Hemodynamic monitoring is important in the care of any hospitalized patient. Frequent monitoring is of utmost importance in critically ill patients and surgical patients with an increased risk of morbidity and mortality. This can be achieved through intermittent monitoring, which is non-invasive but only provides snapshots in time, or through continuous invasive monitoring. The most common way to do this is arterial pressure monitoring via the cannulation of a peripheral artery. Each cardiac contraction exerts pressure, which results in mechanical motion of flow within the catheter. The mechanical motion is transmitted to a transducer via a rigid fluid-filled tubing. The transducer converts this information into electrical signals which are transmitted to the monitor. The monitor displays a beat-to-beat arterial waveform as well as numerical pressures. This provides the care team with continuous information about the patient's cardiovascular system and can be used for the diagnosis and treatment.
While arterial blood pressure monitoring can provide invaluable information, arterial cannulation is not routine in patient care as it is not required for every patient in the ICU or every patient undergoing surgery. For certain patients, the cannulation of an artery is contraindicated. These include infection at the site of insertion, an anatomic variant in which collateral circulation is absent or compromised, presence of peripheral arterial vascular insufficiency, and peripheral arterial vascular diseases such as small to medium vessel arteritis. Additionally, while not absolute contraindications, careful consideration should be made in patients who have coagulopathies or who are taking medications that prevent normal coagulation.
Complications of arterial cannulation for blood pressure monitoring include:
There is controversy regarding the catheterization of the brachial artery. There are several reasons for this. One reason is its proximity to the median nerve, and therefore inadvertent injury to the median nerve is a possibility. Another concern for catheterization at the brachial artery is thrombosis. The third reason for concern is limb ischemia due to a lack of collateral vessels. However, rates of these complications are low, and catheterization at this site is associated with a lower risk of infection compared with femoral artery catheterization. In cardiac surgery patients, brachial artery catheterization is more reliable compared with radial artery catheterization. This is especially true after cardiopulmonary bypass. Therefore, due to the low risks associated with brachial artery catheterization, this site is a viable option for catheterization of the upper extremity.
Arterial pressure monitors provide continuous information on a patient's hemodynamics. This information is invaluable to assist in timely clinical decision-making and intervention in the care of critically ill patients. Analysis of the arterial pressure wave can allow the clinician better to understand the function of the patient's heart as the arterial pressure wave corresponds with the cardiac cycle. During systole, the aortic valve opens, and there is a rapid ejection of blood from the left ventricle into the aorta. The arterial waveform will show an upswing followed by a downward turn. The closure of the aortic valve marks the beginning of diastole. The arterial waveform will show a notch on the downward stroke; this notch is called the dicrotic notch and is due to the closure of the aortic valve. The remainder of the downward stroke is the diastolic flow of blood into the arterial tree. A patient with abnormal heart rhythm or valvular abnormalities will have an abnormal arterial waveform.
Arterial cannulation can provide invaluable information in the care of critically ill patients. However, it is not without risks, one of which is often overlooked as a significant problem in patient care. Any time the integrity of the skin is violated, the risk of infection is present. This is especially true in procedures involving placement of plastic cannulas into vessels where they may stay for extended periods. In the United States, an estimated 80,000 catheter-related bloodstream infections occur each year. While cannulation of a radial artery may not be as serious as cannulation of a major vessel like those in the neck or groin, both have the potential for catheter-related bloodstream infection, and this risk is not insignificant. This is especially true considering that the patient population in whom an arterial line is indicated are more likely to be critically ill and with multiple comorbidities. In these patients, a catheter-associated infection can be catastrophic. Therefore, it is of vital importance that any time an arterial line is placed, meticulous care is taken to minimize infection during placement and for the duration of the time that the line is in situ. Multiple studies have been conducted to compare the rates of bloodstream infection between different intravascular devices. While the location of catheter and duration of catheter placement affect infection rates, the resounding conclusion is that infection rates from arterial catheters are comparable to those of central venous catheters. [Level I]
Several guidelines exist to prevent intravascular catheter-related infections. One is available through the CDC. Emphasis is placed on choosing sites with lower rates of infection. In adults, this would be the radial, brachial, and dorsalis pedis. In children, it is more appropriate to use the radial, dorsalis pedis, posterior tibial arteries. Other points of emphasis are clinician attire of mask and cap, proper hand hygiene, adequate cleansing of patient skin, and placement of drapes to create a sterile field.
One aspect that cannot be overlooked is the importance of nursing care in the management of catheter-associated infections. While nurses are typically not the ones placing these catheters, they are at the front-line when it comes to infection prevention. This is because they are usually the ones accessing the ports for lab draws in addition to performing dressing changes and routine skin care. Through proper education of nursing staff on how to maintain sterility of arterial pressure monitoring circuit as well as continued care of the skin around the cannulation site, the infection rate associated with arterial catheters can be minimized.
Lastly, it should be everyone's responsibility to ensure that lines are not kept for longer than they are necessary as there is a direct relationship between the duration of a line and the infection risk. There should be open communication between nursing staff and ICU providers to discuss the removal of any lines that are in place but are no longer required for patient care. When a patient no longer has an indication for arterial pressure monitoring, the catheter should be removed, and the area should be dressed to allow the cannulation site to heal.
To ensure that a patient receives optimal treatment, it is crucial that staff are aware of factors that affect the safety and accuracy of arterial monitoring. This starts with the training of providers on proper technique for placement of the arterial line, including sterile technique and proper dressing of the line to minimize infection. This also includes training on the correct use and care of the equipment to obtain accurate data. The clinical utility of the pressure monitoring system is dependent on the accuracy of the data that it provides. This accuracy is affected by several factors which are covered below.
To accurately measure arterial blood pressure, the system must be correctly set up. For patients who are lying down, the transducer is usually positioned at the level of the right atrium or the midaxillary line. For patients who are sitting, the cerebral pressure is less than at the level of the heart, so the transducer should be placed at the level of the brain.
The weight of the column of fluid within the tubing exerts hydrostatic pressure on the transducer which can affect the blood pressure reading. Proper leveling of the transducer minimizes the effect of hydrostatic pressure on the transducer and ensures the accuracy of the measurement. For every 2.5 cm, the transducer is above or below the level of the catheter, the pressure in the system changes by 1.877 mm. If the transducer is positioned too low relative to the catheter, the fluid within the tubing above the transducer exerts greater pressure on the transducer and produces an abnormally high-pressure value. If the transducer is positioned too high relative to the catheter, the fluid within the tubing above the transducer exerts less pressure on the transducer and produces an abnormally low-pressure value.
Simply put, if the transducer is positioned too high, the readings will be lowered, and vice versa for a transducer that is positioned too low. This can be dangerous if healthcare providers are recording inaccurate measurements and making treatment decisions based on inaccurate data. Therefore correct positioning of the transducer is very important for accurate blood pressure monitoring and patient care. The position of the transducer should be checked every time the OR or patient bed is repositioned.
The measuring system must also be zeroed in order to obtain accurate data. Zeroing the system provides a reference point of pressure. Most commonly, this is atmospheric pressure. To zero the transducer, the stopcock is opened to the atmosphere. The "zero" button is pressed to indicated on the monitor that this is the zero reference pressure.
Another factor that affects the accuracy of data is the damping of the system. This is the amount of resonance in the system and will affect the systolic and diastolic pressure while maintaining the correct mean arterial pressure. If the system has inadequate damping, there will be an excess of resonance which will result in an overestimation of the systolic pressure and an underestimation of the diastolic pressure. If the system is overdamped, there will be a falsely-low systolic pressure, but the diastolic pressure is usually accurate. Overdamping can be due to a clot or buildup of fibrin in the catheter tip. A system that is not optimally damped will be apparent on waveform analysis. An overdamped trace will show less than 1 1/2 oscillations below the baseline with an unclear dicrotic notch. And overdamped tracing will show oscillations both below and above the baseline, known as "ringing". An easy way to test the damping is to flush the system and assess the dynamic response. This is done by quickly flushing the system via the snap or pull tab. When this is activated, the monitor will display a square waveform which rises suddenly, flattens, and sharply declines. This will be followed by one or two oscillations appearing above and below the baseline after the release of the flush tab. This indicates an optimal dynamic response. A dicrotic notch should also be clearly present.
Taking the above steps will optimize the arterial pressure monitoring system to accurately reflect the patient's actual arterial pressure.
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