Published in Current Opinion in Anaesthesiology: February 2015 – Volume 28 – Issue 1 – p 2–9
Authors: Kozian, Alf et al
Purpose of review: The mean age of patients presenting for thoracic surgery is rising steadily, associated with an increased demand for thoracic surgical treatments by geriatric patients. With increasing age, physiologic changes and comorbidities have to be considered. Thoracic anesthesia for elderly patients requires greater specific knowledge.
Recent findings: Respiratory mechanics change progressively during aging, and the pharmacology of different drugs is also altered with increasing age. This has implications for the preoperative, intraoperative and postoperative management of elderly patients scheduled for thoracic surgery. Special focus has to be placed on preoperative evaluation, the ventilation regime and general intraoperative management. Effective postoperative pain treatment after geriatric thoracic surgery requires careful pain assessment and drug titration.
Summary: Considering key points of physiology and pharmacology can help to provide best possible care for the increasing number of elderly patients in thoracic surgery. Management of geriatric patients in thoracic surgery offer opportunities for anaesthetic interventions including protective ventilation, use of different anesthetics, anaesthesia monitoring, fluid management and pain therapy.
Lung cancer is one of the leading causes of cancer death amongst men in all European countries, except Sweden. Women die from lung cancer more commonly than from breast cancer in a growing number of European countries. The median age of patients presenting with lung cancer is 71 years. In octogenarians, lung cancer is the major cause of cancer-related death. Miyazaki et al. and others have recently reported of successful pulmonary surgery in nonagenarians, which demonstrates how far the age barrier is currently being pushed. The main comorbidities are smoking history, pulmonary obstructive diseases, hypertension, coronary artery disease and BMI above 30. Congestive heart failure and renal insufficiency are reported below 5%.
Surgery for lung cancer may, however, result in serious life-threatening complications, and surgery itself may become the cause of death . Perioperative mortality is reported to be about 5.6%, with age above 65 years, presence of preoperative congestive heart failure and forced expiratory volume in 1 s (FEV1) less than 60% (predicted) as strong risk factors for adverse events. Anaesthetic management thus considers the age-related physiological changes and comorbidity (concomitant diseases) in geriatric patients, as well as acute and chronic pulmonary diseases. Additionally, the interference of the central anesthesia target, that is the lung, with surgical manipulation requires particular anaesthetic techniques including partial or complete airway separation, and is commonly associated with profound pathophysiological changes that may affect the postoperative outcome.
The management of geriatric patients undergoing thoracic surgery may offer opportunities for anaesthetic interventions including protective ventilation approaches, use of volatile anaesthetics, anaesthesia depth monitoring, restrictive fluid management and sufficient pain therapy.
PHYSIOLOGICAL CHANGES TO THE AGING RESPIRATORY SYSTEM
As a consequence of muscle atrophy and decreased elasticity of the connective tissues, increasing age leads to progressive deterioration of various functions of the respiratory system.
The upper airway is often altered by atrophy of the hypopharyngeal and genioglossal muscles, leading to obstruction of the upper airways associated with obstructive sleep apnoea. The risk of occult aspiration is increased by dampening of protective cough and swallow reflexes. Neurological disorders such as stroke or Parkinson’s disease may further reduce these reflexes.
Respiratory mechanics is altered in the elderly due to the loss of elasticity of the thoracic wall, caused by degeneration of the costovertebral joints and reduced width of the inter-vertebral spaces. Total muscle mass is reduced leading to reduced strength of the respiratory muscles. The diaphragm flattens resulting in lower trans-diaphragmatic pressure. All these factors can lead to early exhaustion of the respiratory muscles under strain; one of the reasons that elderly are more difficult to wean from the respirator. Congestive heart failure may be another cause for respiratory muscle weakness, as described by Hammond et al.
The FEV1 as well as the forced vital capacity (FVC) decline around 30 ml per annum, beginning at an age of 27 years for male patients and of 20 years for female patients, whereas the total lung capacity (TLC) remains more or less unaltered, leading to an increase of the residual volume.
The gas exchange area and capillary lung perfusion decrease with age, and permeability of the alveolo-capillary membrane is reduced, together resulting in an increased right to left pulmonary shunt and consequently reducing the arterial partial pressure of oxygen (paO2). The partial pressure of carbon dioxide (paCO2) is practically unaltered by these changes. The normal paO2 can be estimated by the equation paO2 = 103 − 1/3 age. In addition, geriatric patients are at risk of a reduced breathing incentive if they become hypoxic or hypercarbic.
PREOPERATIVE ANAESTHETIC EVALUATION
Respiratory complications significantly contribute to the outcome after thoracic surgery; therefore the preoperative lung function has to be evaluated carefully by clinical examination and lung function testing. The single most important parameter is FEV1. Patients with an FEV1below 70% (predicted) offer an increased incidence of postoperative pulmonary complications. The most important variable is the global cardio-pulmonary reserve. A maximal oxygen consumption less than 10 ml/kg/min and the inability of the patient to climb two flights of stairs indicate a high perioperative risk.
Evidence has been provided that premedication with benzodiazepines may be associated with an increased incidence of postoperative cognitive dysfunction (POCD). Whether or not alternative regimes with, for instance, oral clonidine have an effect on POCD has, to date, not been assessed in randomized controlled trials. Data obtained from patients in ocular surgery point to improved recovery after premedication with clonidine.
The anaesthetic framework includes induction of anesthesia with a hypnotic drug, a potent opioid and a muscle relaxant, and is maintained either with an inhalational anaesthetic (desflurane or sevoflurane) or with propofol and an opioid. Most patients benefit from early tracheal extubation after surgery. For that reason, remifentanil combined with a regional technique such as thoracic epidural anaesthesia (TEA) or paravertebral blockade may be an advantageous anaesthetic strategy.
Intravenous fluid should be given with care to avoid pulmonary oedema. Goal-directed fluid management has the potential to reduce and optimize overall fluid administration. The elderly have a decreased ability to tolerate haemodynamic instability; thus, patients with compromised cardiac function should be more readily treated with positive inotropic drugs .
Elderly patients show increased sensitivity to anaesthetic drugs. Altered pharmacokinetics, pharmacodynamics or both are to be held responsible. Reasons can be found in a decreased volume of distribution, which results in a greater plasma concentration for a given dose, decreased clearance accompanied by increased brain sensitivity. In general, the required minimal alveolar concentration (MAC) of different anesthesia gases is also reduced, and errors in adjusting the volatile concentration will often result in depressed cardiac output and loss of systemic blood pressure. In a study in orthopaedic patients, anesthesia depth monitoring led to reduced vasopressor use and, moreover, to reduced wake-up times. The adequate dosage of analgesic and hypnotic components can be facilitated with anesthesia depth monitoring, thereby attempting to avoid under-dosage-associated awareness.
Thoracic surgery patients are especially at risk for perioperative hypothermia; furthermore, thermoregulatory mechanisms are compromised with increasing age. The total muscle mass is often reduced and a slower metabolism rate leads to less thermogenetic capabilities. Moreover, the shivering threshold during regional anesthesia, especially during spinal and thoracic epidural blockade, is reduced in geriatric patients. Therefore, these patients are at risk for hypothermia-related consequences such as increased perioperative blood loss, increased incidence of myocardial ischaemia, delayed drug metabolism and prolonged Post-Anaesthetic Care Unit admission, as well as delayed wound healing and higher rate of wound infections.
Ventilation of the operated lung following separation of the airways needs to be discontinued temporarily to facilitate thoracic surgery. As a consequence, only about half of the gas exchange area is at one’s disposal, and severe intraoperative hypoxemia was the complication that was feared most in the past. Modern lung separation devices and monitoring techniques, as well as careful preoperative evaluation, enable early detection and anticipation of oxygenation deficits. The focus of intraoperative ventilation has therefore shifted towards a more protective approach, as opposed to the classical ventilation strategy for one-lung ventilation (OLV). For this, tidal volumes (VT) in the range of 8–10 ml/kg and zero end-expiratory pressure to maintain arterial oxygenation and carbon dioxide elimination were used, but this approach has lost its validity.
Moreover, several anesthesia-related risk factors for respiratory failure have been identified, including intraoperative high-tidal-volume ventilation with extremely high ventilation pressures, as well as excessive fluid load. In combination with age-related physiological changes in geriatric patients and pre-existing cardiopulmonary morbidity, anesthesia and surgery-related conditions such as duration of the procedure and the extent of lung resection may worsen the outcome.
As a result, OLV itself may be injurious in terms of increased mechanical stress characterized by alveolar cell stretch and over-distension, increased cyclic tidal recruitment and compression of alveolar vessels. It may thus result in ventilator-induced lung injury with development of permeability-type pulmonary oedema, leukocyte recruitment, cytokine release and neutrophil-dependent tissue destruction.
Alveolar recruitment manoeuvre and PEEP
Atelectasis formation and increased intrapulmonary venous admixture (i.e. intrapulmonary shunt) are common problems during OLV, which may result in intraoperative hypoxemia. The alveolar recruitment manoeuvre (ARM) followed by sufficient Positive End-Expiratory Pressure (PEEP) application to keep the recruited lung regions open improves the pulmonary gas exchange, especially during OLV. PEEP is valuable not only to prevent the lungs from atelectasis formation but also to treat hypoxemia during OLV; more importantly, as part of a protective ventilation strategy, it may decrease cyclic alveolar collapse and reopening. Evidence exists that only a single ARM (30 cmH2O) followed by sufficient PEEP application (5 cmH2O) reduces the amount of atelectasis by 60%.
One-lung ventilation results in increased cyclic recruitment and de-recruitment of the alveolar units, suggesting increased shear stresses with extensively elevated trans-mural pressures. Shearing and stretching have been detected as key factors in initiating an alveolar injury. The highest tidal recruitment is found in the most dependent and basal regions of the lung, whereas accompanying overstretching may occur at the boundaries of atelectatic, and poorly or normally aerated tissue. The cyclic recruitment can be estimated by comparison of the gas/tissue content between expiration and inspiration.
Effects of different tidal volumes during one-lung ventilation
The use of relatively high tidal volumes during OLV has been shown to be associated with impaired prognosis after thoracic surgery. Accordingly, the reduction of tidal volume revealed significant effects on the alveolar release of proinflammatory cytokines after OLV and in the postoperative course. Low-tidal-volume ventilation may further protect the ventilated lung from increased tidal recruitment during OLV. OLV with VT as applied during two-lung ventilation (TLV, 10 ml/kg) or even higher, results in a shift of the radiological lung density to regions with increased gas content. In contrast, OLV with half VT (5 ml/kg) did not impair the lung density distribution as compared with TLV before OLV and did not increase cyclic tidal recruitment in the ventilated lung during OLV.
One-lung ventilation may induce epithelial damage and expression of proinflammatory mediators in the alveoli of the ventilated lung. The immune response was attenuated by volatile anaesthetics (desflurane) as indicated by decreased alveolar expression of pro-inflammatory cytokines. TIVA with propofol resulted in higher alveolar cytokine concentrations and in increased alveolar granulocyte recruitment. Recent experimental data revealed that the use of the volatile anaesthetic desflurane not only attenuated the release of alveolar mediators but also decreased the expression of systemic proinflammatory cytokines. Like other halogenated anaesthetics, desflurane and sevoflurane may have protective effects on mechanical forces during OLV. Furthermore, volatiles may modulate granulocyte recruitment and neutrophil activation, suggesting protective effects during OLV-induced lung injury.
Intraoperative arterial carbon dioxide partial pressures
A target paCO2 of 30–35 mmHg has been a widely accepted goal for anesthesia ventilation in the past. This mild hyperventilation has to be viewed as a risk factor for POCD, as hyperventilation results in reduced cerebral blood flow and reduced tissue perfusion and oxygenation. On the contrary, mild hypercarbic levels (paCO2 40–50 mmHg) may improve tissue oxygenation and perfusion.
A crucial part of the perioperative therapy is facilitating the return to normal physiologic conditions and in this respect, the management of pain after surgical procedures plays a decisive role.
Sufficient pain therapy is essential to minimize complications after thoracic surgery. Systemic application of opioids remains the gold standard; however, thoracic epidural or paravertebral analgesia with local anaesthetics and opioids may provide superior analgesia during the first three postoperative days. Apart from sufficient pain relief, TEA offers beneficial effects such as attenuation of cardiac, respiratory and gastrointestinal complications, and positive effects on immune function and the coagulation system, whereas paravertebral analgesia is associated with fewer major complications. A recent study by Kampe et al. has demonstrated a benefit of TEA as compared to systemic controlled-release oxycodone in preventing chronic or neuropathic pain after anteroaxillary thoracotomy, although one has to note that age ranges and complications had not been reported.
Airway alterations lead to an increased risk of hypoxia and aspiration rates in geriatric patients. Therefore, the following aspects of anesthesia management should be considered:
1. At the end of the thoracic surgical procedure, a backlog of premedication drugs, hypnotics, opioids and muscle relaxants has to be avoided.
2. Postoperative pain therapy has to accommodate for the adverse effects of opioids. Outside the ICUs or intermediate care units, their application should be reduced or substituted with regional or local anaesthetic procedures.
Although further studies lack at this point, the following can be recommended for the intraoperative ventilation of geriatric patients:
1. Avoid hyperventilation and hypocarbia;
2. Remember recruitment manoeuvres and ventilation with sufficient PEEP; and
3. Ventilation should aim for tidal volumes of no more than 6 ml/kg.
The following points should be observed regarding temperature management:
1. Generous use of intraoperative warming techniques such as forced air systems, warm infusions and so on; and
2. Target temperature should be above 36.5°C.
The number of geriatric patients with need for thoracic surgical interventions will further increase in the near future, due to higher incidence of pulmonary carcinomas in this cohort. This will bring new challenges for the anaesthetic management in older patients. Thoracic anesthesia has to consider the high perioperative risk, which is based on age-related physiological changes as well as on main comorbidities, such as smoking history, acute and chronic pulmonary diseases, hypertension, coronary artery disease, and on postoperative pulmonary complications.
To enable surgical manipulation, particular anaesthetic techniques including partial or complete airway separation are required, and this is commonly associated with profound pathophysiological changes.
The management of geriatric patients undergoing thoracic surgery offers opportunities for anaesthetic interventions including protective ventilation approaches, use of volatile anaesthetics, anesthesia depth monitoring, restrictive fluid management and sufficient pain therapy that may affect the postoperative outcome.