Pathophysiological and Perioperative Features of Morbidly Obese Parturients

Yigal Leykin; Tommaso Pellis

Disclosures

Expert Rev of Obstet Gynecol. 2009;4(3):313-319. 

In This Article

Regional Anesthesia & Analgesia for Labor

Regional anesthesia, when compared with general anesthesia, offers several advantages, including minimal airway intervention, minimal cardiopulmonary depression, decreased intra- and postoperative opioid and sedative requirements, decreased postoperative nausea and vomiting, and shortened PACU/hospital stay.[50]

However, regional anesthesia can be technically difficult to perform due to problems with patient positioning, inability to detect usual bony and muscular landmarks, and inadequate needle length (Box 2). According to Jordan et al., successful epidural needle placement in 74% of obese patients required more than one attempt and, in 14%, three attempts.[53] A failed or incomplete block may require general anesthesia and tracheal intubation, often under less than ideal conditions.[54]

Obese patients normally have smaller cerebrospinal fluid (CSF) volumes than normal weight patients, and these changes are further exaggerated in the obese parturient. Decreased CSF volume due to increased abdominal pressure (obesity or pregnancy) may produce more-extensive neuraxial blockade due to diminished dilution of anesthetic. The mechanism by which increased abdominal pressure decreases CSF volume is probably inward movement of soft tissue in the intervertebral foramen displacing CSF.[55] The epidural space volume is also reduced, due to adipose infiltration and increased venous distension from aortocaval compression and increased intra-abdominal pressure, resulting in higher spread of local anesthetic and in higher risk of hypotension and respiratory difficulty.[54]

Although regional anesthesia is much safer than general anesthesia, it is worth reminding that approximately a quarter of all obstetric anesthesia-related deaths are associated with the administration of regional anesthesia (70% with epidural anesthesia, 30% with spinal anesthesia).[45] Therefore, the major challenges in regional anesthesia for obese pregnant women are the identification of appropriate landmarks, adequate patient positioning prior to and after performing the block, choosing a needle of sufficient length and the appropriate dose of local anesthetic (Figure 1 & Figure 2).

Figure 1.

Difficult identification of landmarks required for local anesthesia.

Figure 2.

Always be prepared with longer needles for spinal anesthesia.

In the sitting position, the landmarks and the midline are more easily appreciated compared with the lateral position. However, attention is warranted when returning from the sitting to lateral position, since skin movements over the subcutaneous fat tissue may draw the catheter out of the epidural space. Accordingly, the patient should be allowed to return to a more neutral, relaxed position before fixing the catheter to the skin.[50]

Ultrasonic guidance can aid in performing a neuraxial block in obese patients. Ultrasound studies confirm changes in spinal anatomy and increased skin to epidural distance during gestation.[12] However, it was also demonstrated that, in obese patients, BMI is a poor predictor of the distance to the epidural space, owing to a disproportionate distribution of body fat. Hence, it seems prudent to use, as a first attempt, a standard spinal or epidural needle and only if this fails switch to longer ones (Figure 2).[25,56]

Spinal anesthesia is often used for elective cesarean delivery but, in the obese parturient, this technique may result in a higher level of spinal block, and a single-dose injection may produce, in cases of prolonged surgery, insufficient anesthesia duration.[57] Epidural anesthesia via an epidural catheter can overcome this problem; however, this technique may be inadequate in more than 25% of patients due to difficulty associated with blocking sacral nerve roots and unblocked dermatomes after catheter placement.[25] Continuous spinal anesthesia represents an alternative approach, but it is still not a routinely used method in morbidly obese parturients. A single-shot spinal anesthesia remains, to date, the most common anesthetic technique used for cesarean-section delivery.[58] It is worth mentioning that a morbidly obese woman under spinal anesthesia may experience significant impairment of respiratory function.[59]

A combined spinal-epidural technique represents an attractive alternative, combining the quality of spinal block with the flexibility of an epidural catheter, and also using lower doses of local anesthetics.[12,25] However, there is a potential risk that previously injected epidural drugs may enter the intrathecal space after a combined spinal-epidural technique, resulting in maternal bradycardia and asystole.[60]

While in the obese parturient there is an increased incidence of accidental dural punctures, the incidence of postdural puncture headaches is lower than in nonobese.[61,50] The reduced incidence of postdural puncture headache in the morbidly obese patient may be the result of a decreased pressure gradient between the subarachnoid and the epidural spaces, owing to engorged epidural veins and increased epidural fat, more epidural fat to plug the dural puncture once it occurs and higher intra-abdominal pressure, which reduces spinal fluid leakage.[50]

The importance of labor analgesia is not to be underestimated in the case of morbidly obese parturient owing to, on average, longer labor and a higher rate of oxytocin induction.[12] Prepregnancy weight is positively associated with the incidence of fetal macrosomia and labor abnormalities, such as shoulder dystocia. Each of these is a known risk factor for more painful contractions and complicated labor.[12]

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