Pharmacology, Physiology, and Monitoring in the Perioperative Area - Essay Example

ORDER No: 496483 PHARMACOLOGY, PHYSIOLOGY AND MONITORING IN THE PERIOPERATIVE AREA (THEATRES) The type of major surgery that will be describedis the repair of an abdominal aortic aneurysm [AAA]. The aorta is the biggest blood vessel in the human body; it emerges from the left ventricle of the heart (Holmes-Walker, 2004) and travels down through the chest to a point roughly at the level of the navel, when it divides into the two iliac arteries which supply blood to the lower limbs. Although the aortic walls are thick and elastic in order to cope with the high interior blood pressures, a number of contributory factors – such as age, high blood pressure, smoking or cardiovascular disease – can cause weakening and swelling of the vessel walls (Adams, 2003). If the swelling becomes greater than about 5cm in diameter, the danger of rupture is considerably increased and, although subject to considerable variation, figures show that “ once the aneurysm has ruptured, the chances of survival are low, with 80 to 90 percent of all ruptured aneurysms result in death ( SIR, 2010). At this stage, therefore, surgical intervention is indicated, and there are currently two different techniques available:- (1) Surgical Repair – in which the weakened wall of the aorta is repaired by open surgery, particularly where the damage is extensive, and the damaged region is replaced by a fabric tube or graft (Adams, 2004; Holmes-Walker, 2004) (2) Endovascular [stent-graft] repair – a less invasive method in which a metal/fabric or plastic stent is inserted via an incision in the femoral artery using radiological imaging to guide the stent to the appropriate location within the aneurysm (Adams, 2004; Holmes-Walker, 2004; SIR, 2010). Although both methods are used successfully the individual needs and condition of the patient will dictate which is more appropriate. From the 1990s endovascular stent-graft repair has become more popular particularly for older patients since a shorter stay in hospital is involved and morbidity is considerably lower. However in most cases, especially where the aneurysm is well advanced, open surgical repair is used. The procedure does entail more invasive intervention, though, with consequent possibility of damage to surrounding organs, the brain and the spinal cord (Adams, 2004). Thus, due to the high degree of trauma, a greater degree of invasive monitoring is required. Open surgical repair will now be discussed, and it will be the intention in this essay to discuss the risks involved, the choice of monitoring techniques as well as the drugs used in this method together with their effect on the physiology of the patient. Any surgery carries a risk, generally proportional to the complexity of the operation, and it is appropriate now to consider the major risk factors at the various stages of the procedure, which are as follows:- (1) Preoperative (a) Age/sex – males are more vulnerable (b) Heredity – a first degree relative who has experienced a ruptured aneurysm (c) Rate of growth of the aneurysm (d) Blood clot in the aneurysmic area (e) High levels of MMP-9 in the blood (f) Smoking and/or atherosclerosis history. (2) Perioperative (a) Rupture of the aneurysm (b) Pulmonary infection (c) Cardiac arrhythma (d) Acute renal failure (e) Myocardial infarction (3) Postoperative risk factors These are some of the complications which have been experienced in the post operative stage but may have been caused by events during the preoperative and/or perioperative stages (a) Age (b) Myocardial ischemia (c) Positive fluid balance (d) Renal insufficiency (e) Postoperative bleeding (f) Aortic cross-clamping. Having considered the possible risks, it is now time to discuss in more detail the factors which contribute to the successful outcome of such a major surgical procedure Patients have a greater chance of tolerating invasive surgery if they are received in the operating room with a cardiac index of >2, a SVR [Systematic Vascular Resistance] of about 1000 and a PAWP [Pulmonary Artery Wedge Pressure] optimized based on urine output and cardiac output” (Brantigan, 2001). The value of ensuring that volume loading is achieved in the preoperative stage using hemo- dynamic loading has been emphasised by many authorities. “With monitor guided volume loading the incidence of hypotensive episodes is decreased, renal dysfunction is decreased and mortality decreases as well… Preoperative preparation is better than catch up during surgery” (Brantigen, 2001). The importance of ensuring that hemodynamic monitoring is used appropriately and in the right circumstances is also underlined by Darovic (2002). The wisdom of such preoperative haemodynamic tuning and the consequent benefit to the physiology of patients undergoing surgery is demonstrated by a survey of 100 elderly patients in Australia (Older & Smith, 1988). While in the ICU [Intensive Care Unit] the following tests and instrumentation were administered:- (1) A twelve point ECG (2) Insertion of a silastic urinary catheter (3) The provision of an arterial line (4) The introduction of “a pulmonary artery flotation catheter, usually into the left subclavian vein. The reference point for zero pulmonary artery pressure was established” (Older & Smith, 1988), and the location [of the transducer] marked so as to permit repositioning peri- and postoperatively. A number of significant physiological imbalances were discovered, some of which were severe enough to require postponement or cancellation of the planned surgery:- (a) “Eleven percent had a cardiac index of 2.2l/min/m2 or less [2.41-2.51is normal] (b) Fourteen percent had…hypertension…with a diastolic pressure of over 100 mmHg” (Older & Smith, 1988) (c) Mean oxygen consumption [for 70% of patients] of 121 ml/min/m2 preoperatively rose to 174 ml/min/m2 postoperatively; a figure indicating a significant cardiac output (d) Some patients showed “poor respiratory status…five with significant pulmonary artery hypertension” (Older & Smith, 1988) In order for this type of major surgery to be carried out effectively it is necessary for as much information as possible to be provided at all stages of the operation, and this is best achieved by the technique known as “invasive monitoring”, which operates most effectively at the perioperative stage. Two main procedures are currently available; by means of:- (1) Central line – a catheter providing “ venous access via the superior vena cava or right atrium” (Harvey, 2004) - inserted into the jugular or subclavian arteries in order to measure CVP, and “to serve as a guide to fluid balance…to estimate the circulating blood volume ..[and] to assist in monitoring circulatory failure” (Harvey, 2004). (2) Arterial line – a cannula inserted into (a) “radial, femoral, axillary, or brachial artery..useful for measuring output or blood pressure [systolic, diastolic, and mean]…taking blood samples (Nasir, 2009) (b) “subclavian or jugular artery, through the heart into the pulmonary artery. Allows measurement of PCWP…cardiac output and mixed oxygen venous saturation” (Nasir, 2009). Used for continuous blood pressure monitoring when inotropic or vasoactive drugs are introduced (Harvey, 2004) Open AAA repair requires the blood supply to the aorta to be temporarily interrupted, and this is usually achieved using a bypass system whereby the blood flow is diverted from the heart and the site of the operation. Each surgical procedure will depend upon the “pre-history” of the patient and, to some extent, the consequent complexity of the operation. This means that this type of surgery may be expected to last from about 3-8 hours. Thus, in order to minimise damage to organs – especially the brain – the blood flow in the bypass is cooled [to below 34.5oC] so as to introduce a measure of hypothermia. However, marked hypertension or prolonged cooling were often found to cause some degree of perioperative organ dysfunction; requiring more fluid transfusion, as well as higher vasopressor and inotrope doses (Bush, 1995). In order to overcome these disadvantages, significant improvements were observed by warming the patient during the perioperative stage. “Warming of surgical patients reduced postoperative wound pain, infection, shivering and blood loss” (Sajid et al., 2009). It was also noted that traditional warming methods: cotton blankets “heat moisturiser exchangers, fluid-circulating warming mattresses are rarely effective in maintaining normal temperature. The “Bair Hugger System, consisting of a warming unit and disposable warming blankets” was found to be more effective and posed no discernable risk of infection (Huang, 2003). Warming solutions were also found to be useful. It is now appropriate to consider the range of drugs commonly used in this type of surgery and their physiological effect on the patient. During the surgical procedure for open AAA repair two types of drug are administered: (1) anaesthetics to induce and maintain unconsciousness and (b) drugs which are designed to minimise adverse effects from the surgery on the patients physiology 5.1. Anaesthesia General anaesthesia comprises a number of stages (a) induction - started in the preoperative stage - in which unconsciousness is induced, generally by intravenous injection of a drug such as thiopentone, sometimes in conjunction with analgesics to facilitate the procedure (Raftery, 1992; Markovitch, 2007). Thiopentone, due to its strong anticonvulsant properties, is also indicated for patients susceptible to epileptic seizures, (b) maintenance of anaesthesia - during the perioperative stage - by inhalation of oxygen or nitrous oxide, or by volatile agent such as isoflurane introduced by tube or inhalation [more effective than the traditional halothane since it preserves the “total hepatic blood flow” (Gatecel et al., 2003), (c) reversal or recovery phase [postoperative] in which the effects of any muscle reactants are reversed, volatile agents are withdrawn and “the patient breathes oxygen or oxygen- enriched air” (Markovitch, 2007) The anaesthetic process is becoming more sophisticated and now forms part of the basic training for nurses, especially those intending to become theatre nurses. Many texts on the subject have been produced (Simonsen et al., 2001; Aitkenhead et al., 2001; Allman & Wilson, 2006) Irrespective of the drugs taken by patients before surgery three main drug types are used in open abdominal aortic aneurysm repair during the perioperative stage are “ vasopressors” [vasoconstrictors] and “inotropes”, and both have the effect on the physiology of the patient of combating hypotension (Gilmore, 1999). There are two main classes of vasoconstrictor drugs: (1) sympathominetic – such as methoxamine and oxymetazoline (2) vasopressin analogs – such as arginine vasopressine [AVP] and triglycl lysine vasopressin [Terlipressin] (Klabunde, 2007). All of the drugs in this group operate by acting either directly or indirectly on the SNS, but the mechanism will depend on which of the sympathetic receptors for which the drug has the most affinity. Within this group are noradrenaline and metaraminol; the latter also causing the release of noradrenaline and adrenaline. Noradrenaline has the property, when administered, of raising the blood pressure The function of vasoconstrictors is to raise the blood pressure temporarily, and this is performed “by acting on the receptors that constrict peripheral blood vessels…[they] are also used with anaesthetics to counteract the latter’s vasodilatant effect. Adrenalin, (Markovitch, 2007). In the case of inotropes – while it is accepted that all medication will have different effects on the physiology of patients – their range of physiological effects is wider (Brantigen, 2001). This is a class of drugs which have an influence on the force of the heart muscle contraction, and include Dopamine, Dobutamine and Digoxin (ICU-USA, 2004; By contrast beta-blockers such as Propanolol have the reverse effect in reducing the…force of the contraction (Markovitch, 2007). The third type of drug used is atropine, whose main functions are to reduce “bronchial and salivary secretions, block the bradycardia associated with some drugs used in anaesthesia such as suxamethonium (Pinto Pereiro, 1996). Atropine is generally administered either intravenously or intramuscularly; in both cases although the drug is introduced at the preoperative stage its effects manifest themselves perioperatively. The main effects on the physiology of the patient – in addition to those already mentioned – include a rise in heart rate with occasional tachycardia accompanied by “relaxation of smooth muscle in the gut, urinary tract and biliary tree…[side] effects in the elderly may include amnesia, confusion and excitation. Pupillary dilation and paralysis of accommodation occur, with an increase in intraocular pressure” (Pinto Pereiro, 1996). A small additional incremental dose is able to stabilise the occasionally observed slowing of the heart rate. Metaraminol [Aramine] is used in the prevention of hypotension, and side effects, including brain trauma, may persist postoperatively. Whilst the main thrust of this essay is concerned with the pre- and perioperative stages, the recovery – or postoperative - stage is of considerable importance in ensuring the greatest chance of the patient making a smooth and speedy recovery. Here the contribution is likely to be that of specialist nurses familiar with the demands of vascular disease, and its accompanying complications, rather than of the surgeon and anaesthetist, who are most concerned with the pre- and perioperative stages. An excellent and comprehensive handbook specifically directed at nurses is that written by Al-Khaffaf & Dorgan (2005). This essay has covered the factors surrounding open AAA repair, and extends from the initial condition and risk factors of individual patients, the extent and type of aneurysm and the procedures adopted for invasive monitoring the patients physiological outputs at the perioperative stage. This is followed by a consideration of the types of drugs used, and the reasons for which they are used. The story begins with types of anaesthetic: preoperative injection to induce unconsciousness, inhalation of gaseous medication or intubation for liquids such as isoflurane, and the procedure for monitoring and maintaining the required level of anaesthesia, followed by the method of bringing that patient back to consciousness. The third, in the pre- and perioperative stages is the cocktail of drugs administered: these include vasopressors and inotropes and atropine, together with their effect on the patient’s physiology and their contribution to a successful outcome from the procedure. 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