Introduction
The benefit of the semi-sitting position is still controversial in both neurosurgery
and neuroanesthesia. Neurosurgical procedures in the semi-sitting position are rarely
performed, mainly because of the perception of increased risk related to this position.
While the patient in the vertical position offers a number of advantages to the neurosurgeon,
there are challenges for the anesthetist. Many institutions frown upon the semi-position
technique due to potential serious consequences, such as venous air embolism. The
use of the semi-sitting position is apparently lowest in Japan.[1]
[2]
There are, however, indisputable advantages to the semi-sitting position for posterior
fossa and dorsal cervical spine procedures. The sitting position allows gravitational
drainage of venous blood and cerebrospinal fluid (CSF) and better guidance of surgical
approaches to the midline structures, significantly reducing cerebellar edema, surgery
time, and blood loss. These are key features that could contribute to a better outcome
of patients in semi-sitting position. There are also several advantages to the anesthesiologist,
such as the possibility of venting at lower pressure, less impairment of the diaphragm
motion, and improved access to the tracheal tube.[3]
[4]
[5]
[6]
While there are specific complications associated with the semi-sitting position[7]
[8]
[9]
[10], the use of the semi-sitting position should be based on an interdisciplinary dialogue
between neurosurgeon and anesthesiologist.
Discussion
The use of the semi-sitting position remains controversial, despite its distinct advantages
over the horizontal position. Many institutions are reluctant to use it out of fear
of serious complications, mainly air embolism and paradoxical embolism (PAE). Other
concerns include venous gas embolus in the presence of a patent foramen ovale (PFO),
hypotension, spinal cord syndrome, quadriplegia and pneumocephalus, damage to the
peripheral nerve, quadriplegia, macroglossia, and paradoxical arterial embolism, which
is the most serious complication.[7]
[8]
[9]
[10]
However, embolism during neurosurgery is not related exclusively to the sitting position
and has often been reported in the prone and supine positions. While the advantages
of the sitting position are well documented, the impact and relevance of long-term
embolism has not been well investigated. Reports show there is less blood loss and
better preservation of cranial nerve function in a semi-sitting position.
Embolism is not exclusively associated with the semi-sitting position. One study reported
embolism in 10-17%, also in of the craniotomy performed in the dorsal position.[11] The overall incidence of gas embolism during surgery in semi-sitting position in
cranial surgery is slightly higher, at 21%. These results, which are gathered from
the articles reviewed here, conform with previous studies, which report incidence
of embolism ranging between 7 and 76%.[3]
It is important at this point to stress that the potential advantages of the sitting
position are offset, in emergency cases (reduction of cerebellar swelling), even by
the additional time spent by the surgical team positioning the patient, about 15-20
minutes, when compared to the upright position. In these cases, medical teams should
consider alternative positioning techniques, such as semi-inclined.
The overall incidence of embolism in the sitting position is 39% for operations in
the posterior fossa and 12% for cervical procedures. Detection rate also depended
on the monitoring method, with transesophageal echocardiogram rate was 25.6%, while
with the esophageal Doppler it was 9.4%.[12]
[13]
[14]
[15]
[16]
Stendel et al performed transesophageal echocardiograms in neurosurgical patients
in the lying and semi-sitting positions. Air embolism was observed in 35% of cervical
(foraminotomy) and 75% of posterior fossa operation cases.[17]
Some authors argue that the measurement of ETCO2 alone is an appropriate method for monitoring during the semi-sitting position. The
incidence of embolism while monitoring for the fall in ETCO2 was 11% for cranial surgery and 6.8% for cervical spine surgery.[9]
Simply detecting embolism, however, does not necessarily mean the patient is at risk.
It is just as important to investigate the sequelae associated with hemodynamic or
respiratory embolism.[14]
[18]
Duke et al investigated the incidence of air embolism in a study of patients undergoing
surgery for vestibular schwannoma in the sitting and supine positions. They found
an incidence of 28% in the semi-seated position versus 5% in the supine position.
Serious embolism with hypotension rates were 1.8% and 1.4%, respectively, with no
significant difference between the two groups.[7]
There are several other ways to detect venous air embolism, including transesophageal
echocardiography, reduced end-tidal CO2 (ETCO2) pressure, precordial Doppler changes, increased pulmonary artery pressure in the
catheter, and direct observation of clinical parameters. The transesophageal echocardiogram
(TEE) is the most sensitive test, capable of detecting 0.02 mL/kg of injected air
bubbles, or 5 to 10 micrometers. However, it is expensive and impractical to keep
trained professionals available continuously monitoring via TEE intraoperatively.
Moreover, TEE may lead to complications such as esophageal bleeding, displacement
of the endotracheal tube, and risk of glottis injury with prolonged use.[15]
[17]
Some measures to reduce the occurrence of embolism may actually induce an increase
pulmonary artery pressure, which may lead to the occurrence of air embolism. Specifically,
the positive end-expiratory pressure (PEEP), although it may reduce the occurrence
of air embolism may also, through the increase in right atrial pressure, increase
the risk of an intracardiac shunts from right to left, causing an increase pulmonary
artery pressure.[19]
[20]
The influence of patient posture on the respiratory system has been studied in different
situations. When healthy humans switch from the sitting to the supine posture, functional
residual capacity reduces and breathing resistance increases. In other words, esophageal
pressure values are slightly less negative when sitting compared to supine, but the
transdiaphragmatic pressure remains the same.[21]
[22]
While such postural changes have limited relevance to healthy individuals, they may
influence the breathing pattern and dyspnea in patients with severe chronic lung disease.
The supine posture can increase intrinsic positive end-expiratory pressure, dynamic
hyperinflation, airway resistance, expiratory flow limitation, or orthopnea in patients
with chronic obstructive pulmonary disease (COPD) or obesity. Conversely, some authors
have reported relief of dyspnea for some patients with COPD non-ventilated while in
the supine position. Nonetheless, the supine position can significantly reduce functional
residual capacity in patients with phrenic or neuromuscular paralysis and, in mechanically
ventilated patients, limitation of expiratory flow and PEEPi may increase.[23]
[24]
[25]
We found no significant change in respiratory effort between the sitting and supine
positions in stable COPD patients under noninvasive ventilation, regardless of their
seniority or obesity. In patients with prolonged or difficult weaning, respiratory
muscle weakness is usually present. The effects of posture on these patients' breathing
effort may be clinically relevant. Although a semi-sitting position in bed is often
recommended for patients on mechanical ventilation because of the lower risk of microaspirations
through the endotracheal tube, it is often difficult to maintain in clinical practice.[26]
[27]
Respiratory effort significantly decreased in the semi-sitting position, more comfort,
and less necessity of high PEEP compared with other positions.[28]
As for PEEPi and dyspnea, there were only minor changes across postures. The functional
residual capacity is reduced by 20-30% when the patient goes from sitting to the supine
posture. This effect relates to the increase in intrathoracic blood volume, the greater
head position of the diaphragm, and the obstruction of small airways. The reduction
in functional capacity leads to increasing resistance against the position to sit.[29]
[30]
[31]
In COPD patients, the supine position is associated with dyspnea, or orthopnea and
expiratory flow. Limitation at rest manifests earlier more in the supine than in semi-sitting
position. Many patients with COPD have more severe dyspnea in the supine position.
In such cases, pulmonary hyperinflation, airway resistance, PEEP, and expiratory flow
limitation may play a role in the genesis of orthopnea. In several patients with heart
failure or obesity, airway resistance, increased PEEP, expiratory flow limitation,
and orthopnea are also more prevalent in the supine position compared to semi-sitting.
Patients with severe phrenic or neuromuscular paralysis experience a large reduction
in functional residual capacity when supine. In mechanically ventilated adults with
acute respiratory distress syndrome, the supine posture offers higher expiratory flow
limitation and work-related PEEPi when compared to the semi-sitting.[32]
[33]
[34]
[35]
The semi-sitting position induces a smaller respiratory effort than the supine and
sitting positions. Weaning patients experience a significant increase in respiratory
load and muscle effort for breathing. Mechanical ventilation is meant to decrease
this breathing effort to a level acceptable to the patient. Weaning failure often
results from an imbalance between themechanical load and respiratorymuscle activity.
Most studies evaluating changes in breathing effort according to position reported
little to no differences. In healthy individuals, transdiaphragmatic pressures remained
unchanged between supine, semi-sitting, and sitting.[36] The same held true for elderly individuals in the semi-sitting and supine positions.
In patients with stable COPD, transdiaphragmatic pressures were lower when sitting
versus in supine. In obese individuals, there was no variation between average transdiaphragmatic
pressures in the supine and sitting positions. While postural changes probably are
not relevant to healthy and stable individuals, it may have relevance in patients
with more severe respiratory difficulties.[36]
[37]
[38]
Several studies have indicated that many ICU patients are not positioned in a semi-reclined
position. Although a 45° position in bed is recommended for patients on mechanical
ventilation, mainly because of decreased risk of microaspirations, reduce brain venous
pressure, intracranial hypertension, this is often difficult to maintain in practice.[39]
Conclusion
As shown in the discussion above, the semi-sitting position is a safe and useful adjunct
in neurosurgery of the posterior fossa and the upper cervical spine. Nonetheless,
this requires team effort between dedicated neurosurgeons and anesthesiologists for
a safe fit and seamless positioning. However, there are alternative ways to ensure
adequate and safe positioning.
The semi-sitting position at a 45° angle decreases the inspiratory effort, and is
as comfortable or more for patients that suffer from difficult weaning in ventilation.
Reports show that PEEP is moderately higher in the supine position, whereas the supine
yields lower PEEPi values, but higher breathing effort, possibly because of a central
command of the upper breathing. There is also the potential role of decreased chest
wall compliance when sitting, as seen in paralyzed patients. These results are clinically
relevant because they demonstrate that the semi-sitting position reduces PEEP and
relieves respiratory muscles in difficult weaning situations.
