Progressive hypotension unresponsive to fluids & vasopressors but responsive to thiosulphate
Cardiovascular collapse
No specific laboratory test but lethal cyanide level in blood is approx 500 μg/dl

25. Cyanide toxicity - Management
Sodium thiosulphate
Bolus injection - 30mg/kg
Continuous infusion - 60mg/kg/hr
Hydroxocobalamine (B12)
Prevents ↑ in CN conc. in erythrocytes
Bolus - 50mg/kg
Infusion - 100mg/kg/hr
Fluid replacement
Acidosis correction

26. Monitoring during induced hypotension
ECG – myocardial perfusion, drug effects
Accurate BP monitoring
Gold standard is direct intra-arterial measurement
Capnography – disconnection alarm, indicator of air embolism
Value as an index of PaCO2 is limited due to ↑dead space
Pulse oximetry – allows adjustment of FiO2 to compensate for V/Q mismatch
EEG/evoked potential – reserved for neurosurgery, CPB, severe hypotension
Transcranial doppler velocimetry

27. Safe conduct of induced hypotension
Onset & degree of hypotension
A reliable intravenous line
Horizontal position
Monitoring
Initiate only after intubation & steady anaes state
Hypotension to be induced slowly over min
Proper posture and optimal venous drainage
Adequate fluids but prevent overhydration

28. Safe conduct… Maintenance of near normal PaCO2 & acid base balance
Hypocapnia – vasoconstriction
Alter blood PH
Increase oxygen consumption
Inhibit hypoxic pulmonary vasoconstriction

29. Safe conduct… Oxygenation Increased intrapulmonary shunt
Decreased cardiac output
A high FiO2 ( or higher) during induced hypotension is recommended

30. Contraindications Inexperience Infants Pregnancy Fixed cardiac output
Coronary artery, renal, cerebral disease
Hypovolemia
Severe anaemia

31. Limitation
Risk of tissue hypoxia by reducing microcirculatory autoregulation of vital organs and by inhibiting ANS
The goal is to maintain a pressure sufficiently low to allow reduction of bleeding without suppressing microcirculatory autoregulation of vital organs

32. Complications Coronary artery thrombosis Cerebral thrombosis
Cardiac arrest
Temporary & permanent neurologic deficit
Failure of technique
Resistance & rebound
Post-operative reactionary haemorrhage
Tissue trauma

33. Post-operative care Maintenance of airway
Provision of supplemental oxygen
Analgesia
Warm environment
Respiratory & cardiovascular monitoring
Adequacy of blood volume
Positioning

34. Specific applications
Neurosurgery - Cerebral aneurysm surgery
Spinal surgery- scoliosis correction
Ophthalmic surgery
ENT surgery
Pregnancy

35. Cerebral aneurysm surgery
Low pressure circulation
Reduces blood loss → improves visualisation
Reduces aneurysm transmural pressure
Safe application of clip
Reduces risk of intra-operative rupture
If aneurysm ruptures → less bleeding, rapid identification & control of bleeding point
Vasodilators started after opening dura to avoid ↑in ICP
Avoid the technique in presence of preoperative vasospasm and if a feeding artery is to be temporarily clipped

36. Scoliosis surgery General anaesthesia
Monitoring of spinal cord function using wake-up test or somatosensory evoked potential
Blood loss is massive → BP to be reduced slowly (preferably under SSEP monitoring) and only to the point at which operating conditions become satisfactory

37. Ophthalmic surgery ENT surgery
Uveal dissection - prevention of bleeding into vitreous
Choroidal melanoma - requires period of choroidal ischaemia
ENT surgery
High level of difficulty and impossibility of controlling bleeding by using clamps on afferent vessels, vertebral-basilar artery and external carotid artery makes induced hypotension a choice in ENT surgery

38. Pregnancy ↓BP → ↓ uterine blood flow → fetal asphyxia
Hypotension not to be induced unless considered as life saving measure.
Nitroglycerine - least toxic
Close fetal heart rate monitoring mandatory – serves as a guide to safe level for hypotension and hyperventilation

39. Current status Surgeries with low/moderate bleeding potential
e.g. middle ear, eye, neurosurgery
Induced hypotension used in light of benefit/risk ratio
Surgeries with moderate bleeding potential
e.g. vascular, major orthopaedic surgery
Benefit from combination of hemodilution with autologous transfusion with induced hypotension
Surgeries with high bleeding potential
e.g. cardiac surgery/liver transplantation
Use of induced hypotension has decreased

40. Current status…
The best efficacy combined with ease of use and lack of toxicity belongs to techniques of hypotensive anaesthesia that associate analgesia and hypotension at clinical concentration
Epidural anaesthesia – not always practical
Remifentanil along with propofol or inhalational agents at clinical concentration are preferred techniques because of their safety and ease of use

41. References
Wylie and Churchill Davidson’s ‘A Practice of Anaesthesia’ 6th edition
Wylie and Churchill Davidson’s ‘A Practice of Anaesthesia’ 7th edition
Miller’s Anesthesia 5th edition
International Practice of Anaesthesia by Prys-Roberts and Brown
Degoute CS. Controlled hypotension. Drugs 2007; 67(7):