Outlines
- Blood Pressure in Intensive Care Unit
- Adjunctive Therapies in Acute Myocardial Infarction
- Echocardiography in the Critical Care Unit
- Assessment of Nutritional Status in Critically ill
- Prognosis and Risk Factors for Poor Outcome in Critically Ill Patients with Respiratory Illness
- Blood Transfusion in Critically Ill Patients
- Antibiotics in Intensive Care Unit
- Sedation in the Critically Ill
- Delerium: Prognosis and Outcome
- Brain Death
- Organ Donation
INTRODUCTION
Both high and low blood pressure can cause organ dysfunction. Monitoring blood pressure remains a vital component of hemodynamic monitoring and management in intensive care unit (ICU). Blood pressure monitoring, in combination with other monitoring, has shown to detect more than 90% adverse events when used in anesthesia.1 This chapter looks into different methods of blood pressure monitoring and which is preferred method, invasive or noninvasive. It also attempts to answer which is the better target mean or systolic blood pressure and if there is any ideal target.
METHODS OF BLOOD PRESSURE MONITORING IN INTENSIVE CARE UNIT
Blood pressure monitoring is an essential component of managing hemodynamically unstable patients in ICU. The methods of monitoring can be classified broadly into (1) noninvasive and (2) invasive.
Noninvasive Blood Pressure Monitoring
Noninvasive blood pressure monitoring can be intermittent or continuous. Intermittent noninvasive blood pressure (NIBP), measurements can further be classified into (1) manual and (2) automated.
Manual Noninvasive Blood Pressure Measurement
Manual NIBP can be measured using auscultatory technique. The auscultatory technique has been mainstay of clinical blood pressure recording for as long as blood pressure has been recorded but is now gradually replaced by automated versions.
The auscultatory method uses either mercury, aneroid, or hybrid sphygmomanometers. Korotkoff's technique, which is used for measuring blood pressure, has not changed for more than 100 years. A blood pressure cuff is placed around the arm and the cuff is inflated above the systolic pressure so that brachial artery is occluded. As the cuff is deflated the blood flow is re-established and is accompanied by sounds which can be heard by holding a stethoscope above the brachial artery just below the cuff. The sounds are thought to originate from turbulent flow and oscillations in the arterial wall. The Korotkoff technique tends to give lower systolic and higher diastolic blood pressure reading higher, than intra-arterial pressure.2,3
A mercury sphygmomanometer is regarded as the gold standard in clinical blood pressure measurement although it is gradually fading out due to the ban on use of mercury in some countries due to environmental reasons.4 The simplistic design of the mercury devices makes less room for errors. Though this should not be any reason for being complacent, studies have found that a significant number of devices can be inaccurate due to technical reasons.5,6
Aneroid sphygmomanometers are devices with mechanical system of metal bellows that expand with the increase in cuff pressure and a series of levers record this pressure on a circular scale. The inaccuracies associated with aneroid devices can be as high as 44%; therefore, they need frequent recalibration.54
Hybrid sphygmomanometers combine features of both electronic and auscultatory devices. The mercury column is replaced by an electronic pressure gauge. Blood pressure is taken in the same manner as with mercury or aneroid using auscultatory technique. The cuff pressure can be displayed as simulated mercury column, simulated aneroid display, or digital display. The hybrid sphygmomanometer has the potential to replace mercury devices because it combines best features of mercury and automated devices.7
Automated Blood Pressure Measurement
The automated blood pressure recorder uses the oscillometric method. It has been demonstrated that when the oscillations of the pressure in the sphygmomanometer are recorded, the point of maximum oscillation corresponds to mean arterial pressure (MAP).8,9 The oscillation starts before the true systolic blood pressure and continues beyond the true diastolic blood pressure. The manufactures use algorithm to detect the systolic and diastolic blood pressure. The algorithm used can vary between manufacturers. This can cause considerable variations between devices.10 But overall it is seen that oscillatory technique compares well with intra-arterial and auscultatory method.11
Noninvasive Continuous Blood Pressure Measurement
Currently, there are two different techniques to record continuous NIBP.
- Arterial applanation tonometry
- Volume clamp method
Arterial applanation tonometry: An arterial pulse wave can be obtained by strapping a transducer to an artery with a bone underneath.12 This technique has been refined and is now available. T-Line system is the device available, allowing automated radial artery applanation tonometry (Tensys Medical, San Diego, CA, USA).13 MAP and diastolic arterial pressure (DAP) have been shown to correlate well when compared with intra-arterial measurements, but systolic arterial pressure (SAP) measurements need further improvement.14
Volume clamp method: It was developed by Penáz15 on the principle of vascular unloading. It combines an inflatable cuff with a photodiode. The photodiode sensor measures the changes in the blood volume based on the amount of light transmitted through the finger. The cuff pressure closely follows the blood pressure changes in the finger arteries. This is then calculated beat to beat, after calibration with built-in oscillometric measurement and a real-time arterial pressure, waveform is displayed. Commercially available devices based on this technology are Infinity® CNAPTM SmartPod®-Drager and ClearSight—Edwards, USA. This technology has been validated in different studies and is found to be comparable to invasive arterial blood pressure monitoring within limitations.16,17
The continuous noninvasive devices have an appeal in its noninvasiveness and can be used in situations where there is need to assess, document, and maintain hemodynamic stability, and arterial blood gas measurements are not required. On the other hand, it has its limitations, in cases with severe vasoconstriction, peripheral vascular disease, and distorted fingers due to arthritis where there may be difficulty in obtaining a proper waveform.
Invasive Blood Pressure Monitoring
Invasive blood pressure (IBP) is the gold standard in blood pressure measurement as it gives beat-to-beat information. It is useful when rapid changes in blood pressure are anticipated due to cardiovascular instability or when NIBP measurements are not possible or unreliable, e.g. obesity, arrhythmias, nonpulsatile flow on extracorporeal membrane oxygenation. It is also recommended when the patients are extremely sick and repetitive cuff inflations can cause localize tissue damage and when there is requirement of repeated blood gas analysis.
The Hemodynamic Monitoring System
The basic principle in any invasive pressure monitoring system is to provide a continuous column of liquid connecting the arterial blood to the transducer. The components required are as follows:
- Intra-arterial cannula
- Tubing
- Transducers
- Microprocessor and display screen
Intra-arterial Cannula
A short cannula is inserted into an artery. A 20-G cannula is generally used. Preferably a nonend artery is cannulated. A radial artery is the most commonly cannulated artery. The advantage of choosing a nonend artery is that, should thrombosis occur, arterial sufficiency is maintained via collateral supply. The collateral supply to the hand can be assessed by performing modified Allen's test although it is not 100% reliable.18 If cannulation of nonend artery such as radial or dorsalis pedis is not feasible then brachial or femoral may be used with due monitoring of signs of distal insufficiency.
Tubing
Correct setup and maintenance of the tubing system and transducer are crucial to avoid erroneous readings. With 5improperly and inadequately prepared monitoring system, the hemodynamic indices measured will be inaccurate and invalidate patient's entire hemodynamic profile misleading the treatment. Three steps need to be followed to prepare the monitoring tubing to ensure accuracy: priming the tubing, leveling and zeroing, and dynamic response testing.
Priming the Pressure Tubing
An arterial catheter is connected to the monitoring system by fluid-filled tubing. The fluid column in the tubing carries the mechanical signal created by the arterial pressure wave to the diaphragm of the transducer. The transducer converts the mechanical signal into electrical signal.
Air transmits mechanical impulses differently than fluid. Air bubbles in the tubing are one of the most common sources of error in IBP monitoring. Air bubbles blunt or damp the propagation of the mechanical signal, causing erroneous readings.19 Therefore air-free priming of the entire tubing is one of the most important steps to avoid error. The entire system should be flushed to achieve air-free priming.
Leveling and Zeroing
The zero reference point is set at the atmospheric pressure and this must be referenced to the level of the heart. This process is called leveling and zeroing. Leveling of the catheter system is achieved by aligning the air–fluid interface of the monitoring system with the external reference point of the heart. The external reference point is called the phlebostatic point, and the axis passing through this point is the phlebostatic axis. This can be located by finding the junction of two lines—the vertical line drawn from the fourth intercostal space and a horizontal line drawn through the midpoint of a line going from anterior to posterior side of chest.20,21 Arterial blood pressure is the key determinant of organ prefusion, and the phlebostatic axis accurately reflects the level of the heart.
Zeroing is a simple but important process and is performed by opening the air–fluid interface to the atmosphere and then electronically zeroing the system. Zeroing establishes the atmospheric pressure as the zero reference point for the monitor.
Leveling and zeroing should be done every time after the air–fluid interface and reference point are changed to ensure accuracy and consistency. It is important to appreciate that small offsets from the phlebostatic axis and zero reference point can cause large errors, and this can lead to inappropriate treatment. Therefore once the reference point is identified, it should be marked.
Dynamic Response Testing
In order to ensure that the monitoring system accurately reproduces hemodynamic characteristics, it is important that the system is tested for its dynamic response. The dynamic response can be defined by its resonance and the damping coefficient.
Each system has its own natural oscillatory frequency or resonant frequency. If this resonant frequency of the monitoring system is the same as the frequency making up the arterial waveform then the subsequent signal will be distorted. It is therefore important that the natural frequency of the system is kept very high. If the frequency of the system is 25 Hz or higher, the system will mostly function properly. The natural frequency of the system can be increased by using a wide bore, high pressure tubing with low compliance, and its length limited to less than 122 cm.19,22 Most systems available have natural frequency of around 200 Hz, but this is reduced by addition of three-way stopcocks, air-bubbles, clots, and additional length of tubing.
The resonance of the system can be measured by performing “Fast flush” test (Fig. 1.1).
The damping coefficient of the monitoring system is a reflection on how soon the oscillations excited by the shock of arterial pressure wave eventually come to rest.19,22 Some degree of damping is intrinsic to the system and is essential. The damping coefficient can be measured by using the fast flush test (Fig. 1.2).
Based on the damping coefficient factors, the monitoring system can be as follows:
- Optimally damped: The monitoring system responds quickly to any change (damping coefficient of 0.7).
- Critically damped: No overshooting but system is slow to respond (damping coefficient 1.0).
- Under damped: Due to occurrence of resonance, the overshooting is required (damping coefficient <0.7).
- Over damped: The signal takes long time to reach equilibrium and will not overshoot (damping coefficient >1.0).
The over damping can exist because of multiple factors, such as bubbles/clots in the system, severe vasospasm of the artery, narrow long soft tubing and kinks in the cannula/tubing. These may be major sources of erroneous recording of blood pressure. The systolic blood pressure is underread and the diastolic is overread. There is no much change to the mean blood pressure.
Under damping can also pose problems. There is an overshoot of pressure waves; therefore, the systolic readings are excessively high and diastolic readings are low.
Transducers
The pressure wave of the arterial impulse is transmitted via the fluid interface in the tubing to a transducer. There is a diaphragm that moves in the response to this pressure wave and the movement is converted into an electrical signal. The transducer uses the strain gauge to convert mechanical energy into electrical energy.
Microprocessor and Display Screen
A numerical and graphical display of the arterial blood pressure is provided on a beat-to-beat basis. This allows us to perform waveform analysis. Information regarding volume status and cardiac output (CO) can be determined by looking at the morphology, position of the dicrotic notch, and the “swing.”
Some systems, such as PICCO, LiDCO, and FloTrac, use pulse contour analysis to derive stroke volume (SV), CO, and systemic vascular resistance (SVR) (Fig. 1.3).
FIGURE 1.3: PICCO monitor (pulsion medical systems) uses the AUC to calculated the stroke volume. (AUC: area under the curve).
MEAN ARTERIAL PRESSURE
Mean arterial pressure is a very important hemodynamic index. It has more influence on the blood flow regulation and organ perfusion. MAP can be calculated by direct and indirect measurement of arterial pressure. When measured by looking at the arterial pressure trace, due to the shape of the trace, the MAP value is more geometric mean then arithmetic average (Fig. 1.4).
It is important to understand the relationship between MAP, CO, and SVR. Cardiovascular system is a hydraulic circuit, and it follows the same principles that are defined by Ohm's law to define the equation between pressure, flow, and resistance.
According to Ohm's law:
Pressure = flow × resistance
This can be rewritten as follows for cardiovascular system:
MAP = CO × SVR
As CO can be derived by multiplying SV by heart rate (HR), the above equation can be further expanded as:
MAP = SV × HR × SVR
So as per the above derivation, MAP values are dependent on the interplay between the three parameters: SV, HR, and SVR (Fig. 1.5).
Mean or systolic, invasive, or NIBP measurements:
There is significant clinical difference in systolic blood pressure recordings by invasive and noninvasive methods. As discussed above, noninvasive oscillometric methods measure MAP and the systolic and diastolic recordings are estimated by algorithms. Lehman et al.23 conducted a retrospective study, which looked into blood pressure 7monitoring techniques in ICU and whether SAP or MAP should be targeted for therapeutic intervention. The study noted that MAP corresponds well between invasive and noninvasive methods; in comparison, the systolic pressure readings were discordant. The MAP value is a most consistent unit for recording blood pressure in ICU, and it does not vary with measurement modality. However, certain pathologies may still require recording of systolic and diastolic pressures (e.g. aneurysms and dissections, stroke, acute coronary syndromes, and valvular regurgitation).
FIGURE 1.4: Mean arterial pressure—geometric average when derived from arterial pressure trace.
NOTE: MAP can also be calculated by using the formula: MAP = [(2 Diastolic BP) + systolic BP]/3
FIGURE 1.5: Factors determining MAP.
(MAP: mean arterial pressure; CO: cardiac output; SVR: system vascular resistance; HR: heart rate; SV: stroke volume; LV: left ventricular)
Invasive blood pressure monitoring is preferred in critically ill patients as NIBP overestimates the systolic blood pressure in hypotensive and low flow states and vice versa. This can be clinically significant, hypotensive systolic NIBP values (<70 mm Hg) are associated with a higher rate of acute kidney injury (AKI) and mortality as compared to IBP of the same value.23
WHAT IS AN IDEAL MAP?
As important as this question is, there is no clear answer to define effective MAP. It is important to understand the physiological changes during hypotension. When the pressure drops below critical pressure (approximately 50 mm Hg), the blood flow to the brain which is maintained by cerebral autoregulation ceases, and as the pressure drops, so does the cerebral blood flow. The critical point of oxygen delivery varies hugely between different organs; gut is the first organ to get affected during shock state. In patients who are chronically hypertensive, the autoregulatory curve is shifted. Critical pressure is higher than what it is in normal individuals. With long-term treatment and blood pressure control, the autoregulatory curve also normalizes.
In a large, multicentered, randomized clinical trial,24 which looked at high (80–85 mm Hg) versus low (65–70 mm Hg) MAP targets in septic patients found that there was no significant mortality difference between the two groups. The high MAP target group had less risk of developing AKI and need for renal replacement therapy in patients with known hypertension. It is also important here to remember how the MAP targets are achieved, either by preload optimization or vasopressors. Referring to the interplay between MAP, CO, and SVR, adequate fluid resuscitation is important; otherwise, excessive use of vasopressors without adequate fluid resuscitation increases the risk of kidney injury.
At the other end of the spectrum, in patients with traumatic brain injury, MAP needs to be maintained so that cerebral perfusion is adequate. Cerebral perfusion pressure (CPP) is defined as:
CPP = MAP – ICP
ICP is the intracranial pressure.
Cerebral perfusion pressure target should be maintained above 60 mm Hg to prevent secondary brain injury.
Therefore, there is no fixed value of MAP. The MAP target needs to be individualized depending on the circumstances of the patient.
In a recent retrospective analysis of patients with sepsis, Maheshwari et al. found that the risk for myocardial injury, ICU mortality, and AKI were present even at an MAP of 85 mm Hg. When MAP remained less than 65 mm Hg, it had relation to mortality. The longer the time spent below MAP 65 mm Hg, the higher the risk of renal and myocardial injury and mortality.25 They suggested that MAP be maintained well above the usual threshold of 65 mm Hg.25
CONCLUSION
Invasive blood pressure monitoring remains the “gold standard” in ICU. Automated continuous blood pressure 8monitoring using oscillometric techniques have comparable MAP recordings with IBP monitoring, but their role probably remains limited for assessment and documentation in relative stable patients. Invasive monitoring with absolute numbers is preferred in patients with hemodynamic instability.
Although both invasive and noninvasive methods give SAP, DAP, and MAP, guidelines have often recommended systolic rather than mean as the target and which modality is used to monitor is usually not appreciated.26–29 MAP is more relevant for organ prefusion, and it is more constant across measuring methods and should be preferred parameter to target therapy although having a single MAP target is difficult and therapy should be personalized to the patient.
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