Neurosurgical procedures in sitting position need advanced cardiovascular monitoring.
There is an ongoing debate about the sitting position (SP) in neurosurgical patients. The SP provides a number of advantages as well as severe complications such as commonly concerning venous air embolism (VAE). The best monitoring system for the detection of VAE is still controversial.
In a retrospective analysis Günther et al compared 208 patients. Transesophageal echocardiography (TEE) or transthoracic Doppler (TTD) were used as monitoring devices to detect VAE; 101 cases were monitored with TEE and 107 with TTD.
The overall incidence of VAE was 23% (TTD: 10%; TEE: 37%), but the incidence of clinically relevant VAE (drop in end-tidal carbon dioxide above 3 mmHg) was higher in the TTD group (9 out of 17 VAE, 53%) compared to the TEE group (19 out of 62 VAE, 31%). None of the patients with recorded VAE had clinically significant sequelae.
In this small sample they found more VAE events in the TEE group, but the incidence of clinically relevant VAE was rare and comparable to other data. There is no consensus in the definition of clinically relevant VAE. 1).
Invasive arterial blood pressure measurements for systolic (ABPsys), diastolic (ABPdiast) and mean arterial pressure (MAP) are established monitoring technologies for these kind of procedures. A noninvasive device for continuous monitoring of blood pressure and CO based on a modified Penaz technique (volume-clamp method) was introduced.
The surgical field remains clean during the whole procedure. The venous pressure is very low and there is less venous bleeding. The main disadvantage of this position is the occurrence of air embolism which is rapidly and precisely diagnosed with the trans-esophageal Doppler. The air can be aspirated from the right atrium with the central lines placed in the correct position pre-operatively.
The sitting position is generally preferred for the supracerebellar approach because it allows gravity to work in the surgeon’s favor by facilitating cerebellar retraction and reducing venous bleeding and pooling in the operative field. The disadventage involving the risk of air embolus can be minimized by taking proper precautions.
The sitting position can be employed with acceptable rates of morbidity and mortality. However, these reports were prepared by groups performing 50 to 100 or more of these procedures per year, and the hazards of the sitting position may be greater for teams who have less frequent occasion to use it. With increasing frequency, the sitting position is being avoided through the use of one of its alternatives (prone, semilateral, lateral positions). However, we are likely to continue encountering it because even surgeons who are inclined to use alternative positions may opt for the sitting position when access to midline structures (the floor of the fourth ventricle, the pontomedullary junction, and the vermis) is required. Nonetheless, alternative positions for posterior fossa surgery exist and should be considered when a patient has contraindications to the sitting position.
Because controversy exists regarding continued use of the seated position for neurosurgical procedures, this prospective (1981-1983) and retrospective (1972-1981) analysis of 554 seated patients was done to establish the incidence and severity of venous air embolism (VAE) related to type of surgical procedure and anesthetic technique; to examine the impact of specific monitoring practices on detection, morbidity, and mortality; and to establish the incidence of other complications related to the seated position (hypotension, quadriplegia, and arterial air embolism (AAE)). The overall morbidity and mortality related to the seated position was 1% (2 VAE, 1 AAE, 2 hypotension, 1 myocardial infarction) and 0.9% (1 VAE, 1 AAE, 2 hypotension, 1 quadriplegia), respectively. There has been no mortality since 1975. N2O did not seem to increase the incidence or severity of VAE. The seated position is safe in experienced hands if appropriate surgical and anesthetic skills are exercised in patient selection and management. Caution is advised in patients with atherosclerotic cardiovascular disease, severe hypertension, cervical stenosis, and right to left intracardiac shunts 3).