The role of neuromonitoring in cardiovascular surgery.

This review describes the techniques currently used for quantitative neurophysiologic measurement during cardiac surgery and their potential impact on clinical outcome. Electroencephalography (EEG) characterizes cerebrocortical neuronal electrical activity and was part of some of the earliest cardiopulmonary bypass procedures, yet today it is not widespread use. Each of the common misunderstandings regarding a supposed limitation of this technology is explained. Its major genuine shortcoming, a lack of selectivity, may now be overcome with the combined use of additional monitoring modalities. The influence of intracranial hemodynamics on observed EEG changes may be determined continuously and noninvasively with transcranial Doppler (TCD) ultrasound. TCD provides an indication of sudden change in either blood flow or vascular resistance as well as the detection of emboli. In addition, the metabolic status of cortical neurons can be monitored by regional cerebral venous oxygen saturation (rCVOS) using noninvasive transcranial near-infrared spectroscopy. The % rCVOS tends to remain remarkably stable over a wide range of temperatures, perfusion pressures, and anesthetic states. Marked change in either direction signifies a serious imbalance between oxygen delivery and consumption. Measurement of rCVOS does not require blood flow, pulsatile or otherwise, so that it offers the only means of monitoring during circulatory arrest. By characterizing the dynamic interplay among cerebral hemodynamics, metabolism, and electrogenesis, these technologies permit the rapid detection and correction of potentially hazardous conditions.

Postbypass effects of delayed rewarming on cerebral blood flow velocities in infants after total circulatory arrest.

Cerebral perfusion is reduced after prolonged periods of total circulatory arrest in infants. Methods of rewarming after arrest may modify the flow pattern of recovery, and a single report has suggested that using cold reperfusion to delay rewarming could mitigate abnormalities in cerebral blood flow. Cerebral perfusion was evaluated by transcranial Doppler sonography in 16 infants who required periods of total circulatory arrest of 35 minutes or more. In group A (n = 9) rewarming was begun immediately on reperfusion, whereas in group B (n = 7) a 10-minute period of cold reperfusion was instituted before rewarming was begun. The mean and end-diastolic flow velocities were measured before incision (baseline) and at 20, 45, and 90 minutes after conclusion of cardiopulmonary bypass. Mean arterial pressure, hematocrit value, and arterial carbon dioxide tension were controlled, with no significant differences between the two groups (p > 0.05). In group A, the mean cerebral blood flow velocity was below the baseline level at all three postbypass measurements (p < 0.001). In group B, however, mean velocity did not differ significantly from the baseline value (p > 0.05). Twenty minutes after bypass, 89% of the patients in group A had no diastolic Doppler signal, indicating absence of perfusion during diastole, compared with only 28% in group B (p = 0.02). These preliminary results suggest that a delay in rewarming on reperfusion may be beneficial in infants after circulatory arrest.

A prospective study of rectal methohexital: efficacy and side effects in 648 cases.

Rectal methohexital has been used for nearly 30 yr in pediatric anesthesia. Despite this long and increasingly varied use, no large prospective series has been published detailing safety and efficacy. This study prospectively evaluated the efficacy, safety, and side effects of this medication in a series of 648 cases. On 553 of 648 occasions (85%), the child fell asleep after a single 30-mg/kg dose of 10% methohexital. Sleep was less likely in patients with myelomeningocele or who were receiving oral phenobarbital or phenytoin. When sleep occurred, the average time to onset of sleep was 6 min. Most patients who remained awake 15 min after drug were sedated. Defecation (10%) and hiccups (13%) were common but benign side effects. Partial airway obstruction and/or desaturation to Spo2 < or = 93% occurred in 26 patients (4%), but was resolved with blow-by oxygen and/or jaw-thrust in all but two cases. These two patients (0.3% of total) required aggressive airway intervention by the supervising anesthesiologist. Apnea did not occur in any patient. Methohexital has a high efficacy rate for sleep (85%) or sedation (96%), and has a relatively rapid onset. Significant respiratory side effects occur infrequently, but can be life threatening if not properly managed.

Auditory evoked responses in children during hypothermic cardiopulmonary bypass: report of cases.

Variations in core temperature and cerebral blood flow during open heart surgery may affect auditory brainstem responses (ABRs) and middle latency responses (MLRs) in both adults and children. We documented the changes in ABRs of two infants (ages 3 and 11 weeks, respectively) with variations in core temperature during hypothermic cardiopulmonary bypass and total circulatory arrest and compared them with those of a 19-year-old adult. Changes in MLRs that occurred in association with reductions in cerebral blood flow as monitored by transcranial Doppler are also reported in a 6-year-old child. With the reductions in temperature in both infants and the young adult, ABR latencies increased and amplitudes decreased. Effects of hypothermia on ABR latencies were completely reversed by rewarming. MLR amplitudes were transiently reduced during periods of normothermic hypoperfusion. Hypothermia partially prevented these changes, and normoperfusion after rewarming recovered MLRs. Monitoring ABRs and MLRs may be a useful technique for assessment of brain function during hypothermic cardiopulmonary bypass in children and infants.

Cardiorespiratory effects of premedication for children.

Cardiovascular and respiratory effects of pediatric preanesthetic premedication have received only minimal attention, probably because most children tolerate such drugs without apparent ill effect. In children with congenital heart disease or other serious illness, there is often reluctance to use premedication. We sought to determine whether different premedication regimens produced significant cardiorespiratory effect. A randomized prospective study of the cardiovascular and respiratory effects of different oral, nasal, and rectal premedication regimens was conducted. Fifty-eight young children (average age 2.7 yr) were studied. Oral meperidine (3 mg/kg) with pentobarbital (4 mg/kg) decreased heart rate, mean arterial pressure, cardiac index, respiratory rate, and oxygen saturation. Stroke volume was maintained. Nasal ketamine (5 mg/kg) with midazolam (0.2 mg/kg) produced no significant cardiovascular or respiratory effects. Rectal methohexital (30 mg/kg) increased heart rate with a coincident decrease in stroke volume but had no other positive or negative cardiac or respiratory effect. This information documents disparate cardiorespiratory effects of different preanesthetic medications in normal children.

Cardiovascular effects of rectal methohexital in children.

STUDY OBJECTIVE: To define the cardiovascular effects of rectal methohexital in children with normal cardiac function. DESIGN: Cardiovascular evaluation of each patient was performed before and after medication. Each patient's predrug results were used as control measurements for comparison with measurements made after methohexital administration. SETTING: Inpatient operating room induction area in a privately endowed philanthropic children's hospital. PATIENTS: Forty-seven children age 35 +/- 22 months (mean +/- SD) scheduled for elective orthopedic or plastic surgery, free of cardiac or pulmonary disease, and receiving no medication with central nervous system activity. INTERVENTIONS: Control measurements of heart rate (HR), blood pressure (BP), and echocardiographic evaluations were obtained on the day before scheduled surgery. Repeat measurements were performed after the onset of methohexital-induced sleep. The time span of the measurements was designed to include the period of peak plasma methohexital concentration. In the preoperative holding area, 30 mg/kg of a 10% methohexital solution was administered rectally. If sleep did not occur in 15 minutes, an additional 15 mg/kg was given. MEASUREMENTS AND MAIN RESULTS: HR increased markedly after rectal methohexital [126 +/- 23 beats per minute (bpm) to 144 +/- 21 bpm, p less than 0.001], and stroke volume (SV) decreased (24 +/- 9 ml to 21 +/- 8 ml, p less than 0.01). There were no significant changes in BP or cardiac index. The shortening fraction and ejection fraction remained within the normal range for this age-group. CONCLUSIONS: Rectal methohexital induces sleep in healthy pediatric patients with minimal cardiovascular side effects. The primary effects are increased HR and decreased SV.

Application of the Mizus endotracheal obturator in tracheostomy and tentative extubation.

A variety of catheters, stylets, and obturators have been used to assist with problems in airway management. Obturators specifically designed for airway use are now available in different sizes. The pediatric-size obturators (2.2 mm diameter) can be placed into the airways of most patients without apparent respiratory impairment. Use of these obturators to preserve a path to the airway is described in cases of tracheostomy and tentative extubation. The advantages and disadvantages inherent in the use of such obturators are described.

Endotracheal tube leak pressure and tracheal lumen size in swine.

Endotracheal tube "leak" is often estimated in children to judge the fit of uncuffed endotracheal tubes within the trachea. Twenty-five swine were intubated with uncuffed tracheal tubes to determine whether a more sensitive measurement of leaks could be devised and whether leak pressure estimates fit between tracheal tube and trachea. We compared leak pressure measurement using a stethoscope and aneroid manometer with a technique using a microphone, pressure transducer, and recorder, and found no differences between the two methods. The tracheas were then removed and slides prepared of tracheal cross-sectional specimens. Regression analysis revealed a linear relationship between tracheal lumen size and tracheal tube size for both low leak pressure (y = -0.4 + 0.79x, r = 0.88, P less than 0.05) and high leak pressure (y = -2.9 + 0.71x, r = 0.92, P less than 0.05) groups. We conclude that leak testing with a stethoscope and aneroid manometer is sensitive and accurate, and that tracheal tube leak pressure accurately portrays fit between tube and trachea.