Hypotensive anesthesia reduces intraoperative blood loss. Studies have demonstrated a 2-4 fold reduction in intraoperative blood loss if mean arterial pressure (MAP) is reduced to 50 mmHg during surgery. If MAP is maintained at 60 mmHg rather than 50 mmHg, blood loss is about 40% greater. Thus the lower the pressure is reduced, the lower the blood loss. Blood loss during hypotensive anesthesia is related to MAP rather than cardiac output or central venous pressure. Postoperative wound drainage is not increased following hypotensive anesthesia. Consequently, most studies have shown a 50% reduction in in-hospital transfusion using hypotensive anesthesia.
By reducing intraoperative blood loss, surgical exposure is enhanced thereby reducing surgical time. Furthermore, the dry surface facilitates penetration of cement into cancellous bone. In a matched-pair study comparing the radiographic appearance of cemented cups, hypotensive anesthesia was associated with improved cement fixation. For this reason, it is possible that hypotensive anesthesia may influence long term outcome of cemented hip arthroplasty by improving the quality of the cement-bone interface.
Hypotensive anesthesia may have other advantages. Firstly, intraoperative blood loss is reduced, so that less fluid is used making fluid overload unlikely. The reduced blood loss limits dilution and consumption of coagulation factors and subsequent postoperative rebound hypercoagulability. This may lessen the risk of DVT.
Hypotensive anesthesia can be induced using either general or regional anesthesia. With general anesthesia, hypotension is achieved using deep inhalation anesthesia which acts to dilate the arterial system and depress cardiac contractility or with vasodilators. The literature suggests that blood loss is reduced to a similar degree whether hypotension is induced with vasodilators alone or by combination of vasodilatation and cardiac depression. The vasodilators act upon both venous and arterial vessels. One advantage of vasodilation is that it maintains cardiac output better through deep inhalation anesthesia. Whether this has any effect on DVT rates or outcome is unknown.
An array of vasodilators have been used including intravenous infusions of sodium nitroprusside, nitroglycerine or adenosine. Other agents include Apresoline, calcium channel blockers and ACE inhibitors. Beta blockers maybe used to control heart rate. Labetalol is a useful agent combining beta blockade with arterial dilatation.
Hypotensive Epidural Anesthesia
This technique was developed to combine the virtues of epidural anesthesia (avoidance of airway problem and reduced rate of DVT) with the benefits of induced hypotension. The technique has been described in detail.
Briefly, it entails injecting 20-25 mL local anesthetic at an upper lumber interspace to provide an extensive epidural block. This produces analgesia to T4 or above and a near complete sympathectomy (including the cardiac sympathetics). The sympathectomy results in a reduction in arterial pressure. Concurrently, a low dose epinephrine intravenous infusion is used to stabilize the circulation. The initial dose is 2 µg/min but the dose is adjusted, in combination with intravenous fluid, to allow the MAP to fall to 50 mmHg while maintaining a stable heart rate. If a central venous pressure (CVP) catheter is inserted, fluid management is simplified by infusing fluid to preserve CVP in a normal range (2-5 mmHg).
With this technique, arterial pressure can be reduced whilst maintaining heart rate, CVP, stroke volume and cardiac output in normal range. In addition, if necessary, patients can be kept awake to monitor brain function. This maybe helpful in patients with neurological impairment in order to document preservation of cognitive function intraoperatively.
This technique can be used in the majority of high risk cases: patients with hypertension, advanced age, ischemic heart disease or patients with poor cardiac function. In-hospital mortality using this technique at the Hospital for Special Surgery is 0.1% which demonstrates that it is not associated with adverse outcome. The technique should be used with caution in patients with valvular heart disease, carotid occlusive disease and in patients with advanced renal disease.
Regional anesthesia is considered the ideal technique for THR as it avoids many of the complex airway problems, reduces blood loss and transfusion requirements and is associated with a lower rate of DVT and pulmonary embolism than general anesthesia. A variety of regional anesthetic techniques can be used.
Spinal anesthesia provides intense rapid onset anesthesia. Bupivacaine provides at least three hours of surgical anesthesia. With larger doses, higher sensory levels can be achieved facilitating induced hypotension. If epinephrine infusions are used during spinal anesthesia, most of the benefits of epidural hypotensive anesthesia can be achieved including a low intraoperative blood loss and DVT rate. Continuous spinal anesthesia is used in some centers but offers little advantage over continuous epidural and may increase the risk of adverse neurological outcome.
Recently, a technique of combined spinal epidural anesthesia has been developed. A small gauge spinal needle is inserted through an epidural needle. The virtues are the rapid onset of spinal anesthesia, decreased risk of local anesthetic toxicity with the flexibility of having an epidural catheter in place.
Either of the three techniques can be used and the advantages of one over another are minor. Epidural catheter techniques offer flexibility to extend or prolong the anesthetic or to provide postoperative analgesia. On the other hand, for straightforward primary THR, spinal anesthesia using small gauge pencil-point needles provides an excellent anesthetic.
Regional anesthesia techniques can be combined with general anesthesia. Following placement of an epidural catheter, patients can be intubated or a laryngeal mask airway inserted. If patients are ventilated, some of the advantages of regional anesthesia over GA are lost. On the other hand, in prolonged procedures such as complex revision surgery, a combined technique makes it easier to manage patients, particularly by eliminating the problem from the dependent shoulder.
Sedation is an important part of management during regional anesthesia. With midazolam, amnesia can be secured. In longer cases, intravenous infusions of propofol, thiopental or midazolam can provide a stable sedated state. Oxygen supplementation should be provided via nasal cannulae or face mask and patients must be monitored with pulse oximetry.
Sharrock NE. Anesthesia. In: Callaghan JJ, Rosenberg AG, Rubash HE, eds. The Adult Hip Philadelphia: Lippincott - Raven Publishers, 1998.