Perioperative Pulmonary Management

Postoperative respiratory physiology in upper abdominal and thoracic surgery

• Thoracic and upper abdominal surgery is associated with a reduction in vital capacity by 50% and functional residual capacity by 30%. Diaphragmatic dysfunction, postoperative pain, and splinting cause these changes.

• Following upper abdominal surgery, patients shift to a breathing pattern where ribcage excursions and abdominal expiratory muscle activities increase. This shift is attributed to decreased central nervous system output to the phrenic nerves, thus inhibiting the diaphragmatic stimulation. A reflex mechanism arising from the sympathetic, vagal, or splanchnic receptors is thought to be responsible. In humans, this reflex inhibition is reversed partially by epidural anesthesia.

• Following upper abdominal and thoracic surgery, the patients maintain adequate minute volume, but the tidal volume is smaller and the respiratory rate increases (ie, rapid shallow breathing). These breathing patterns, along with the residual effects of anesthesia and postoperative narcotics, inhibit cough, impair mucociliary clearance, and contribute to the risk of postoperative pneumonia.

• Other factors that may contribute to increased respiratory complications are electrolyte imbalance (eg, hypokalemia, hypophosphatemia, hypocalcemia), general debilitation, and underlying lung disease (eg, chronic obstructive lung disease [COPD]).

PATIENT AND PROCEDURE RELATED RISK FACTORS

Patient-related risk factors

• Age: Despite early suggestions of an increased risk of pulmonary complications with advanced age, this is not an independent risk factor for pulmonary complications. In a study of patients over 80 years of age, overall 30-day mortality was 6.2%, patients who belonged to ASA class II had less than 1% mortality (Djohovic JL, 1979). Several other studies have shown that age is not a predictor for postoperative pulmonary complication, therefore, surgery should not be declined on the basis of advanced age alone.

• Obesity: Obesity decreases the expiratory reserve volume and functional residual capacity of lungs; morbid obesity causes restrictive lung disease and decreases thoracic compliance and leads to alveolar hypoventilation.

  In severe cases, obesity is associated with pulmonary hypertension, cor pulmonale, and hypercapnic respiratory failure (Pickwickian syndrome). Obesity causes a reduction in lung volume, ventilation-perfusion mismatch, and relative hypoxemia, which are accentuated after surgery. Obesity (ie, body mass index of more than 27 kg/m²) increases the risk of postoperative pulmonary complications and respiratory failure in patients undergoing abdominal surgery, but may not be a risk factor in thoracic surgery.

  A recent review article (Smetana GW, 1999), the risk of postoperative pulmonary complications was not excessive in seven studies of obese patients, who underwent abdominal or peripheral procedures. Several other studies have reported no association between obesity and postoperative pulmonary complications as well. Another recent study (Phillips EH, 1994) did not report excessive pulmonary complications in obese individuals following laparoscopic cholecystectomy.

• General health status: The American Society of Anesthesiologists (ASA) classification and the Goldman Cardiac Risk Index have predicted successfully the pulmonary risk. Patients who have poor exercise capacity are at increased risk of developing postoperative pulmonary complications. In a study by Gerson MC, (1990) inability to raise heart rate by a simple exercise predicted 79% of pulmonary complications.

• Smoking: Current smokers have a 2-fold increased risk of postoperative complications, even in the absence of COPD. The risk is highest in patients who smoked within the last 2 months. Patients who quit smoking for more than 6 months had a risk similar to the nonsmokers. Warner MA et al(1989, prospectively investigated the role of preoperative smoking cessation on postoperative pulmonary complications in patients undergoing coronary artery bypass surgery. Current smokers developed postoperative complication at a rate of 33%, compared with 57% for individuals who quit for less than 8 weeks. The complications rates were 11.9% in never smokers and 14.5% in patients who were ex-smokers of more than 8 weeks.

  Postoperative morbidity was not decreased in patients who quit smoking for less than 8 weeks. The beneficial effects of smoking cessation including improvement in ciliary and small airway function, and decrease in sputum production, occur gradually over several weeks. The increased incidence of postoperative complications in patients who recently stopped smoking has not been shown in other studies. The likely mechanism is that abrupt absence of the irritant effect of cigarette smoke in postoperative period, inhibits coughing and leads to retention of secretions and small airway obstruction.

• Chronic obstructive pulmonary disease (COPD): This is one of the most important risk factors. Patients with severe COPD (FEV₁ less than 40% predicted) are 6 times more likely to have a major postoperative complication. Despite the increased risk, there appears to be no prohibitive level of pulmonary function for an absolute contraindication. The benefits of surgery must be weighed against these complications. A careful preoperative evaluation of patients with COPD should include identification of high-risk patients and aggressive treatment. Elective surgery should be deferred in patients who are symptomatic, have poor exercise capacity or if acute exacerbation is present.

• Asthma: Inadequate control of asthma preoperatively may increase risk of postoperative complications. Optimal asthma control is defined as absence of symptoms and forced expiratory flow (FEV ₁) more than 80% predicted or personal best should be achieved.

• Sleep apnea: Patients with sleep apnea are at increased risk of developing deterioration of sleep disordered breathing, severe hypoxemia, and hypercapnia in the postoperative period. Individuals with sleep apnea are also obese and may present difficulties with endotracheal intubation, or early postoperative upper airway obstruction, requiring reintubation or other therapies.

  In patients with known or suspected sleep apnea, the intraoperative and postoperative use of sedatives and narcotics should be minimized. Careful monitoring in postoperative period is required for worsening of sleep apnea, development of airway obstruction or CO₂ retention. In patients suspected to have sleep apnea, the diagnosis should be confirmed and severity should be assessed preoperatively, with a formal polysomnographic sleep study. The severity of sleep apnea is judged based on apnea-hypopnea index (AHI) and the lowest oxygen saturation during sleep. Whenever possible, patients should be adequately treated with nasal CPAP preoperatively. Furthermore, patients with sleep apnea often benefit from regional anesthesia rather than general anesthesia.

Procedure-related risk factors

• Surgical site: The incidence of complications is inversely related to the distance of the surgical incision from the diaphragm. The complication rates for upper abdominal surgery range from 17-76%. For lower abdominal surgery the rate is 0-5%, and for thoracic surgery the rate is 19-59%. Laparoscopic cholecystectomy is associated with a lower incidence of complications; the mean decrease in FVC was reported at 23%, as compared to 50% with laparotomy.

• Duration of surgery: Patients undergoing procedures lasting for more than 3-4 hours have a higher incidence of pulmonary complication than for surgeries less than 2 hours (40% versus 8%).

• Type of anesthesia: Data are inconsistent about whether the complication rate is lower with spinal or epidural anesthesia compared to general anesthesia. A study published in 1984, (Celli BR) reported no difference in patients anesthetized with spinal or general anesthesia for abdominal surgery. A study of high-risk patients shows that respiratory failure is significantly higher with general anesthesia (Trahan S, 1973). Several other studies (Pedersen T, 1990 and Yeager MP, 1987) found high rates of respiratory failure and other postoperative complications in patients undergoing general anesthetic compared to spinal or epidural anesthesia. The spinal or epidural anesthesia is safe and should be considered in high-risk patients. Regional nerve block is associated with a low-risk and should be considered whenever possible for high-risk patients.

• Key hole surgery: Laparoscopic abdominal surgery, particularly the cholecystectomy, is associated with fewer postoperative pulmonary abnormalities and a shorter hospital stay. These techniques use small incisions and the reduced manipulation of visceral organs minimizes the adverse effects on respiratory muscles. Laparoscopic surgery led to 23% decrease in FVC and 16% decrease in FEV₁ (Torrington KG, 1996), therefore, making it possible for patients with severe COPD to tolerate surgery Video-assisted thoracoscopic surgery utilizes much smaller incisions, consequently, the hospitalization time is substantially reduced. Smaller incisions, performed without separation of ribs and less postoperative pain leads to early ambulation and reduced pulmonary complications.

PREOPERATIVE RISK ASSESSMENT

History

Perform a complete history and physical examination to identify risk factors. Seek any history of smoking, exercise intolerance, unexplained dyspnea, or cough. Note evidence for COPD, such as decreased breath sounds, wheezes, crackles, or a prolonged expiratory phase.

Workup

• Pulmonary function tests (PFT): Several retrospective studies of routine preoperative PFT found only a marginal benefit in predicting postoperative complications in patients other than for lung resection; therefore, these should not be the sole reason to alter plans for necessary surgery. These could be used to identify high-risk patients for whom aggressive perioperative management is warranted. The American College of Physicians consensus statement suggests the following indications for preoperative PFT:

1. Patients undergoing cardiac or upper abdominal surgery with a history of smoking or dyspnea

2. Patients undergoing lower abdominal surgery if dyspnea or history of smoking anticipating prolonged surgery

3. Patients undergoing orthopedic surgery with uncharacterized lung disease

4. All patients undergoing lung resection

• Data from studies of Kroenke K (1993) and Wong D (1995) concluded that patients with severe COPD, classified as very high risk may undergo surgery with moderate risk of postoperative complications (29%). Pulmonary function tests do not identify either the high-risk patients or the severity of dysfunction when the risk of surgery is forbidding. Therefore, surgery should never be withheld based on results of pulmonary function testing.

• Spirometry: Bedside spirometry is often underutilized but an extremely useful tool for objectively evaluating the respiratory status of patients preoperatively. Spirometry can be employed to predict postoperative complications and to guide optimization of airflow obstruction in preparation for surgery. Gass GD (1996) suggested high postoperative risk in patients who had FEV₁ < 70% predicted, FVC < 70 percent predicted and FEV₁/FVC ratio <65%.

• Arterial blood gases: A PaCO₂ of more than 45 mmHg often occurs in severe COPD and indicates a high risk, although it is not necessarily prohibitive. Hypoxemia is not a significant predictor of complications. Patients undergoing cardiac or abdominal surgery who have dyspnea or are smokers and thoracic surgery patients should have arterial blood gas analysis.

• Chest radiograph: Chest x-rays add little to the clinical evaluation in healthy patients. All patients older than 60 years, or with clinical findings of cardiac or pulmonary disease should have a preoperative chest x-ray unless one was done in the last 6 months.

Risk indices

• Pulmonary Risk Index/Cardiopulmonary Risk Index (CPRI): A combined Cardiopulmonary Risk Index is proposed for risk stratification of pulmonary complications. The pulmonary risk factors were added to Goldman Cardiac Risk Index; patients with a combined score greater than 4 (out of a total of 10 points) were 17 times more likely to develop complications. These pulmonary risk factors include the following:

1. Obesity (ie, body mass index more than 27 kg/m2)
   
2. Cigarette smoking within 8 weeks of surgery
   
3. Productive cough within 5 days of surgery
   
4. Diffuse wheezing within 5 days of surgery
   
5. FEV₁/FVC ratio less than 70% and PaCO₂ within 45 mmHg
   

• Lawrence Risk Index: This test was developed based upon clinical information such as abnormal physical exam and abnormal chest radiograph. The test needs further validation in prospective studies.

• American Society of Anesthesiology (ASA) Classification: This score is based on simple clinical criteria and is easy to quantify. Although subjective, the scores of 2-5 indicate increasing level of severity, and increased postoperative morbidity. The ASA classification along with examples of each class are described below:

• ASA Class I: A normal, healthy patient without organic, physiologic or psychiatric disturbance; e.g., healthy with good exercise tolerance.

• ASA Class II: A patient with controlled medical conditions without significant systemic effects; e.g. controlled hypertension or controlled diabetes without systemic effects, cigarette smoking without COPD, anemia, mild obesity, age less than 1 or greater than 70 year, pregnancy.

• ASA Class III: A patient having medical conditions with significant systemic effects, intermittently associated with significant functional compromise; e.g., controlled CHF, stable angina, old MI, poorly controlled hypertension, morbid obesity, bronchoscopastic disease with intermittent symptoms, chronic renal failure.

• ASA Class IV: A patient with a medical condition that is poorly controlled, associated with significant dysfunction and is a potential threat to life; e.g., unstable angina, symptomatic COPD, symptomatic CHF, hepatorenal failure.

• ASA Class V: A patient with a critical medical condition that is associated with little change of survival with or without the surgical procedure; e.g., multiorgan failure, sepsis syndrome with hemodynamic instability, hypothermia, poorly controlled coagulopathy.

• ASA VI: A patient who is brain dead and undergoing anesthesia care for the purposes of organ donation.

PREOPERATIVE EVALUATION: THORACIC SURGERY

Preoperative Evaluation - Lung Resection

• Surgical resection remains the only potential curative therapy for patients with localized nonsmall cell lung cancer. These patients often have COPD from long history of smoking. Development of COPD is considered a risk for bronchogenic carcinoma. The first successful pneumonectomy for treatment of lung cancer was performed by Graham and Singer in 1933. Complete resection of Stage I and II non-small cell lung cancer leads to a 5-year survival of approximately 70%. Overall 5-year survival from lung cancer is dismal at 14%. Hence, every patient with limited lung cancer should be rigorously evaluated for potentially resectability and cure.

• Preoperative assessment identifies patients at greatest risk for postoperative complications and those patients with severe impairment, where risk of surgery is prohibitive.

• A multicenter study found in-hospital patient mortality rate of 3.8% after wedge resection, 3.7% after segmental resection, 4.2% after lobectomy, and 11.6 % after pneumonectomy (Romano PS, 1992). The significant predictors of mortality were older than 60 years, extended resection, chronic heart or lung disease, and low FEV₁.

• Some suggest measuring preoperative pulmonary function, the calculation of predicted postoperative pulmonary function, gas exchange, and exercise testing.

Preoperative pulmonary function

• Forced expiratory volume in 1 second (FEV₁) is the primary value used for resectability. FEV₁ predicts pulmonary reserve and is a strong predictor of postoperative complications. For full pneumonectomy, a preoperative FEV₁ of greater than 2 L/sec is required, whereas the suggested threshold value for lobectomy is 1 L/sec. Absolute numbers may be less helpful in female patients than percent predicted values.

• Diffusion capacity is a noninvasive method to assess pulmonary circulation, which reflects volume of pulmonary capillary bed. Diffusion capacity is reportedly a good predictor of morbidity and mortality after lung resection. A diffusion capacity of below 60% predicted was found to have a patient mortality rate of 24%. A diffusion capacity of < 40% with borderline FEV₁ criteria is associated with high mortality and morbidity and may be prohibitive (Markos I, 1989).

• Predicting postoperative pulmonary function
  Predicted postoperative function = (Preoperative function) X (% of function contributed by the lung that will remain postoperatively)

• This measurement should improve the predictive value of preoperative testing. Based upon a combination of spirometry and quantitative perfusion lung scan, a predicted postpneumonectomy FEV₁ more than 0.8 L/sec. is suggested as the lower limit. Postpredicted FEV₁ > 0.8 L/sec as a measure of resectability is based on a study by Segall and Butterworth (Segall JJ, 1966) that showed the hypercapnia (PaCO₂ > 45 mmHg) developed in patients with an FEV₁ < 0.8 L/sec.

• The percentage of predicted value is a better measure as it reflects differences in size, age, gender, and race. The predicted postoperative FEV₁ of 40% more is associated with the least mortality. A predicted FEV₁ of 40% predicted is required for performance of minimal activities of daily living without dyspnea. Lung resection leaving patient with a lower functioning lung will certainly lead to a respiratory invalid. FEV₁ of less than 30% is associated with 20 to 30% 5-year survival rates. Therefore, decline of postoperative FEV₁ to less than 30% will not only cause immediate postoperative morbidity and mortality but also excessive longer-term mortality. Use of radionuclide lung scanning to calculate this value is required. If the predicted postoperative FEV₁ is less than 0.8 L/sec or less than 40% predicted, the patient is unresectable.

• Radionuclide quantitative lung scanning: Radionuclide scanning is extensively used in evaluation of suspected pulmonary embolism. This technique has also been used to quantitate the function of a lung or a lobe, which will be resected. Therefore, the function of the remaining lung can be calculated Olsen and Associates (Olsen GN, 1974) reported that postpneumonectomy FEV₁ < 0.8 L/sec was associated with prohibitive risk. Another study concluded that predicted postoperative FEV₁ > 0.8 resulted in acceptable survival following pneumonectomy. A threshold value of predicted postoperative FEV₁ of 35% is suggested (Gass GD 1986), but has not been tested either retrospectively or prospectively.

• The gas exchange is assessed by measuring diffusing capacity and an arterial blood gas analysis.

• High morbidity and mortality have been associated with a predicted postoperative diffusion of less than 40% predicted.

• A low resting arterial PaO₂ is not a strong predictor, but hypercapnia (PaCO₂ > 45 mmHg) has been considered a significant risk factor, though not proven.

Exercise testing

• A comprehensive physiologic evaluation is dependent on interaction among pulmonary function, cardiovascular function, and oxygen utilization. This may take the form of stair climbing or complete cardiopulmonary exercise testing.

• Stair climbing, though poorly standardized, has been shown to identify patients at increased risk for lung resection. Patients capable of climbing 3 or more flights of stairs have lower complication rates.

• Complete cardiopulmonary exercise testing may identify patients who achieve a maximal oxygen consumption (VO₂ max) of less than 1 L/min. These patients have excess mortality. Expressing VO₂ max as ml/kg may be more useful. A VO₂ max of more than 20 ml/kg/min is associated with least postoperative complications, whereas a value of less than 10 ml/kg/min may be prohibitive because of the high morbidity and mortality. The patients who exercise to a VO₂ max of between 10-20 ml/kg/min may have increased but an acceptable risk.

• Caution: Patients who do not meet these criteria should not be summarily refused surgery if they are willing to accept possibility of earlier death or prolonged disability over certainty of cancer death.

Preoperative evaluation - Cardiac surgery

• The incidence of left lower lobe atelectasis has been reported in up to 90% of patients, which may increase the postoperative morbidity and prolong the hospital stay. The incidence is attributed to phrenic nerve injury and may last from 30 days to 2 years. Operative factors associated with this complication are a longer bypass time, entrance in to pleural cavity, direct injury during mobilization of internal mammary artery, and cold cardioplegia.

• The overall incidence of pulmonary complications after coronary bypass surgery is 7.5%. Patients with abnormal pulmonary function studies are more likely to have prolonged mechanical ventilation and postoperative pneumonia. No studies indicate a value of pulmonary function below which cardiac surgery is precluded.

PREPARATION FOR SURGERY

Smoking cessation

• Instruct patients undergoing elective surgery to abstain from smoking for 8 weeks before surgery.

• Use of counseling, nicotine replacement therapies, and bupropion (Zyban) have improved quit rate and should be used aggressively.

Chronic obstructive pulmonary disease

• Aggressively treat patients with COPD to achieve the best possible baseline function.

• Use of bronchodilators, smoking cessation, antibiotics, and chest physical therapy may reduce significantly pulmonary complications.

• Treat patients who have a persistent wheeze, functional limitation, or severe air flow obstruction with perioperative steroids.

Asthma

• Optimize asthma control prior to surgery. Perioperative systemic corticosteroids are recommended for persistent symptoms if the peak flow rate and FEV₁ are less than 80% predicted or previous best.

• The safety of perioperative corticosteroid use is well established in asthmatic patients.

• Perioperative steroids were not associated with death, serious infections, or adrenal suppression.

• Hypothalamic-pituitary-adrenal axis suppression should be assumed to be present in patients who receive systemic steroids for more than 3 weeks in the past 6 months. These patients should receive stress dose coverage perioperatively (Hydrocortisone 100 mg I/V Q8H).

Preoperative antibiotics

• Indiscriminate use of prophylactic antibiotics does not lead to a reduction in pulmonary complications. These drugs may be used in patients with a clinically apparent respiratory infection.

• Cancel elective surgery if patient has active infection.

Patient education

• Lung expansion, deep breathing and coughing, and incentive spirometry are best taught to the patient prior to surgery.

INTRAOPERATIVE STRATEGIES

Type of anesthesia

• The type of anesthesia and neuromuscular blockage affect the incidence of postoperative pulmonary complications. Intermediate and shorter acting agents (e.g., vecuronium, rocuronium) are preferred, as residual neuromuscular blockade from longer acting agents may contribute to pulmonary complications.

• Based on available literature, the spinal anesthesia may be safer than general anesthesia; therefore, spinal anesthesia should be considered for high-risk patients. Depending on the type and duration of surgery, endotracheal intubation and mechanical ventilation may be preferable, because of the ability to monitor and control respiratory rate and tidal volume.

Type of neuromuscular blockade

• Pancuronium, a long-acting neuromuscular blocker, may lead to residual effects, cause hypoventilation, and increase complications.

• Use the intermediate-acting agents (eg, vecuronium, atracurium) in high-risk pulmonary patients.

Duration and type of surgery

• When available, a less ambitious, shorter procedure should be considered in extremely high-risk patients. The duration of surgical procedure is known to affect rate of postoperative complications.

• As upper abdominal and thoracic operations carry the greatest risk, a percutaneous (laparoscopic) procedure should be substituted for an open procedure if possible.

POSTOPERATIVE STRATEGIES

Lung expansion maneuvers

• Deep breathing and incentive spirometry: Deep breathing exercises and incentive spirometry appear to be equally effective. These are components of chest physical therapy and have been shown to reduce postoperative pulmonary complications in high-risk patients. These maneuvers are facilitated greatly by regular physiotherapy visits.

• Intermittent positive pressure breathing: Although used commonly in the 1960s and 1970s, the postoperative complications rate with this is similar to deep breathing exercises. The cost and potential for abdominal distension does not warrant its use.

• Continuous positive airway pressure: The literature has shown the benefit of continuous positive airway pressure (CPAP) therapy in a select group of patients as a secondary intervention for refractory atelectasis. When administered via face mask, close patient supervision is required in case of vomiting or difficulty clearing secretions.

Pain control

• Pain is a highly complex process involving specialized nociceptor fibers in the peripheral tissues; neurotransmitters and neuromodulators at all levels of neuraxis; integration of information in central nervous system; and learned behavior, affect and cognitive status.

• Adequate postoperative pain control will minimize the pulmonary complications by encouraging earlier ambulation and performance of lung expansion maneuvers.

• Management of postoperative pain includes narcotics and narcotics like medications administered peripherally, in to the epidural or intrathecal space. Intrathecal administration of narcotics is associated with longer duration of analgesia (15 to 22 hours), respiratory depression and headaches. Intercostal nerve blocks have also been shown to be beneficial in upper abdominal surgery.

• Recent studies have popularized the use of epidural analgesia as an alternative to parenteral narcotics. In upper abdominal procedures, lower rates of pulmonary complications and shorter duration of hospital stay were found in patients who received epidural analgesia. Epidural catheters can be used for patients undergoing thoracic or upper abdominal surgery by placement of the catheter at thoracic vertebral level.

• Epidural narcotics provide a longer duration of action, lack of excessive sedation and respiratory depression, minimal or no sensory motor loss. Addition of a local anesthetic provides more rapid onset of action and may help localize correct placement of catheter, although hypotension and motor blockade are the potential side effects. Epidural narcotics are morphine, fentanyl, sufentanyl and hydroxymorphine; the local anesthetics used for epidural analgesia are bupivicaine and ropivacaine. Adding small doses of local anesthetics to narcotics is a preferred approach, which potentiates pain relief, minimizes nerve blockade and reduces side effects from both agents.

• Postoperative epidural analgesia and intercostal nerve blocks improve pain control, and help reduce postoperative complications with little risk. Cuschieri RJ (1985) reported postoperative pulmonary complications in 24% of postoperative patients receiving epidural analgesia, compared to 64% randomized to intramuscular morphine.

• Epidural hematoma is a rare complication except when concomitant anticoagulation is prescribed. There are reports of epidural hematomas in patients receiving low molecular weight heparin, who had placement of epidural catheters. Warnings have been issued on this issue by the national advisory panels. A safe practice is not to place the epidural catheter for at least 12 hours after the last dose of low molecular weight heparin.

• Perioperative use of nonsteroidal anti-inflammatory drugs may complement other pain management strategies. Nonsteroidal agents are known to decrease narcotics requirement in the postoperative period. The newer agent ketorolac maybe administered intramuscularly as needed. Other nonsteroidal agents are given orally or rectally. Caution is advised in patients at risk of bleeding, with history of peptic ulcer disease and established renal dysfunction.

Prevention of Thromboembolism

• Prevention of deep venous thrombosis and pulmonary embolism is important for all major surgeries but more so for hip or knee orthopedic procedures, where the incidence may be as high as 50-60%. Risk also is high in the elderly and in those patients with malignancies.

• Heparin prophylaxis: The incidence of venous thrombosis, PE, and death can be significantly reduced by embracing a prophylactic strategy in high-risk patients. Prevention of DVT in lower extremities will inevitably reduce the frequency of PE; therefore, populations at risk must be identified, and safe and efficacious prophylactic modalities should be used. The risk groups identified in clinical practice and the prophylaxis recommended by the Sixth Consensus Conference on Antithrombotic Therapy is described in the following table:

  Table: Prophylaxis Against Venous Thromboembolism
  
  

  Condition	Risk(%)*	Recommendation
  General Surgery
  Low risk	3	Early ambulation
  Moderate risk	29	Unfractionated heparin: 5000 U SC given 2 h preoperatively and q12h postoperatively or LMWH: Dalteparin, 2500 U 1-2 hr before surgery, then once daily Enoxaparin, 2000 U before surgery, then once daily Nadroparin 3100 U 2 hr before surgery, then once daily Tinzaparin 3500 U 2 hr before surgery, then once daily
  High risk	39	Unfractionated heparin: 5000 U SC given 2 h preoperatively and q8h postoperatively; or Dalteparin, 5000 U 10-12 before surgery, then once daily Enoxaparin, 4000 U 10-12 hr before surgery, then once daily
  Very high risk	80	(1) Unfractionated heparin: 5000 U SC given 2 h preoperatively and q8h postoperatively; dalteparin: 2500 U given 2 h preoperatively and qd; plus, intermittent pneumatic compression applied intraoperatively (2) Dalteparin, 5000 U 10-12 before surgery, then once daily Enoxaparin, 4000 U 10-12 hr before surgery, then once daily (3)Perioperative warfarin (INR, 2.0-3.0)
  Orthopedic Surgery/Neurological Surgery/Trauma
  Total hip replacement	51	(1) Dalteparin, 5000 U 1-2 hr before surgery, then once daily Enoxaparin, 3000 U 10-12 hr before surgery, then once daily Nadroparin 40 U/kg U 2 hr before surgery, then once daily Tinzaparin 50 U/kg 2 hr before surgery, then 75 U/kg once daily (2) Warfarin: preoperatively and adjusted to INR of 2.0-3.0 postoperatively, continue up to 4 wk after surgery
  Total knee replacement	61	(1) Dalteparin, 5000 U 1-2 hr before surgery, then once daily Enoxaparin, 3000 U 10-12 hr before surgery, then once daily Nadroparin 40 U/kg U 2 hr before surgery, then once daily Tinzaparin 50 U/kg 2 hr before surgery, then 75 U/kg once daily (2) Warfarin: preoperatively and adjusted to INR of 2.0-3.0 postoperatively, continue up to 4 wk after surgery
  Hip fracture surgery	48	(1) Dalteparin, 5000 U 1-2 hr before surgery, then once daily Enoxaparin, 3000 U 10-12 hr before surgery, then once daily Nadroparin 40 U/kg U 2 hr before surgery, then once daily Tinzaparin 50 U/kg 2 hr before surgery, then 75 U/kg once daily (2) Warfarin: preoperatively and adjusted to INR of 2.0-3.0 postoperatively, continue up to 4 wk after surgery
  Neurosurgery	24	(1) Intermittent pneumatic compression or (2) unfractionated heparin: 5000 U SC q12h and intermittent pneumatic compression for high-risk patients
  Acute spinal cord injury with leg paralysis	40	(1) Unfractionated heparin: SC in doses adjusted to paralysis produce APTT=1.5 X control 6 h after dose (2) Enoxaparin 3000 U twice daily (3) Warfarin adjusted to INR of 2.0-3.0 in rehabilitation phase (4) Intermittent pneumatic compression plus unfractionated heparin: 5000 U SC q12h
  Multiple trauma	53	Intermittent pneumatic compression until further bleeding is unlikely; then, give (1) enoxaparin: 30 mg SC q12h or (2) warfarin: adjusted to INR of 2.0-3.0
  Medical Conditions
  Acute Myocardial infarction	24	Unfractionated heparin: 5000 U SC q12h unless therapeutic anticoagulation used
  Ischemic stroke with paralysis	42	Unfractionated heparin: 5000 U SC q12h
  Medical patients (cancer, bedrest, CHF, severe lung disease)	20	Unfractionated heparin: 5000 U SC q12h; Dalteparin 2500 U once daily, Enoxaparin 2000 U once daily

  *Approximate risk without prophylaxis for all and/or proximal DVT or symptomatic PE

• Pleural effusion

   

• Paradoxical embolism

• Approximately 3% of patients receiving unfractionated heparin may develop immunoglobulin G–mediated thrombocytopenia. Heparin-induced thrombocytopenia (HIT) may manifest clinically as extension of the thrombus or formation of new arterial thrombosis. HIT should be suspected whenever the patient's platelet count falls to less than 100,000/mm³ or less than 50% of the baseline value, generally after 5-15 days of heparin therapy. The definitive diagnosis is made by performing a platelet activation factor assay.

   

• The treatment of patients who develop HIT is warfarin, IVC filter, or rapidly acting anticoagulant drugs (eg, danaparoid sodium, ancrod, Hirudin). Clinical cross-reactivity is uncommon with danaparoid sodium, therefore making it a reasonable choice for patients with HIT.