Abstract
Hemodynamic instability is a common issue faced in the neurological intensive care unit (neuro-ICU). Practitioners often find themselves pondering about the patient’s volume status and whether to give or to withhold fluids in order to optimize cerebral blood flow and brain tissue oxygen delivery. The physical examination, a vital component of care, is often limited in this patient population. Each patient has a different and unique combination of cardiac function, systemic and pulmonary vascular resistance, concurrent infections, and confounding medications, which vary hour by hour. It is not surprising that simple bedside vital signs and urine output measurements are often inadequate estimates of volume status. In this chapter we will discuss advanced hemodynamic monitoring and neuromonitoring techniques that are available, along with respective advantages and limitations.
17 Advanced Hemodynamic and Neurological Monitoring in the Neuro-ICU
17.1 Hemodynamic Monitoring
Patients who are critically ill in the neurologic intensive care unit (neuro-ICU) can quickly become hemodynamically unstable. Having the appropriate tools to adequately assess hemodynamics beyond basic vital signs is imperative in preventing organ failure or death. It is not uncommon for patients to require continuous blood pressure monitoring or vasopressors which require arterial lines and central venous catheters. When initial resuscitation efforts fail, advanced hemodynamic monitors are implemented to provide additional information to further guide treatment. The choice of monitor is dependent on what specific information is needed, what system is available, and the experience of the practitioner (Table 17‑1). Regardless of the monitor, the practitioner must have a strong understanding of physiology and knowledge of normal hemodynamic values (Table 17‑2) in order to interpret the data provided effectively.
Mean arterial blood pressure (MAP) | 70–100 mm Hg |
Mixed venous O2 saturation (SVO2) | 60–75% |
Central venous O2 saturation (SCVO2) | 70% |
Right atrial pressure (RAP) Central venous pressure (CVP) | 2–6 mm Hg |
Stroke volume (SV) | 50–100 mL |
Cardiac output (CO) | 4–8 L/min |
Cardiac index (CI) | 2.5–4 L/min/m2 |
Systemic vascular resistance (SVR) | 800–1,200 dynes·sec/cm-5 |
Pulmonary artery pressure (PAP) Systolic Diastolic Mean | 15–30 mm Hg 8–15 mm Hg 9–18 mm Hg |
Pulmonary artery occlusion pressure (PAOP) | 8–12 mm Hg |
Oxygen delivery (DO2) | 950–1,150 mL/min |
Oxygen consumption (VO2) | 200–250 mL/min |
Extravascular lung water index (ELWI) | 0–7 mL/kg |
Global end diastolic volume index (GEDI) | 650–800 mL/kg |
17.1.1 Invasive Monitoring: Pulmonary Thermodilution
Pulmonary Artery Catheter
The pulmonary artery catheter (PAC) or Swan-Ganz catheter was considered the goal standard for hemodynamic monitoring for decades but has lost favor with intensivists with the advent of newer minimally invasive monitors. The PAC is inserted through the jugular or subclavian vein into the right heart and placed directly into the pulmonary artery. This allows measurements of the right atrial pressure (CVP), pulmonary artery pressure, and pulmonary artery occlusion pressure or wedge pressure, which is a reflection of the left atrial filling pressure. There are disadvantages to using a PAC. First, it is invasive and technically difficult. It requires a high degree of expertise to properly interpret the arterial waveform tracings as it advances into the pulmonary artery. In a survey of 534 critical care physicians working in 86 European ICUs, a significant portion felt knowledge was less than adequate. 1 A Cochrane review of randomized trials from 1954 to 2012 demonstrated that there was no alteration in mortality, length of stay, or cost saving associated with PAC usage. 2 The best indication for PAC remains right ventricular heart failure or pulmonary hypertension because there is currently no other method capable of direct measurements of right heart or pulmonary pressures.
17.1.2 Less Invasive: Transpulmonary Thermodilution
Transpulmonary thermodilution (TPTD) is a way to measure cardiac output (CO) that is less invasive and modified from the pulmonary artery catheter (PAC) right-heart thermodilution method introduced by Drs. Swan and Ganz in the early 1970s. A known amount of solution with known temperature, typically 15 to 20 cc of cold saline, is bolus into the central venous circulation where it travels through the right heart, the lungs, the left heart, and ultimately into the central arterial circulation where a thermostat, at the tip of a femoral or axillary arterial catheter, depending on the manufacturer, measures the rate and degree of temperature change. The CO is inversely proportional to the area under the thermodilution curve. This method has been compared to pulmonary artery thermodilution with clinically acceptable accuracy. 7 , 8
Because the injector solution passes through all four chambers of the heart and the pulmonary vasculature, it is possible to calculate several additional volumetric parameters as well as fluid status in the lungs:
Global end diastolic volume (GEDV) and global end diastolic volume index (GEDI)
These are related to the volume in all four chambers of the heart at the end of diastole and serves as a proxy of preload.
A low GEDV or GEDI may suggest responsiveness to preload resuscitation.
Global ejection fraction (GEF) and cardiac function index (CFI)
GEF is a marker of cardiac contractility with normal ranges between 25 and 35%.
CFI is another marker of cardiac contractility and is mathematically calculated by dividing CO by GEDV.
A low GEF and CFI suggests that poor ventricular performance is the cause for a low stroke volume (SV) or CO; this may prompt addition of inotropic support.
Extravascular lung water (EVLW) and extravascular lung water index (EVLWI) and pulmonary vascular permeability index (PVPI)
Fluid build-up in the lung can affect oxygenation. EVLW and EVLWI assess pulmonary edema and can be helpful in monitoring disease progression.
Elevated EVLW or EVLWI may be seen in heart failure, volume overload, or lung injury.
Pulmonary vascular permeability index (PVPI) can be used to differentiate the mechanism for elevated EVLW: high PVPI is seen in pulmonary edema as a result of lung injury, such as in acute respiratory distress syndrome (ARDS), whereas normal PVPI is seen in hydrostatic and cardiogenic pulmonary edema.
In general, TPTD is useful in patients with unstable hemodynamics, unclear volume status, or impairment in tissue oxygen delivery and demand. Typical conditions include septic shock, cardiogenic shock, pulmonary edema, and ARDS. It can also be considered if strict maintenance of hemodynamic parameters (i.e., euvolemia for subarachnoid vasospasm) is critical. The additional volumetric data is helpful to guide intervention selection such as fluid resuscitation, diuresis, inotropic or vasopressor support, etc.
Pulse Contour Cardiac Output (PiCCO)
PiCCO is a system which combines the principles of arterial pulse contour analysis with transpulmonary thermodilution to measure CO.
Access Requirements
Central venous catheter
Femoral, brachial, or axillary arterial line
Technology
Similar to VolumeView, the pulse contour analysis provides continuous hemodynamic information while calibration with thermodilution provides intermittent volumetric parameters.
Data Output
Hemodynamics (CO, SV, systemic vascular resistance [SVR], stroke volume index (SVI))
Preload (GEDV, GEDI)
Contractility (GEF)
Volume (pulse pressure variation [PPV], stroke volume variation [SVV])
Lung water (EVLW, EVLWI)
Afterload (SVR, systemic vascular resistance index [SVRI])
Intrathoracic blood volume index
Limitations
Need to be on controlled mechanical ventilation; no spontaneous breathing
Requires regular calibration three to four times a day
Maximum duration of 10 days
Not recommended with intracardiac shunts
Data may be inaccurate with severe tachyarrhythmias, valvular insufficiency, pulmonary embolism, partial lung resection, and possibly with aortic aneurysms
VolumeView/EV1000
The VolumeView system consists of the VolumeView sensor and the EV1000 Clinical Platform monitor. It combines the concept of pulse contour analysis and TPTD into one system.
Access Requirements
Central venous catheter, in the subclavian or internal jugular vein
Femoral arterial line
Technology
TPTD is used to measure a calibrated cardiac output (COTT). The COTT is then used in the calibration of a proprietary algorithm to estimate a continuous CO (COCont) via pulse contour analysis, similar to FloTrac.
Data Output
Hemodynamics (COCont, SV, SVI, and SVV)
Preload (GEDV/GEDI)
Afterload (SVR/SVRI)
Contractility (GEF/CFI)
Lung water (EVLW/EVLWI and PVPI)
Advantages
Unlike FloTrac, TPTD is not affected by the mode of ventilation and it is more accurate in patients suffering from septic shock. 10
Demonstrated to have no significant difference compared to PAC-thermodilution when used in cardiogenic shock patients, irrespective of mitral or tricuspid regurgitation, intra-aortic balloon pump, or therapeutic hypothermia. 11
Limitations
It requires both a central venous and central arterial line.
Calibrations must be done at least every 8 hours in order for the continuous pulse contour derived estimation of COCont to be reliable.
The error between COCont and COTT was shown to be lower with shorter recalibration intervals, 1 and 2 hours, compared to the standard 8 hours. 12 This suggests that recalibration should be done more frequently in unstable patients and after any significant clinical intervention.
Dilution Cardiac Output (LiDCO)
It is operator-dependent system that combines the concepts of pulse power analysis (rather than pulse contour analysis) with lithium dilution to measure CO. It has been shown to be at least as accurate as bolus thermodilution via pulmonary artery catheter. 9
Access Requirements
Radial arterial line as well as a venous (central or peripheral) line.
Technology
A bolus of lithium chloride is flushed through the venous line and a lithium sensor, attached to the arterial line, detects the concentration of lithium in the blood.
The lithium concentration “wash-out” time is used to calibrate the arterial waveform analysis providing continuous readings for SV, SVV, and CO.
Data Output
SV, SVV, PPV, and CO are obtained.
Advantages
Similar to other TDTP, more accurate than pulse contour analysis especially in patients with unstable hemodynamics
CO correlates well with PAC
Injectate can be given via central or peripheral venous line
Limitations
It does not provide any advanced volumetric variables such as GEDV, GEF, CFI, or EVLW.
It requires calibration every 8 hours with approximately 5 mL sample of blood each time. Hemoglobin and sodium levels need to be entered for the software calculations.
For proper calibration the dose of lithium chloride may require adjustment based on patient size.
Although the typical lithium dose (0.15–0.30 mmol/dose for average adult) is small and toxicity is unlikely, the risk remains higher than cold saline injection.
LiDCO cannot be used in patients on lithium therapy (i.e., bipolar).
17.1.3 Minimally Invasive Monitoring: Pulse Contour Analysis
Pulse contour analysis is based on the concept that the SV is proportional to the area under the arterial pressure waveform. Once the SV is obtained, the CO can be easily calculated using CO = SV × HR. Various companies have developed methods of analyzing the waveform and calculating CO via proprietary algorithms. One of the main disadvantages of pulse contour analysis is that it becomes more inaccurate when a patient is unstable on pressors.
FloTrac/Vigileo
Uncalibrated, minimally invasive pulse contour analysis sensor. The FloTrac sensor is connected to the Vigileo monitor but is also compatible with the newer EV1000 Clinical Platform monitor.
Access Requirements
Peripheral or central arterial line, typically radial.
Technology
The sensor measures the arterial pulse rate (PR) instead of the HR; therefore, it captures perfusing beats. SV is calculated and updated every 20 seconds utilizing a proprietary formula which incorporates: age, gender, body surface area, waveform skewness and kurtosis, and standard deviation of arterial pulse pressure. The PR is multiplied by the calculated SV to return CO.
Data Output
CO, SV, stroke volume variation (SVV), and stroke volume index (SVI)
SVR (if combined with CVP reading from central venous catheter)
Stroke Volume Variation (SVV)
SVV is determined by assessing the arterial pulse pressure changes during the respiratory cycle.
SVV above 10 is associated with an increase in SV after fluid bolus.
Note: SVV is not a measure of volume status but rather preload responsiveness. A patient with SVV below 10% suggests low likelihood of fluid responsiveness but does not discern between euvolemia versus hypervolemia.
SVV has only been validated for use in patients that are on controlled mechanical ventilation with fixed respiratory rates and tidal volume of >8 cc/kg. SVV is not reliable in spontaneous breathing due to the natural fluctuating rates and tidal volumes.
Advantages
It is quick and simple to set up.
It does not require central vascular access (unless SVR is desired).
The output is well validated in hemodynamically stable patients 3 , 4 and not affected in patients undergoing therapeutic hypothermia.
The sensor monitors arterial vascular changes continuously and calibrates automatically, making the system operator-independent.
Limitations
Multiple variables in calculating SV makes this method the most mathematically complex and therefore greater potential for sources of error. Accuracy of input parameters (age, gender, height, weight) as well as a good arterial pressure waveform is vital to the algorithm’s calculation of SV and CO.
Accuracy may be compromised in severe shock states due to peripheral vasoconstriction 5 , 6 dampening the arterial waveform leading to a falsely low CO. It is recommended to use a central arterial access, such as femoral artery, in these situations.
Arrhythmia will affect the accuracy of SVV; CO and SV are not affected.
17.1.4 Noninvasive Hemodynamic Monitoring
These products are completely noninvasive with no venous or arterial access required.
ClearSight System
ClearSight utilizes the volume clamp method to provide real-time, noninvasive, continuous monitoring of key hemodynamic parameters. It consists of a finger blood pressure cuff outfitted with infrared sensing technology. The blood pressure cuff deflates and inflates while the infrared sensors continuously monitor finger arterial volume and pulsations. This occurs 1,000 times per second and arterial variations are displayed as a waveform similar to that obtained by a traditional arterial line. The system then uses a proprietary algorithm to analyze the arterial waveform to return hemodynamic variables.
The ClearSight system can utilize one finger cuff for up to 8 hours, or two finger cuffs for up to 72 hours by alternating between each finger hourly. Movement of the patient’s hand above or below the heart level is automatically adjusted for without affecting the waveform measurements.
Data Output
Blood pressure, PR, SV, SVV, SVR, and CO are obtained.
Advantages
CO measurement comparable to PiCCO TPTD in stable patients undergoing elective coronary artery bypass grafting (CABG). 13
Noninvasiveness and easy installation make this an attractive method for continuous perioperative CO monitoring in hemodynamically stable patients.
Limitations
Reliability of this method during times of hemodynamic instability or significant changes in peripheral vascular resistance is limited due to finger hypoperfusion. 14
Compared to PiCCO TPTD in critically ill patients, there was poor estimations of cardiac function and unreliable detection of CO change in response to volume expansion. 15
Reliability during therapeutic hypothermia is unknown.
Cheetah NICOM
Cheetah NICOM is a noninvasive monitoring system consisting of four sensor pads applied to the thorax utilizing the concept of bioreactance. An electrical current is applied across the thorax by the transmitting sensors, blood flow in the thorax leads to a time delay in the current, and the signal is detected by the recorded sensors. The signal recorded is analyzed using a proprietary algorithm and converted into the aortic waveform. SV is obtained by calculating the area under the systolic portion of the waveform. Electrical propagation in the thoracic cavity is influenced by body surface area. Therefore, CO calculation incorporates SV and HR, as well as age, weight, and height.
Data Output
CO, cardiac index (CI), SV, SVI, and SVV
Advantages
Easy to set up
Works on spontaneously breathing or mechanically ventilated patients
Provides changes in SV in response to fluid challenge or passive leg raise
Studies performed on relatively stable post-cardiac surgery patients have shown good correlation when Cheetah NICOM was compared to PAC-thermodilution 16 or FloTrac. 17
Limitations
Certain conditions may affect the accuracy of the monitor. These include severe aortic insufficiency, severe anatomic abnormalities of the thoracic aorta (i.e., aortic graft, large aortic aneurysm, or aortic dissection), left ventricular assist devices, and external and some older models of internal pacemakers.
Because the system relies on electrical currents passing through the thorax, it may be affected by conditions that increases the thoracic volume (i.e., obesity, chronic obstructive pulmonary disease, pulmonary edema, or pleural effusion) and studies on this are limited.
Current data regarding the accuracy of Cheetah NICOM in the critically ill shows poor correlation when compared to PAC-thermodilution 18 or PiCCO-TPTD. 19
17.2 Neurological Monitoring
The critical care management of neurologically injured patients extends beyond the initial stabilization. It is well known that these patients are at risk of a secondary brain injury which contributes to an increase in morbidity and mortality. Neuromonitoring has a key role in the early recognition and management/prevention of secondary brain injury. Often referred to as multimodality monitoring, advanced hemodynamic monitoring is utilized along with invasive and noninvasive neuromonitoring (Table 17‑3) to optimally manage a patient.
17.2.1 Noninvasive Monitors
Transcranial Doppler
Transcranial Doppler (TCD) (Fig. 17‑1) is an important bedside tool in the neuro-ICU. Details on the use have been described in Chapter 18.
Uses
Detection of vasospasm in subarachnoid hemorrhage (SAH)
Prediction of stroke risk in sickle cell patients
Ancillary test to support the diagnosis of death by neurologic criteria
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