Arterial Blood Pressure

Intermittent noninvasive blood pressure monitoring is standard during all anesthetic procedures and is considered to be enough control for most moderately invasive spine surgeries. A beat-to-beat control of blood pressure is necessary when managing fragile patients or during potentially bleeding surgeries, as well as during all intracranial procedures. Indeed, such a tight control and invasive monitoring allows maintaining adequate cerebral perfusion and limiting the risk of bleeding, either during the course of the procedure or upon recovery. Invasive arterial blood pressure monitoring may also be of help when assessing the intravascular volume status and for drawing arterial blood samples and performing gas analysis.

The entire set for invasive blood pressure monitoring includes an arterial cannula, a continuous irrigation system through infusion of saline under pressure, a pressure transducer that transforms the mechanical signal into an electrical one, and a monitor with a display screen. Zeroing is required at starting of use, with the transducer placed at the horizontal level of the heart. Site for arterial cannula insertion may vary, the most frequent being the radial artery at the wrist. Ulnar, pedal, and femoral arteries can also be used. Humeral artery should be avoided because its distal territory cannot be supplied by collateral vascularization. Strict sterile conditions must be respected during insertion, and echo guiding may be of help for difficult cases ( Fig. 9.1 ).

Example of an echo-guided arterial cannula insertion. Localization of the radial artery is made easy using echography and color Doppler, allowing visualization of flux pulsatility and direction (1). After puncture of the artery, the insertion of the metallic guide can be followed online (2) and the adequate positioning of the catheter into the artery can be checked thereafter (3).

Continuous beat-to-beat measurement of blood pressure is also possible noninvasively, using volume clamp or arterial tonometry methods. The volume clamp derives blood pressure according to pressure variations in a finger cuff, so as to keep finger artery volume constant. Arterial tonometry uses fingertip plethysmography and pulse volume amplitude. Some authors have evaluated the use of such systems during neurosurgical procedures, in patients where arterial blood sampling is not expected to be necessary. The idea was to avoid the well-known complications of an arterial cannula, including vessel or nerve trauma, pseudo-aneurysm formation, bleeding due to unnoticed disconnection, local hematoma, infection, arterial thrombosis, distal embolism, and ischemia. Although some studies have reported acceptable bias as compared to invasive intra-arterial measurement, a recent meta-analysis has warned about out of acceptable range inaccuracy and imprecision.

Blood pressure management during a neurosurgical procedure must take account of patient comorbidities, risk of bleeding, and brain perfusion. Large hemodynamic variations frequently occur during induction of and recovery from general anesthesia, and practitioners should seek after stability as much as possible. However, perfusion pressure is not the only determinant of cerebral blood flow. In addition to CPP, autonomic regulation, metabolism coupling, and cerebrovascular reactivity to CO ₂ and O ₂ , acute or chronic changes in global cardiac output play a role. Cerebral blood flow autoregulation should be regarded as a dynamic integrated process, displacing the lower limit, upper limit, and plateau height of the autoregulatory perfusion pressure curve as a function of circumstances. Hence, combining blood pressure monitoring with estimates of cardiac output and cerebral circulation seems to be sound and might reveal efficient at ameliorating patient outcome in the future.

The respiratory variation of the blood pressure curve also offers simple means for assessing the intravascular volume status. This point is developed later in the chapter.

Cardiac Output

Assessment of cardiac output during intracranial neurosurgery has recently been advocated as a potential interesting monitoring to help optimizing cerebral hemodynamics. However, systematic cardiac output monitoring in a neurosurgical context is not the rule, except for very specific and rare procedures such as giant aneurysm clipping under circulatory arrest and in patients with severe cardiovascular comorbidities.

Gold standard methods of cardiac output measurement are the classical thermodilution techniques through a pulmonary artery catheter and transthoracic or transesophageal echocardiography. For neurosurgical procedures, and in otherwise healthy patients, transesophageal echocardiography is mainly used during surgery in the sitting position in case of patent oval foramen to detect eventual paradoxical air embolism. Transesophageal Doppler of the aorta can also estimate the stroke volume from the aortic velocity waveform and is sometimes considered as a reference measure of cardiac output. Its accuracy highly depends on probe placement and on any modification of the aortic diameter such as in case of intra-aortic balloon or coarctation. Several noninvasive devices are also proposed with variable performance, accuracy, and precision. Electrical thoracic bioimpedance relies on the assumption that thoracic impedance changes parallel to those of stroke volume. The limitations of the technique include electrode positioning, electrical interference, abnormal fluid inside the thorax, changes in peripheral vascular resistance, age, gender, arrhythmias, and movement. Thoracic bioreactance is a variant, which takes account of thoracic voltage phase shift and has almost the same limitations, except that it can still provide an estimate of cardiac output in case of cardiac arrhythmia because it averages values over several seconds. Cardiac output may also be derived from the arterial pressure contour, obtained either directly or from the noninvasive methods described above, and using variable algorithms. All these techniques are highly dependent on arterial pressure waveform signal quality, and therefore on eventual finger edema, intense vasoconstriction, sensor position, and movement. Some of them necessitate calibration and others not.

The clinical utility and reliability of cardiac output measurement in neuroanesthesia have been relatively poorly studied. Thoracic bioimpedance has been demonstrated to reliably mirror the hemodynamic changes associated with crucial events during neurosurgical procedures such as induction, scalp infiltration, changes in anesthetic depth, or mannitol administration. Stroke volume monitoring is also sometimes used to guide fluid administration during intracranial neurosurgery. Up to now, scientific evidence does not support equivalence between invasive and noninvasive methods of hemodynamic monitoring and the lasts cannot replace the formers when close monitoring is needed, such as during surgery in the sitting position.

Intravascular Volume Status

Substantial fluid shifts can occur during neurosurgical procedures, either as a consequence of blood loss or following the administration of medications aiming at improving brain relaxation, such as mannitol or furosemide. Hence, controlling for the intravascular volume status is essential. Several methods are available of which some are simple and easy to implement. Indeed, central venous pressure and pulmonary capillary wedge pressure are no longer considered as gold standards, at least in patients under positive pressure ventilation.

The invasive arterial pressure waveform allows measuring the delta pulse pressure (DPP) or the delta-down (DD) intermittently ( Fig. 9.2 ). To do so, the patient must be in cardiac sinus rhythm and under mechanical ventilation. The tidal volume should be set at least at 8 mL/kg, and the frequency rate reduced to values as low as 4 per min to mimic respiratory pauses. DPP corresponds to the difference between the maximal and minimal pulse pressure during one breathing cycle, divided by the mean of those two pulse pressures, while DD is the difference between the systolic arterial pressure at the end of a 5-s respiratory pause, immediately before lung inflation, and its minimal value during the course of one respiratory cycle. Values of DPP higher than 13%, and values of DD higher than 5 mmHg are indicative of an increased probability of preload cardiac output dependency and a positive hemodynamic response to fluid loading. Both indices have been demonstrated to be accurate in the context of intracranial neurosurgery. Their main disadvantages are their intermittent nature and susceptibility to confounding factors, including nonsinus cardiac rhythm, ventilation volume and pressure, vasopressor administration, abdominal pressure, and cardiac failure. Dynamic parameters may have additive value, insofar as they offer a continuous assessment of the intravascular volume. They are based on systolic pressure or stroke volume variability as assessed by pulse contour analysis of an arterial or plethysmographic waveform. Using such technology, several commercially available devices provide indexes of susceptibility to fluid loading. Closed-loop systems of fluid administration have even been proposed. Accumulating evidence suggests that goal-directed fluid administration is beneficial for enhanced recovery after specific types of surgeries, but this question has still not been fully addressed for neurosurgical procedures.

Illustration of delta-down (DD) and delta pulse pressure (DPP) calculation. For this measurement, respiratory rate should be set at 4 per min or lower to mimic a respiratory pause. The patient should be in cardiac sinus rhythm, and the respiratory tidal volume should be 8 mL/kg or higher. DPP corresponds to the difference between maximum and minimum pulse pressure over a respiratory cycle, divided by their mean. DD is the difference between maximum and minimum systolic blood pressure over a respiratory cycle.

Hemoglobin Concentration

The main oxygen transporter in the blood is hemoglobin. During potentially bleeding neurosurgical procedures and in patients with preexisting anemia, it is necessary to have prompt access to rapid, accurate, and reliable hemoglobin concentration measuring devices. Aside from automated laboratory hemoglobin measurement systems, which may be too time demanding particularly in the context of an emergency, several point-of-care devices are proposed.

Blood sample–requiring systems at the bedside use reagent-based or nonreagent-based absorption photometry at multiple wavelengths or conductivity. Other noninvasive, and hence non–blood sample–requiring devices involve multiple wavelength co-oximetry, occlusion spectroscopy, or transcutaneous reflection spectroscopy technologies. Several studies have been performed to compare the results obtained with those devices and those obtained using automated laboratory systems or between them ( Table 9.1 ), not necessarily in the context of neurosurgery. Although bias as compared to laboratory values is generally small, the limits of agreement may be large with a significant amount of outliers. In addition, discrepancies may vary according to real hemoglobin level and several confounding factors. Pulse co-oximetry may be confounded by ambient light, bilirubin, hemoglobin variants, intravascular dyes, motion, sensor positioning, temperature, venous congestion, edema, high tissue lipid content, and hypovolemia. Although having been demonstrated to be indicative of hemoglobin level during complex spine surgery, pulse co-oximetry remains less accurate than conventional laboratory measurements. Accuracy may improve when calibrating with a baseline laboratory testing at the beginning of the procedure, particularly in children undergoing neurosurgery. Those noninvasive monitors provide more timely information, but should be considered as trend monitors rather than absolute indicators of transfusion need.

Jugular Venous Oxygen Saturation

Near infrared spectroscopy (NIRS) combined with transcranial Doppler ultrasonography (TCDU) and jugular venous oxygen saturation (SjvO ₂ ) monitoring are means for appreciating brain hemodynamics and oxygen delivery. SjvO ₂ monitoring requires the retrograde placement of a catheter into a jugular vein, up to the jugular bulb, to monitor the mixed cerebral venous blood and above the C1/C2 level to limit contamination by the facial venous blood. Side for placement should preferably be the dominant side of the brain or the side with the most prominent pathology, if any. Oxygen saturation may be measured in regularly drawn blood samples or using a fiber-optic catheter and light absorption in the red/infrared wavelength range. Although generally quite easily placed, the catheter may be the source of rare complications, including hematoma, infection, thrombosis, raised ICP, or incidental wrong vessel puncture.

SjvO ₂ reflects the balance between oxygen delivery to the brain and cerebral metabolic consumption of oxygen. Normal values range between 50% and 75%. Decreases in SjvO ₂ can be seen in case of systemic or local oxygen supply deficiency, including hypoxia, low blood pressure, decreased CPP, embolism, or vasospasm ( Table 9.2 ), and in case of increased oxygen consumption including hyperthermia and seizure. Causes of SjvO ₂ values above normal range can be classified into those related to a decrease in cerebral metabolism, restricted oxygen diffusion or extraction, shunting, increased oxygen supply, or a combination of these. For example, a global decrease in cerebral metabolism can be observed during hypothermia. Infarction or inflammation causes restriction in oxygen diffusion and extraction, as well as microvascular shunting. Increased oxygen supply is observed during hyperoxia, and hypercarbia can be responsible for substantial shunting.

Interpretation of SjvO ₂ values may be uneasy in some instances. It is a global measure and it may miss eventually occurring focal brain ischemia. When brain tissue death is massive, SjvO ₂ can be high as a consequence of shunting. Classical causes of wrong SjvO ₂ values are displacement of the catheter, thrombus around its tip, and contamination by facial venous blood. Anesthesia may also have an influence on SjvO ₂ values. Transient desaturations have been reported under propofol anesthesia, more frequently in normothermic than in hypothermic patients, while SjvO2 increases have been observed under sevoflurane anesthesia. This is due to the different properties of those agents on cerebral vasculature, propofol being vasoconstrictive, and sevoflurane vasodilating. Side of recording may also be important, insofar as cerebral venous drainage usually predominate on one side through the dominant jugular bulb.

SjvO ₂ has mostly been used to guide therapy in the intensive care unit for traumatic brain injury and subarachnoid hemorrhage patients. However, its intraoperative use during intracranial neurosurgery allows commonly detecting desaturation episodes, and its combination with TCDU may impact on clinical decision-making to optimize cerebral physiology. Large randomized clinical trials investigating the impact of intraoperative SjvO ₂ monitoring on patient outcome are lacking.

Near Infrared Spectroscopy

NIRS consists in measuring near infrared light absorption by the superficial part of brain tissues. Light is emitted through the skin and the skull, entering the brain tissue to a depth of approximately 1–2 cm. Absorption at different wavelengths allows estimating the mixed capillary–venous regional cerebral oxygen saturation (rScO ₂ ). rScO2 depends on the balance between local metabolism and oxygen delivery, which in turn depends on regional cerebral blood flow and blood oxygen content. If metabolism is constant, rScO ₂ can be regarded as a surrogate measure of regional cerebral blood flow changes. The system necessitates the use of a noninvasive scalp probe and allows continuous monitoring of rScO ₂ over a restricted area of approximately 1 cm ² .

Normal rScO ₂ values at atmospheric oxygen levels range from 62% ± 6% in cardiac surgery patients to 71% ± 6% in healthy young men. Low preoperative baseline rScO ₂ values are predictive of increased 30-day morbidity and mortality. There is no consensus in the literature on the rScO ₂ threshold value indicating significant desaturation, but a decline of more than 20% as compared to baseline is generally accepted as a predictor of cerebral ischemia. It is also generally agreed that both depth and duration of desaturation are important. A less extreme threshold of 10% should probably prompt therapeutic intervention to raise rScO ₂ .

Transcranial Doppler Ultrasonography

In 1982, Aaslid was the first to describe the use of transcranial Doppler for measuring cerebral blood flow velocity. Ultrasound brain exploration has long been limited to Doppler exploration, but technological advances now allow grossly imaging the main brain structures using echography. Through the visual recognition of the insonated vessel, it has considerably facilitated Doppler use and has added morphological elements that can be useful to diagnosis. TCDU is the only noninvasive and cost-effective bedside tool that provides real-time information on cerebral hemodynamics. It has no contraindications, and its technical limitations can often be overcome by the intravenous administration of a sonography contrast agent. In adults, insonation usually occurs through the four commonly defined acoustic windows, including the temporal, orbital, suboccipital, and submandibular window. These windows are characterized by a thinner bone layer, permitting ultrasound penetration.

Ultrasound Basic Physics

Echography necessitates an ultrasound transducer, which acts both as a transmitter and a receiver. Reflection of ultrasound waves occurs at the interface between different tissue types, and depends on the resistance to ultrasound breakthrough, or acoustic impedance. When ultrasounds are reflected, and thus sent back to the transducer, the piezoelectric crystals convert the mechanical energy of the returning echoes into an electrical current, which is processed by the ultrasound machine to produce a two-dimensional gray scale image. During their travel into the tissues, ultrasound waves are attenuated. Attenuation depends on three factors, namely the attenuation coefficient of the tissue, the traveled distance, and the ultrasound frequency. Insofar as bones are substantially impervious to ultrasounds, and that attenuation increases with ultrasound frequency, a low-frequency transducer in the order of 2 MHz is necessary for transcranial ultrasonography.

The flow within vascular structures can be studied using color flow Doppler. Red blood cells moving toward the transducer return waves at a higher frequency than the emitted signal. The corresponding image of flow is displayed in red on the device. Contrarily, when those cells are moving away from the transducer, the returned waves have a lower frequency and are displayed in blue. Color flow Doppler helps identifying arteries through the integration of information coming from the anatomical view, as well as direction and pulsatility of flow. Pulse wave Doppler is based on the Doppler principle described by Christian Doppler in 1843. Exactly like in color flow Doppler, the reflected ultrasound wave has a higher or a lower frequency than the emitted one. This is termed as a positive or negative shift in frequency, respectively. The velocity of red blood cells as measured by a pulse wave Doppler depends on the cosine of the angle of insonation ( Fig. 9.3 ). As a consequence, when the angle of insonation is 0°, the measured velocity equals the true velocity. When the angle is 90°, the measured velocity is 0, irrespective of the actual velocity.

Illustration of the importance of the insonation angle in pulse wave Doppler.

Practical Applications

In the operating room and during neurosurgery, TCDU can be used to seek for hyperemia, vasospasm, microembolic signals, vasomotor reactivity (VMR), and to estimate ICP. These indications are also common in the neurointensive care unit. Other common indications of TCDU are the identification of sickle cell disease children at the highest risk of first-ever stroke, and those in need of blood transfusion. TCDU is also very sensitive and specific to detect right-to-left shunt and to quantify the functional repercussions of such a shunt. The last indication of TCDU is the diagnosis of a cerebral circulatory arrest. This paragraph will be limited to the indications that are of potential utility in the operating room.

Vasospasm and Hyperemia

Vasospasm is a suddenly occurring and rapidly evolving pathology. TCDU is therefore a key tool for its identification and diagnosis, as well as for daily monitoring, appreciation of severity, and circumscription of incriminated territory. A change in mean flow velocity (MFV) often precedes an eventual neurological deficit. Detecting those changes in velocity authorizes to adapt the treatment precociously, and prevent complications.

The threshold for deciding on an increased MFV varies according to the studied artery ( Table 9.3 ). More than an absolute number, the magnitude of a change in mean velocity is also important. An increase of more than 50 cm/s in 24 h is predictive of the occurrence of a vasospasm-related ischemic lesion. In the middle cerebral artery, when the MFV remains between 120 and 200 cm/s, the sensitivity for diagnosing a vasospasm is weak (67%). In that case, only arteriography may provide certainty. Another useful parameter for diagnosing vasospasm in the middle cerebral artery is the Lindegaard’s ratio. It corresponds to the ratio between the MFV in the middle cerebral artery and the MFV in the submandibular internal carotid artery. The Lindegaard’s ratio may also help differentiating hyperemia from vasospasm, which have completely opposite treatments.

Table 9.3

Proposed Criteria for Diagnosing Vasospasm on Most Frequently Studied Cerebral Arteries

Concerned Artery	MCA	MFV	Lindegaard’s ratio for the MCA
	Moderate vasospasm	>120 and <200 cm/s	Hyperemia: <3 Moderate vasospasm: 3–6 Severe vasospasm: >6
	Severe vasospasm	>200 cm/s
	ACA	MFV
	Discreet vasospasm	>80 cm/s
	Significant vasospasm	>130 cm/s
	PCA	MFV
	Discreet vasospasm	>90 cm/s
	Significant vasospasm	>110 cm/s
	Vertebral artery	MFV
	Vasospasm	>80 cm/s
	Basilar artery	MFV
	Vasospasm	>95 cm/s

 

ACA , anterior cerebral artery; Lindegaard’s ratio, ratio between the mean flow velocity in the MCA and the mean flow velocity in the submandibular internal carotid artery; MCA , middle cerebral artery; MFV , mean flow velocity; PCA , posterior cerebral artery.

Microembolic Signals or High Intensity Transient Signals

During carotid surgery, monitoring an intracranial artery distal to the steno-occlusive site allows detecting spontaneous embolic signals and quantifying the embolic potential of the atherosclerotic plaque. According to the International Cerebral Hemodynamics Society, those signals are characterized by a high intensity, that is more than 10 dB higher than blood flow, a random occurrence during the cardiac cycle, a unidirectional nature, a short duration (10–100 ms), and an audible component (explosive signal) ( Fig. 9.4 ). This kind of surveillance is useful not only during carotid surgery, but also during aortic, cardiac, or even orthopedic surgery. It helps identifying the critical phase of surgery, when the risk of embolism is important. Detecting such a high risk should prompt corrective measures at the level of the equipment (e.g., extracorporeal circulation) or the surgical technique.

Illustration of microembolic signals or high intensity transient signals as evidenced by transcranial Doppler ultrasonography.

Vasomotor Reactivity

VMR represents the ability of cerebral vessels to maintain a near constant blood flow in response to a vasomotor stimulus. TCDU allows a real-time observation of vasomotor changes in response to various stimuli. Concerned stimuli can be hemodynamic stimuli such as a rapid leg cuff deflation or a Valsalva maneuver or the transient hyperemia response to medications such as acetazolamide, breath-holding, and CO ₂ inhalation. CO ₂ is the strongest vasodilation stimulus to the cerebral circulation. Several factors may influence VMR, including gender and level of estrogenic impregnation, as well as chronic cardiovascular pathologies such as chronic high blood pressure and diabetes. In those two last cases, VMR is reduced.

A simple method for measuring VMR has been described by Markus et al. It consists in calculating the breath-holding index (BHI) as follows:

MFVend−MFVbaselineMFVbaseline×100seconds of breath holdingsMFVend−MFVbaselineMFVbaseline×100seconds of breath holdings
MFVend − MFVbaseline MFVbaseline × 100 seconds of breath holdings

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related

Related posts:

Neuroembryology Transesophageal Echocardiography Supratentorial Lesions Neuroendoscopy Geriatric Neuroanesthesia Coexisting Diabetes Mellitus in Neurosurgical Patients

Stay updated, free articles. Join our Telegram channel

Join