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REGIONAL ANESTHESIA AND PAIN MANAGEMENT

Nocturnal Oxygenation During Patient-Controlled Analgesia

J. Gilbert Stone, MD^*, Kathryn A. Cozine, MD, and Alvin Wald, PhD

*Department of Anesthesiology, New York Medical College, St. Vincents Medical Center; and Department of Anesthesiology, Columbia University, College of Physicians & Surgeons, New York, New York

Address correspondence to J. Gilbert Stone, MD, 25 Bank St., New York, NY 10014. Address reprint requests to J. Gilbert Stone, MD, Anesthesiology, St. Vincents Hospital, 153 West 11th St., New York, NY 10011.

	Abstract

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Patient-controlled analgesia (PCA) has become a standard modality for the management of postoperative pain, although anecdotal reports of excessive sedation and respiratory depression impugn its safety. To study the prevalence and severity of nocturnal hypoxemia, we measured arterial oxygen saturation (SpO₂) continuously overnight in 32 postoperative patients who were receiving morphine via PCA. To evaluate the potential benefit of providing concurrent supplemental oxygen, the patients breathed oxygen-enriched air the night of surgery and room air the next night. Patients experienced more pain and consumed twice as much morphine the first night. However, breathing supplemental oxygen that night, the nocturnal mean SpO₂ was 99% ± 1%, 94% ± 4% (P < 0.001), and only four patients had periods of hemoglobin desaturation <90%. In contrast, breathing room air the subsequent night, the mean SpO₂ was lower (94% ± 4%; P < 0.001), and hypoxemia occurred more frequently and was more severe: 18 patients experienced episodes of SpO₂ <90%, 7 patients experienced episodes of SpO₂ <80%, and 3 patients experienced episodes of SpO₂ <70%. One patient required resuscitation for profound bradypnea and cyanosis, but none suffered permanent sequelae. We conclude that when postoperative patients use PCA at night, hypoxemia can be substantial and oxygenation can be improved by providing supplemental oxygen.

Implications: Oxygen saturation was measured postoperatively in patients using morphine patient-controlled analgesia. Substantial nocturnal hypoxemia occurred in half of the patients while they breathed room air. The severity of the hypoxemia was reduced when patients received supplemental oxygen.

	Introduction

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Patient-controlled analgesia (PCA) has attained considerable popularity and has become a therapeutic standard for the management of acute postoperative pain (1–3). Nevertheless, anecdotal reports of excessive sedation and respiratory depression suggest that PCA may sometimes be hazardous (4–15). Narcotics cause hypoventilation and decrease the ventilatory response to both hypercapnia and hypoxemia (16,17). They also make patients drowsy (18), and carbon dioxide retention occurs even during unmedicated sleep (19,20). In fact, many patients report that they induce sleep in the postoperative period with their PCA equipment. Patients are commonly allowed to breath room air while they use PCA, and without supplement oxygen (O₂), hypoventilation can cause hypoxemia. However, neither the severity nor the prevalence of hypoxemia has been well documented in individuals receiving narcotics via PCA to relieve nocturnal postoperative pain. We performed this study to obtain that information and to evaluate the potential benefit of providing concurrent supplemental O₂.

	Methods

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With institutional approval, consenting patients were enrolled in the study. All patients underwent total hip replacement and received morphine via a PCA device for acute postoperative pain relief. Patients were excluded if they required intensive care after surgery or if they could not maintain an arterial O₂ saturation (SpO₂) of 97% either while breathing room air before the induction of anesthesia or later while receiving supplemental O₂ in the recovery room.

After surgery was completed and patients emerged from anesthesia, they were made comfortable in the recovery room with small IV doses of morphine. When alert and oriented, they were instructed in the use of a PCA infusion device and were not discharged from the recovery room until proper usage was demonstrated. Baxter Healthcare PCA II pumps (Deerfield, IL) were programmed to deliver single 1- or 1.5-mg IV morphine bolus doses on demand, with a lockout time of 6 min and an hourly dose limitation of 8–12 mg. A timed record of the dose delivery and attempts during lockout was made automatically by the PCA system. Study patients did not receive basal infusions, sedatives, or additional respiratory depressants while using PCA. Data acquisition took place in a quiet, private room and did not begin until a study nurse again explained the operation of the PCA equipment several hours later. At that time, nasal O₂ prongs were taped securely to the face and recording began. To permit undisturbed sleep throughout the night, the monitor screen was darkened and the pulse oximeter was made inaudible.

For the first postoperative night, all study patients breathed room air supplemented with humidified O₂ delivered via nasal prongs at an acceptable flow rate of 3–6 L/min. Supplemental O₂ was discontinued the next morning; thereafter, only room air was breathed. Monitoring was also suspended at that time and was not resumed until evening. After the second postoperative night, patients were asked during which night their pain had been worse.

SpO₂ was determined continuously by pulse oximetry from evening until morning using a disposable oxisensor affixed to a toe on the operative side and a modular monitoring system that sampled data every 6 s and computed a median SpO₂ value every minute. These SpO₂ values were stored in memory and subsequently displayed for visual inspection. Unsustained SpO₂ values were judged to be artifactual and were rejected if they varied by >3% over a 5-min period or if they were not accompanied by a stable heart rate (obtained with five-lead electrocardiograph). A nocturnal SpO₂ nadir was then determined for each study night by taking the mean of the five lowest consecutive sustained values. A nightly mean SpO₂ was also calculated from approximately 720 data points.

Data were collected from each study patient on both the first and second nights after surgery. SpO₂ comparisons between the two nights were made by using paired t-tests and 2 analysis. Linear regression analysis was used to correlate the SpO₂ nadirs that patients attained breathing room air with their O₂-supplemented SpO₂ nadirs, their age, body mass index, morphine dose, and the number of postoperative days spent in hospital. Data are expressed as mean ± SD. P < 0.05 was deemed significant.

In a pilot study, two of the authors recorded their own SpO₂ values on five separate occasions while sleeping overnight in a hospital on-call room. They used the same monitoring equipment, breathed room air, and were not medicated. Data were obtained to test the feasibility and consistency of nocturnal SpO₂ monitoring.

	Results

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The authors who monitored their own SpO₂ while sleeping overnight in the hospital recorded consistent values of 97%–99% breathing room air.

The study was originally designed to randomly supply patients with either room air or O₂-enriched air for the duration of the first postoperative night, then to switch environments the following night. However, after examination of the data from the first four patients, the protocol was altered so that subsequent patients received supplemental O₂ the first postoperative night and room air thereafter. Two of these initial study patients had breathed room air the night of their surgery and had experienced episodes of arterial hemoglobin desaturation <80%. A segment of the overnight SpO₂ recording from one of these patients is depicted in Figure 1. Shortly after monitoring began, the patient was given supplemental O₂ by a concerned study nurse.

View larger version (21K): [in this window] [in a new window]   Figure 1. SpO2 recording from a patient who could not maintain satisfactory oxygenation breathing room air and who was given supplemental O2.

Fifteen female and 17 male patients completed the study. Ages ranged from 45 to 86 yr (mean 66.0 ± 12.0 yr). Five patients were classified as ASA physical status I, 21 as ASA physical status II, and 6 as ASA physical status III. Seven patients were obese. Most patients took nonsteroidal antiinflammatory drugs on occasion before surgery, and one patient was medicated with intermittent preoperative oxycodone. Excluding arthritic joint disease, all were judged to be healthy and fit for their age. The SpO₂ of these patients averaged 98.3% ± 1.1% breathing room air in the supine position before the induction of anesthesia.

Although the study protocol did not dictate an anesthetic technique, every patient but one received a combination of spinal (n = 6) or epidural (n = 25) block and general anesthesia with controlled ventilation via an endotracheal tube. Intrathecal fentanyl was administered to the patients who received spinal anesthesia, and all patients received IV intraoperative fentanyl (225 ± 196 µg) and midazolam (1–5 mg) as premedication. Longer acting drugs, such as droperidol, were avoided. Surgery began at 8 AM and lasted 3.3 ± 1.0 h. All patients had their tracheas extubated in the operating room, and all were admitted to the recovery room by 2 PM, where they remained for 3.4 ± 1.3 h before going to their hospital rooms. By that time, the effects of both regional and general anesthesia had mostly dissipated.

Of the 32 patients, 24 reported having more pain during the first night after surgery compared with the second night, and 8 believed that their pain level was about the same both nights. Patients self-administered a larger total dose of morphine the first postoperative night versus the second (23.7 ± 12.2 vs 11.8 ± 9.2 mg; P < 0.001) and tried to activate the PCA device more often during lockout intervals that night. Consequently, the ratio of morphine doses received to morphine dosing attempts was smaller the first night (0.77 ± 0.19 vs 0.87 ± 0.18; P < 0.005), which indicates more discomfort.

The overnight mean SpO₂ was 5% higher the first night when patients breathed O₂-supplemented air than that the second night, when they were exposed to room air only (98.9% ± 1.2% vs 93.7% ± 3.9%; P < 0.001). Likewise, the mean of each patient's individual nightly SpO₂ nadir was significantly lower without O₂ supplementation (86.8% ± 8.4% vs 95.1% ± 3.4%; P < 0.001).

Successive overnight SpO₂ recordings from a patient breathing first with, then without supplemental O₂ are shown in Figure 2. Figure 3 depicts a segment of an SpO₂ recording from a patient who began the second night breathing room air only. Part way through the study period, however, he became somnolent and difficult to arouse, and a nurse noted periodic hemoglobin desaturation to <80% before she reestablished nasal O₂ supplementation.

View larger version (14K): [in this window] [in a new window]   Figure 2. Successive overnight SpO2 recordings from a patient breathing air with and without supplemental O2.

View larger version (21K): [in this window] [in a new window]   Figure 3. SpO2 recording from a patient who began the night breathing room air and later received supplemental O2.

View larger version (22K): [in this window] [in a new window]   Figure 4. Individual nocturnal SpO2 nadirs breathing O2 supplemented air the first postoperative night and room air the second night (n = 32).

Hospital discharge of those patients who became hypoxemic was not delayed. The 18 patients with nocturnal SpO₂ nadirs 90% were discharged after 7.8 ± 2.6 postoperative days, and those whose overnight SpO₂ was >90% went home after 9.0 ± 2.7 days. There was a poor correlation between individual nocturnal SpO₂ nadirs and the number of days spent in hospital after surgery (r = -0.03).

SpO₂ always fluctuated a little throughout both nights, and all patients exhibited intervals of minor hemoglobin desaturation at one time or another while using narcotic PCA. Some patients, however, sustained substantial nocturnal hypoxemia breathing room air, and to be able to identify those patients in advance would be of value. Thus, each patient's first night SpO₂ nadir breathing with supplemental O₂ was compared with their second night's SpO₂ nadir breathing room air (Fig. 5), and a good correlation was found (r = 0.64; P < 0.001). In fact, our study patients can be subdivided into three groups based on their nocturnal SpO₂ nadir with O₂ supplementation. In the seven patients who maintained their SpO₂ >98% breathing supplemental O₂, none became even mildly hypoxemic (SpO₂ <90%) when exposed to room air. Of the 12 patients who achieved SpO₂ nadirs of 95%–98% breathing nasal O₂, 7 became mildly hypoxemic breathing room air, and 1 patient displayed severe hypoxemia (SpO₂ <80%). Of the 13 patients with SpO₂ nadirs <95% with nasal O₂, 6 became severely hypoxemic the next night while breathing room air, and only 1 patient maintained an SpO₂ >90%.

View larger version (17K): [in this window] [in a new window]   Figure 5. Correlation between each patient's first night nocturnal SpO2 nadir breathing with supplemental O2 and their second night's nocturnal SpO2 nadir breathing room air only. r = 0.64, P < 0.001.

	Discussion

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When postoperative pain is managed with PCA rather than by intermittent IM injection, patient satisfaction is high and narcotic consumption is reduced (1). No associated mortality or any permanent morbidity has ever resulted from properly delivered PCA. A few anecdotal incidents of excessive sedation or respiratory depression seem to be the only serious complications that have arisen (4–15). Nevertheless, in this study, hypoxemia often developed overnight while patients attempted to control their own postoperative narcotic administration.

Narcotic analgesics decrease respiratory rate and minute volume. Hypoventilation results in carbon dioxide retention, and the ventilatory response to hypercapnia and hypoxemia are both reduced by narcotics (16,17). Sleep itself is affected by narcotics in the postoperative period; rapid eye movement is nearly eliminated, and slow wave activity is severely suppressed (21). Sleep also becomes fitful and fragmented, and patients often display erratic breathing patterns (21–24). These nocturnal episodes of abnormal ventilation are similar in character to those seen in individuals with sleep apnea; they, too, are accompanied by profound hypoxemia (22–24). Providing supplemental O₂ to patients after surgery improves oxygenation, but the narcotic-induced abnormal intervals persist (23,24), and their occurrence in the postoperative period far exceeds that of sleep apnea in the general population (25). Moreover, nocturnal hypoxemic episodes are rare in postoperative patients who receive continuous regional analgesia with local anesthetics (22). Nocturnal breathing patterns were not examined in our study, but we often observed SpO₂ recordings with periods of significant hypoxemia (Figs. 1–3).

After surgery, patients experience discomfort, anxiety, and frequently periods of oppressive pain. Postoperative suffering stimulates ventilation and tends to counteract narcotic-induced respiratory depression (26). Indeed, the safety of PCA depends on the balance between these two opposite effects. Nevertheless, some patients develop significant narcotic-induced respiratory depression from PCA while still complaining of persistent, unrelieved pain (9). Others find deep breathing, coughing, and postoperative movement of any kind extremely painful. Their breathing becomes restricted and shallow, and narcotic therapy can actually improve ventilation (2,3). Individuals vary greatly in their response to both postoperative pain and to opioids (1), and it is difficult to predict which patients treated with PCA will be susceptible to narcotic overdose.

In addition to modifying the pain threshold, narcotics cause dose-dependent drowsiness, mood changes, and mental clouding (18). If sedatives are prescribed in addition to narcotic PCA, respiratory depression is not uncommon after surgery (10,12,13). The same sedated state is often induced in patients who use narcotic PCA alone. These patients act like inebriated individuals without inhibitions and without prudence. In fact, this behavior is not infrequently promoted by the hospital staff, who encourage patients to dose themselves liberally with their PCA devices at the first sign of discomfort and preemptively before bedtime.

Narcotic-induced hypoxemia is not a new or unexpected finding; it occurs with disturbing frequency when patients breathe room air in the postoperative period. SpO₂ levels 80% have often been reported in patients receiving IM narcotic injections after surgery on a PRN schedule (7,9,24,27,28). Likewise, dangerous hemoglobin desaturation can develop in postoperative patients treated with continuous IV or epidural narcotic infusions (22,23,29–31). The present study is the first to demonstrate a similar degree and incidence of hypoxemia in patients titrating their own narcotic administration via PCA.

Oxygen supplementation in operating theaters, in recovery rooms, in intensive care units, and during transport has become standard practice today for obtunded or recently sedated patients. Studies have repeatedly shown oxygen therapy to decrease postoperative hypoxemia in patients receiving narcotics by intermittent IM injections or by constant IV infusion (22–24,29,32). Our study affirms the benefit of postoperative oxygen therapy when narcotics are administered at night via PCA devices. It remains to be determined whether hypoxemia is as severe in the daytime and for how long after surgery supplemental O₂ is of value at night. This and another study have found benefit on the first two nights after hip surgery (24), and others have demonstrated narcotic-induced hypoxemia for 4 nights after other major operations (29,30,32). However, O₂ therapy can be uncomfortable, and its cost and that of PCA are issues that must be considered (33).

Central to our thesis, and still without definition, is the critical, life-threatening hypoxemic threshold (34,35). However, when the PaO₂ acutely decreases to <40 or 50 mm Hg and does not soon return to normal, central nervous system function declines, and some deficits persist (35). Adaptive physiologic modification also begins at that hypoxemic level: patients hyperventilate and compensatory cardiovascular mechanisms are initiated (36), erythropoietin synthesis increases (36), and tissues most sensitive to oxygen deprivation alter their genetic makeup so that vascularity increases (37). A substantial stress response is also elicited, and an increased incidence of myocardial ischemia has been reported after surgery in elderly patients who become severely hypoxemic (27,38,39). Perhaps postoperative myocardial infarction occurs more frequently on surgical floors rather than in intensive care units because supplemental O₂ is not routinely supplied there (40–42).

PCA-induced hypoxemia did not cause any patient permanent harm, but one patient did consume an inordinate amount of morphine overnight and required resuscitation. This near tragedy emphasizes the need to limit cumulative dosing over many hours. Perhaps patient monitors and alarms will one day be incorporated into a new generation of safer PCA devices, but until that time, we must recognize that individuals demonstrate large variation in their response to narcotics and to pain (1) and that ventilatory and cognitive function are not always affected equally (43,44).

We originally planned to study 100 patients, with each breathing room air one night and O₂-supplemented air another. The O₂ environment was to be randomly supplied the first night and reversed the next. However, when we examined our initial results, fear for patient safety dictated that we abandon randomization; consequently, the study conditions differed the two nights. Moreover, clinicians in our hospitals began to prescribe concurrent supplemental O₂ with PCA whenever they suspected that their patients might develop hypoxemia; thus, our investigation ended prematurely. However, this small study has other serious weaknesses. No SpO₂ monitoring was performed in the preoperative period, and the possibility that these patients always become hypoxemic at night breathing room air cannot eliminated. We also lacked control groups who underwent the same monitoring but were treated with alternate acute pain therapies. Such patients may experience more or less hypoxemia. Moreover, this study does not address the possibility that surgical pain and suffering rather than narcotic administration may be the cause of the observed postoperative nocturnal hypoxemia.

Nevertheless, the extent of the hypoxemia that occurred in a high percentage of our patients who were breathing room air while using PCA is a remarkable and disturbing finding. PCA may be the safest mode yet devised for delivering narcotics, but we believe that our data suggest that both more careful monitoring and the provision of supplemental O₂ should be considered whenever narcotic PCA is prescribed.

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