Challenges of Cerebral Perfusion Pressure Measurement.
Secondary neuronal injury occurs in the hours and days after the primary brain injury from complex biochemical and physiological responses to the initial injury, including excitotoxicity, production of free radicals, electrolyte shifts, and inflammation. (10) Cerebral autoregulation occurs in the vasculature of the brain and operates to maintain constant cerebral blood flow depending on systematic blood pressure and other factors. However, in the presence of cerebral insult or injury, cerebral autoregulation is often impaired, warranting monitoring of CPP to ensure adequate blood flow to the brain. (10) Mean arterial pressure is a key element in calculating CPP. Intra-arterial MAP is measured with a strain gauge transducer, which can be set either at the phlebostatic axis (midaxillary) level or at the tragus (external auditory meatus of the ear) to calculate CPP. Differences in transducer placement may result in overestimating or underestimating CPP, which may guide treatment decisions that inadvertently cause further cerebral ischemia or adult respiratory distress syndrome. (11-14)
Lassen (15) wrote in 1959 that the CPP should be derived using MAP measured at the tragus. However, there is no evidence to substantiate this claim. There are conflicting opinions regarding placement of the transducer when monitoring intra-arterial blood pressure (16,17) primarily rooted in different interpretations of underlying physiological processes. A 2009 conceptual framework proposed by Jones (16) highlights the underpinnings of the physiological debate regarding measurement location for CPP The model proposes that CPP is a reflection of downstream perfusion pressure exerted on the cerebral tissue, whereas MAP reflects upstream pressure exerted from the aorta. (16) Unfortunately, there is lack of consensus on where optimal effective downstream pressure (EDP) should be measured. Effective downstream pressure is measured using either ICP values or central venous pressure (CVP) readings. The transducer for measuring intra-arterial blood pressure, and obtaining MAP and calculating CPP, is zeroed at the location of the purported EDP site. It is argued that, if ICP is greater than CVP, then ICP should be considered the parameter for EDP and the transducer should be leveled at the tragus; however, in the absence of intracranial hypertension, the optimal EDP should use CVP values, and the transducer would be zeroed at the midaxillary region.
These parameters become particularly important if a physiologic transducer is set lower than the true reference point. In this scenario, there is an increase in hydrostatic pressure, and a higher value is reported. Conversely, if the transducer is set higher than the true reference point, hydrostatic pressure is decreased, resulting in a lower value. These values are critical when considering CPP. False high values for MAP result in false high values for CPP, placing patients at risk for cerebral hypoperfusion and ischemia. For example, if a patient's head of bed is at 50[degrees], measuring MAP at the midaxillary region could result in a calculated CPP of up to 18 mm Hg higher than CPP values obtained when MAP is measured at the tragus. (18) Therefore, a CPP reading of 60 mm Hg at the midaxillaiy level may actually be a "true" CPP of 45 mm Hg, causing severe cerebral ischemia due to incorrect reporting and treating of CPP calculated at 60 mm Hg. Data are needed to determine specific numerical differences and investigate outcomes by measurement locations for specific CPP values.
CPP Recommendations and Practice Variations
Initiation of CPP monitoring in severe neurological injury is a key recommendation by the Brain Trauma Foundation, the American Association of Neurological Surgeons, and the Neurocritical Care Society. (2-4) Recommendations are based on level II and III evidence demonstrating benefit when CPP monitoring is instituted among critically ill patients with neurological insult. Guidelines recommend CPP of 50 to 70 mm Hg, because evidence shows worsening cerebral ischemia and outcomes if values fall less than 50 mm Hg. (11) Similarly, CPP values greater than 70 mm Hg are associated with worsening disability and mortality. (12-14) Although evidence supports CPP monitoring for neurocritically ill patients, there is no evidence indicating optimal location of the arterial transducer when obtaining these measurements.
Variations in CPP measurements are widely documented. A specific investigation of references from the Brain Trauma Foundation guidelines revealed few cited studies indicating how CPP was measured. (7) Of those that specify a method for obtaining CPP, 2 studies reference MAP where CPP is calculated at the level of the tragus, whereas 3 separate studies include midaxillary MAP to calculate CPP. (9) A systematic review aimed to identify optimal location for transducer placement for CPP monitoring. (16) Of the 57 studies meeting inclusion criteria, no studies indicated transducer placement for CPP measurement.
Variations in CPP measurement are supported by recent surveys of neurocritical care nurses, advanced practice providers, and physicians. (5,9) Specifically among nurses, a survey of 28 critical care units indicates that 70% routinely measure CPP at the midaxillary line and 20% measure CPP at the tragus. In another survey, 75% of nurses report measurement at the midaxillary line, and 25% measure at the tragus. (16) Surveys of advanced practice providers and physicians indicate that 62.1% measure CPP at the midaxillary region, 36.2% measure CPP at the tragus, and 1.7% measure CPP at another location, likely the arterial catheter site or a combination of all methods. (9) A systematic review by Kosty et al (5) found that 50% of included studies did not report where MAP was measured, 31% measured MAP at midaxillary, and 19% measured MAP at the tragus.
Challenges Associated With MAP Measurement
Cerebral perfusion pressure is a derived variable expressing the difference between MAP and ICP. Certainly, valid measurement is a challenge inherent to every physiologic variable. However, measuring the MAP at the tragus versus the phlebostatic axis, or both at times, presents several key added challenges falling within environmental, treatment, communication, and nursing workflow domains.
The environment domain includes physical constraints associated with arterial blood pressure measurement at the tragus and co-dependence of advanced hemodynamic monitoring reference points. Current considerations of environmental domain include both the transducer reference point and the length of the conduction tubing required. Figure 1 shows that the 2 most common reference points to consider for transducer placement include the phlebostatic axis (right atrium of the heart) and the tragus (external auditory meatus). (8)
There is much heterogeneity in transducer level placement for the calculation of CPP. The length of the tubing to obtain pressure values when measuring CPP should be considered. Excessively long tubing may result in amplification, thus exaggerating the signal resulting in values that exceed true physiologic pressures. (19) Assessment of amplification should be considered when measuring CPP, and equipment should be planned to allow for optimal transducer placement using only standard length tubing (eg, the tubing that is supplied by the manufacturer).
The treatment domain includes measurement challenges that impact treatment, including medication management, zero drift, nonharmonious CPP and cardiac treatment goals, and the need to simultaneously treat MAP from alternate reference points. Because values vary according to transducer location, treatment is dependent on where the transducer is located. Maintaining CPP greater than 60 mm Hg may be impacted by transducer location because MAP is different when the arterial transducer is referenced at the phlebostatic axis compared with the tragus. This may include overtreatment or undertreatment depending on the patient situation and transducer location. Zero drift of the ICP monitoring device may also impact treatment. Depending on the device used to measure ICP, zero drift may occur, resulting in inaccurate measurements. (20) Because treatment of low CPP often includes fluid bolus and vasopressor medications, accurate measurement of CPP to achieve the goal of cerebral perfusion is paramount. Another treatment domain concern is the need to address parameters for both cardiac and neurologic support. Much of the cardiac literature assumes that MAP is measured at the phlebostatic axis. If the patient is treated for both shock and inadequate CPP, an arterial transducer at the tragus may compromise the cardiac goal. Both invasive and noninvasive advanced cardiac monitoring technologies assume MAP measurement at the phlebostatic axis. Therefore, clarity is needed regarding the transducer location when setting cardiac and cerebral pressure goals. The nurse may need to take measurements at each location and work with the care teams to determine treatment goals and priorities.
The communication domain includes the collection, documentation, reporting, and computerized algorithms used to obtain and report physiologic values in electronic health records (EHRs). The primary role of an EHR is to facilitate communication and guide management decisions. Additional functions of the EHR are to facilitate research, quality improvement, and public health initiatives. All functions are predicated on valid data capture. Valid data capture requires that the clinical measurement be accurate and the information be recognized and recorded into the
appropriate field in the EHR. This is of particular concern in both clinical care and research because several studies and reviews for the past 20 years identify wide variation in the correctness and completeness of even basic physiologic EHR data such as blood pressure due to a variety of factors. (21) Several organizational case studies showed improved documentation error rates when vital signs were electronically captured by the EHR rather than manually documented. (22) However, neither case study included ICP or CPP, or any other co-dependent variable, as a vital sign in their assessment of error.
In the case of CPP measurement and recording, EHR data quality is particularly complex. The ICP and MAP measurements must be accurate and recorded at the same time for CPP calculations to be valid. Some centers measure ICP continuously, whereas others measure ICP intermittently. For organizations that do not measure ICP continuously, these values may have to be manually collected by the nurse, potentially rendering any automatically generated data inaccurate. In addition, the EHR may be collecting blood pressure from multiple sources, and most lack the sophistication of dictating which values to base the CPP measurements. The EHR would also need to include documentation of transducer location or develop a unifonn procedure for measurement that specifies location. If additional physiologic monitors such as pulmonary artery catheters or less invasive cardiac output monitors are used that require leveling of the arterial transducer at the phlebostatic axis, the nurse may have to take multiple measurements hourly or more often to reflect both the cardiac values and the CPP at the tragus. Lacking any advanced technology to allow multiple concurrent measurements, this would most certainly be a manual nursing process. In addition, all team members would need to be clear how measurements are obtained when setting goals and treatments.
Nursing Workflow Domain
The nursing workflow domain describes the external forces that impact the normal routines associated with nursing workflow that must be adjusted dependent on where the arterial transducer is referenced. The issues outlined in the environmental, treatment, and communication domains converge to impact nursing workflow. In centers where the arterial transducer is leveled at the tragus, nursing workflow is impacted by the need to adjust arterial line system length to reach the tragus and the potential need to level the arterial transducer at both the phlebostatic axis and the tragus depending on the cardiac and/or neurologic goals for management. Even in centers where CPP is routinely measured at the phlebostatic axis, frequent manual measurement of CPP may be necessary depending on ICP monitoring practices. Ensuring accurate EHR data even in these centers may be challenging. In centers where arterial transducer leveling varies according to practitioner preference or the patient's assessed need, nursing workflow must adapt, and documentation should reflect the changes accordingly.
Implications for Further Research and Technology
An analysis of the challenges of measuring CPP results in several areas for further investigation and technological innovation. A key question that must be answered is whether transducing arterial pressure at the tragus versus the phlebostatic axis makes a significant difference in the values obtained. It is logical to assume that the location of the transducer and the head-of-bed location would impact the measurement of MAP and therefore the measurement of CPP. However, this has not been well studied to date. Findings from a national, multisite trial indicate that differences are substantial and have major implications for CPP-directed therapy. (23) If measurement of CPP using an arterial line transduced at the tragus, it would be important to know whether the addition of several feet of pressurized extension tubing significantly impacts the values obtained. If indeed the measurement of CPP using an arterial line measured at the tragus versus the phlebostatic axis impacts the values obtained, a key resulting question is whether arterial transducer location impacts decision-making in care (eg, interventions) that will ultimately impact patient outcomes. Finally, understanding the implication of ICP and CPP documentation in the current EHR environment is important. It is unclear whether automatic documentation of ICP and calculation of CPP result in valid data capture of these values in the EHR.
Current ICP and arterial line technology does not facilitate easy measurement of CPP. Arterial line setups do not currently allow for transducing multiple pressure points from one line. If they did allow for sampling of the pressure at multiple discreet points, it might allow for targeting both cardiac and neurologic targets for pressure augmentation. The EHR should also include adaptable fields to allow for accurate collection of ICP and CPP values and allow centers to easily indicate the transducer location.
Although CPP measurement is common and essential to the management of critical neurologic illness, agreement on the technique and process for obtaining valid measurements is lacking. Multiple observational studies show significant variation in CPP measurement technique related to arterial pressure transducer location, and this variation impacts both nursing practice and patient treatment. Additional studies are needed to show whether the practice variation impacts patient outcomes.
Questions or comments about this article may be directed to Sarah L. Livesay, DNP RN ACNP-BC ACNS-BC, at email@example.com. She is an Associate Professor, Department of Adult and Gerontological Nursing, Rush University College of Nursing, Chicago, IL.
Molly M. McNett, PhD RN, is Director, MetroHealth, Cleveland, OH.
Monica Keller, BSN RN, is Staff Nurse, UT Southwestern, Dallas, TX.
DaiWai M. Olson, PhD RN, is Associate Professor, Department of Neurology, Neurotherapeutics, and Neurological Surgery, UT Southwestern, Dallas, TX.
Dr. Olson is the Editor of the Journal of Neuroscience Nursing. Dr. McNett is on the editorial board of the Journal of Neuroscience Nursing. The other authors declare no conflicts of interest.
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Caption: FIGURE 1 Comparing CPP With MAP Measured at the Tragus Versus the Phlebostatic Axis With Patient Head of Bed Elevated at 45[degrees]
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|Title Annotation:||Literature Review|
|Author:||Livesay, Sarah L.; McNett, Molly M.; Keller, Monica; Olson, DaiWai M.|
|Publication:||Journal of Neuroscience Nursing|
|Date:||Dec 1, 2017|
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