The primary reason for mechanical ventilation in this group is apnea as a result of unreversed anesthetic agents (Table 21-1). The primary reasons that anesthesia is not reversed are iatrogenic hypothermia, the need to reduce cardiopulmonary stress, or the presence of altered pulmonary mechanics. Some cardiac surgeons favor cold cardioplegia to reduce the likelihood of hypoxic injury. These patients receive narcotic anesthesia throughout the procedure and may require 8 to 16 hours for warming and full reversal of anesthesia. Transplant recipients (heart or lung) are ventilated to ensure cardiopulmonary stress is minimized during the initial acclimation period and to minimize any adverse effects of an increased work-of-breathing in the immediate postoperative period. The most difficult group of patients is those with preexisting lung disease whose pulmonary mechanics are adversely affected by surgery, who require ventilatory support because of compromised cardiopulmonary reserve and bronchial hygiene.
Table 21-1Indications for Ventilation in Postoperative Patients |Favorite Table|Download (.pdf) Table 21-1 Indications for Ventilation in Postoperative Patients
• Apnea—unreversed anesthetic agents
• Minimize postoperative cardiopulmonary stress
• Preexisting lung disease compromising cardiopulmonary reserve
Minimal or no prior pulmonary disease It is usually easy to ventilate these patients. Most simply require postanesthesia recovery. Volume or pressure ventilation in the continuous mandatory ventilation (A/C) mode is acceptable (Figure 21-1). Tidal volume may be normal (6-8 mL/kg ideal body weight [IBW]) since lung function is normal. The rate can be set at 12 to 18/min. Fio2 is titrated to maintain a normal Pao2 (> 80 mm Hg) and low levels of positive end-expiratory pressure (PEEP) (5 cm H2O) is applied to maintain functional residual capacity (Table 21-2). In hypothermic patients, minute ventilation is decreased to avoid hypocarbia and alkalosis. As a result, the initial rate may need to be set low and increased as body temperature increases.
An algorithm for mechanical ventilation of the postoperative patient.
Prior pulmonary disease Patients with a history of chronic pulmonary disease are ventilated in the same manner as any patient with chronic pulmonary disease. Air trapping is a concern with chronic obstructive pulmonary disease (COPD). Moderate tidal volume (6-8 mL/kg IBW) and respiratory rate (12-18/min) should be selected. A long expiratory time is needed to avoid auto-PEEP. PEEP is applied to counterbalance auto-PEEP when spontaneous breathing resumes. Plateau pressure (Pplat) less than 25 cm H2O should be used in patients with COPD. In patients with chronic restrictive pulmonary disease, air trapping is not a problem. Because of reduced lung volumes, however, smaller VT (4-6 mL/kg predicted body weight) and rapid rates (20-30/min) are set to avoid high Pplat.
Single lung transplant Of all the patients ventilated postoperatively, this group is the most troublesome if one lung has relatively normal pulmonary mechanics (transplanted lung) and the other has mechanics reflecting either obstructive or restrictive disease (native lung). In these patients, the ventilator should be set to ensure the maximum function of the native lung, since this will be the lung presenting the greatest challenge. If the native lung has chronic obstruction, ventilate with moderate volume and slow rates. With pulmonary fibrosis in the native lung, a smaller VT and more rapid rate are indicated. In the case of pulmonary fibrosis, there is less concern about air trapping. However, plateau pressure may be high due to reduced compliance.
The greatest ventilatory challenge is the patient with a single lung transplant where the native lung is obstructed and the transplanted lung has become stiff because of fluid, infection, rejection, or acute lung injury. In this setting, it is difficult to dictate ideal ventilator settings because of the differing pathologies in each lung. However, tidal volume should be small. Attention needs to be paid on two variables as adjustments are made. First, concern about plateau pressure because of ventilator-imposed lung injury and damage to the surgical site. Second, air trapping in the obstructed lung resulting in grossly compromised ventilation/perfusion ratios. In this setting, permissive hypercapnia may be necessary with the final ventilator settings being a compromise between conflicting needs.
Many postoperative patients develop respiratory complications postoperatively. Some of these patients can be easily managed with mask CPAP or NIV. Mask CPAP is beneficial in patients who develop respiratory failure following abdominal surgery. In these patients, CPAP can be set at 8 to 12 cm H2O based on patient tolerance with Fio2 set to ensure Spo2 is more than 92%. If patient is hypercarbic, NIV can be applied with PEEP of 5 to 8 cm H2O, inspiratory pressure set to provide a tidal volume of 4 to 8 mL/kg IBW, respiratory rate for an appropriate Paco2, and Fio2 to maintain Spo2 more than 92%. Cardiac surgical patients and transplant patients may also benefit from mask CPAP or NIV if they develop respiratory failure.
For the majority of postoperative patients, monitoring of gas exchange (pulse oximetry and arterial blood gases), level of consciousness, pulmonary mechanics, and the ability to cough and deep breathe are sufficient to determine if there is a need for continued ventilatory support (Table 21-3). However, in patients with COPD, monitoring of auto-PEEP is also important. These patients are often fluid-positive, which can affect respiratory function. Monitoring fluid balance, including central venous pressure, is often useful. In patients with hemodynamic instability or severe cardiac disease, careful monitoring of pulmonary and systemic hemodynamics is also indicated.
Table 21-3Monitoring of the Mechanically Ventilated Postoperative Patient |Favorite Table|Download (.pdf) Table 21-3 Monitoring of the Mechanically Ventilated Postoperative Patient
• Pulse oximetry
• Level of consciousness
• Pulmonary mechanics
• Auto-PEEP and plateau pressure
• Fluid balance
Ventilator discontinuation is a simple process for most postoperative patients. When gas exchange is adequate at an Fio2 of 0.50, the patient is alert and oriented, able to lift the head and take a deep breath, ventilatory support can be discontinued and the patient is extubated. Many clinicians prefer short (30 minutes) spontaneous breathing trials, or a gradual reduction of pressure support to 5 to 10 cm H2O before discontinuation. However, unless the baseline status is abnormal (ie, COPD), a specific weaning protocol may extend the time ventilation is required. In patients with underlying pulmonary disease or lung transplant patients, more prolonged weaning may be necessary.