Karen Turner
RN, BN, Cert/Palliative Studies, Grad Cert. Nsg. (Intensive Therapy) Intensive
Therapy Unit, St.Vincent’s Hospital.
ABSTRACT
In acute care settings such as intensive care, coronary care, operating theatres, and emergency departments the care of critically ill and injured patients often requires invasive monitoring. This article presents an introduction to one such type of invasive monitoring: the intra-arterial line, which measures blood pressure.
This article examines the component parts of the intra-arterial line, their importance and their use. It looks at why the line is used, and the difference between the invasive and noninvasive measures of blood pressure. It explores safety issues as they pertain to the patient and the nurse and concludes with how to interpret the information from the waveform that the arterial line provides.
Introduction
Critically ill and injured patients require constant monitoring so that changes in their condition can be rapidly assessed and early interventions instigated to restore them to as normal physiological functioning as possible. An important monitoring tool routinely used to assess physiological function in critical care settings is intra-arterial blood pressure monitoring.
Intra-arterial blood pressure monitoring has been a theoretical concept since the eighteenth century but has only come into common clinical practice since the electromechanical revolution of the 1960's (Davoric & Vanriper, 1995). It is a valuable monitoring tool with inherent risks and therefore requires competent knowledgeable nursing care for its safe and effective use.
This article explores the technological considerations, safety aspects and nursing care of a patient with an intra-arterial line in situ. It identifies and discusses the components of an arterial monitoring device, namely, the pressurised flush bag, the transducer, the tubing between the transducer and the cannulae, and the cannulae, itself. It then discusses the nursing care and responsibilities in looking after a patient with intra-arterial blood pressure monitoring in progress. Arterial waveform interpretation will be outlined followed by a brief discussion on occupational, health and safety issues pertaining to intra-arterial blood pressure monitoring.
Technical Aspects of Intra-Arterial Blood Pressure Monitoring
The arterial line is usually inserted for two reasons; to provide a constant beat to beat measurement of the systolic, diastolic and mean arterial blood pressures and for the frequent gathering of arterial blood gas samples (Lough, 1987). The patency of the arterial line therefore is paramount and is achieved through the utilisation of a pressurised flush bag. The flush bag, depending upon unit protocol is usually 500 mls of 0.9% sodium chloride solution (normal saline) and is attached to the transducer via tubing. This bag is placed under pressure by placing it in a sleeve which when inflated to 300 millimetres of mercury (mmHg) enables a continuos flow of 3 to 5 millilitres/hour (mm/hr) which prevents arterial blood from tracking up the tubing or the tubing becoming blocked (Hudak, 1998).
While some unit protocols heparinise the flush bag to prevent cannulae thrombosis, Gamby and Bennet (1995) have concluded that this is unnecessary. They conducted a study comparing heparinised and non-heparinised flush bags and concluded that "it is the constant flush under pressure which maintains patency of the cannulae rather than heparinisation of the flush" (Gamby & Bennet, p.149). This research demonstrates that the flush bag does not need to be heparinised, just pressurised.
The flush bag plays a vital role in preserving the accuracy of arterial blood pressure readings by preserving the integrity of the transducer. The transducer is an external, disposable, fluid-air interface which detects changes of pressure in the artery. It is able to detect these pressure changes because there is a column of fluid between the blood in the artery and a diaphragm within the transducer. This diaphragm conveys the fluctuations in pressure to a silicon chip which, when amplified can be displayed on a monitor as an arterial waveform with the correlating blood pressure (Daily & Schroeder, 1995). Air bubbles and blood in the transducer alter this fluid filled column and therefore lead to inaccurate readings. To prevent this, the transducer has a quick flush mechanism which is connected to the pressure bag so that air bubbles and blood can be flushed out of the system via an exit port to ensure accurate readings.
The transducer not only monitors pressure changes it is also a port for obtaining blood gas samples and maintaining the accuracy of the blood pressure readings through the ability to calibrate to atmospheric and hydrostatic pressures. The calibration of the transducer, which is commonly referred to, as "zeroing" is important because without off setting the extraneous atmospheric and hydrostatic pressures, the accuracy of the arterial trace and measurements would be encumbered (Davoric, Vanriper & Vanriper, 1995). Ahrens (1994) suggests that "based on the available evidence, … zeroing needs to be performed only once, unless the transducer is disconnected from the monitor". Many nursing authors however recommend that the nurse practitioner zero the transducer themselves each shift to ensure the accuracy of the transduced readings (Chulay, 1995) and this is common in clinical practice.
The positioning of the arterial transducer is commonly either in close proximity to the insertion site of the arterial cannulae or secured further away, such as on a pole at the head of the patient’s bed. The literature consulted for this article states that the transducer should be secured at the site and level of the catheter tip, with the cannulated extremity placed on the same plane as the right atrium, known as the phlebostatic axis (Daily & Schroeder, 1995; Chulay, 1995; Chulay & Holland, 1997; Campbell, 1997). Securing the transducer at the phlebostatic axis aids in preserving the accuracy of the readings because if the transducer is placed lower than the phlebostatic axis, the readings will be inaccurately high; but if the transducer is higher than the phlebostatic axis, the readings will be erroneously low (Daily & Scheoder, 1995). It is crucial that the nurse be aware of these implications in caring for the arterial cannulae because if inaccurate readings are misinterpreted as accurate, inappropriate interventions may be implemented which may be detrimental to the patient.
The length of the tubing between the transducer and the cannulae has a direct effect upon the accuracy of the values given on the monitor. The tubing must be stiff, non-pliant, less than 120 cm in length and of a 'large' diameter (Daily & Schroeder, 1995). This is important because the tubing must conduct the pulsatile fluid movement from the tip of the cannulae to the transducer without being compressed, and at a similar frequency to the air-fluid interface to remain an accurate reflected value. If tubing is too long or too short, or if the tubing is soft and pliable, the accuracy of the readings will be compromised.
The intra-arterial cannulae itself is usually a twenty gauge cannulae which is connected directly to the non-pliant tubing leading to the transducer. The cannulae is inserted into the artery in the opposite direction to the flow of the blood. This is so that there is a pulsatile movement in the fluid column which is detected by the diaphragm in the transducer and converted to a waveform.
Nursing Care of Patients Undergoing Continuous Intra-Arterial Blood Pressure Monitoring
The cannulae is an important component of the intra-arterial line. It poses significant risk, however, to the perfusion distal to the cannulated artery. For this reason, prior to insertion and regularly while an arterial cannulae is insitu, collateral circulation must be assured to the limb distal to the cannulae. This can be initially confirmed by the Allen test (Anderson, 1998, p.60) for the radial artery and for all the other sites (such as the axillary, brachial, femoral and dorsalis pedis arteries) by checking the colour, warmth, capillary return and movement if the patient is conscious. By having the limb distal to the cannulae in constant view and placing the pulse oximeter probe distal to the insertion site, the nurse may continually observe its perfusion
Having the arterial cannulae itself on view is an important guard against arterial haemorrhage. With the cannulae in the artery there is the potential for a significant haemorrhage should the line become disconnected or the cannulae dislodged. It is for this reason that the arterial cannulae should be on constant display, with a transparent dressing, leur-lock connections and the cannulae insertion site splinted against unwanted movement (Gavenstein & Paulus, 1987).
Preventing infection of the intra-arterial cannulae site is a particularly vital nursing responsibility. The patient is usually critically ill, with an existing pathophysiological problem which will, to varying degrees, compromise his/her immune system. This makes the patient particularly vulnerable to nosocomial infections which must be guarded against. It is therefore an important nursing responsibility to ensure aseptic insertion, regular aseptic dressings and frequent inspections of the site for any signs of infection (Chulay, 1995).
Once the accuracy of the arterial systolic, diastolic and mean pressures are assured through using appropriate tubing which is patent, zeroed, and leveled at the phlebostatic axis the nurse should be confident that the readings are accuarte. However, there is a tendency for clinical nurses to harbor mistrust in their patients' arterial monitoring values and to return to non invasive blood pressures to ascertain the accuracy of the invasive. Chulay and Holland (1997) deem this as being unnecessary and advocate that "the arterial blood pressure measurement will be the most accurate reading as long as the catheter is patent and the transducer system is properly set up and functioning" (Chulay & Holland, 1997, p.14). Common nursing practice however, which is supported by authors such as Lough (1987), is to correlate the mean arterial pressures (MAP) of the noninvasive cuff blood pressure to the intra-arterial pressure.
Arterial and non invasive blood pressures actually measure different components of the circulating blood volume. While "arterial systems measure pressure, the amount of force exerted by circulating blood over a specific area . . . Cuff pressures measure flow, the amount of blood circulating over a specific time" (Campbell, 1997). Despite this difference, Lough (1987) is of the opinion that in a healthy patient there will be no more of a difference in the MAPs than 10mmHg. In this situation, if the accuracy of the arterial line was questionable it could be beneficial to compare the MAP's. However as Lough (1987) explains, in a patient with a low cardiac output, such as in shock, a cuff pressure may be unobtainable and therefore the MAPs may be significantly different.
The most responsible actions a nurse can take amid the controversy about invasive and non invasive blood pressures is to know how to interpret arterial waveforms. A normal waveform, as in Figure 1, is made by the pressure wave which preceedes the flow of blood from the contraction of the heart. The initial steep up swing is called the anacrotic rise and occurs during early systole with the opening of the aortic valve and left ventricular contraction. The rounded peak of the waveform is due to the continued stroke volume ejection from the left ventricular contraction and is therefore still in systole. The other side of the waveform is the downward slope which, represents the peripheral run off following the cessation of the contraction. The notch on the downward slope represents the closure of the aortic valve and is called the dicrotic notch which heralds the onset of diastole and the waveform will continue to decline until the next systole (Darovic & Vanriper, 1995).
Figure 1 : Tracing from Arterial Blood Pressure Monitoring
Occupational Health and Safety Issues
Before concluding this article it is important to acknowledge that occupational health and safety issues are paramount for the protection of the clinical nurse caring for the patient with an arterial line. The fact that the arterial cannulae directly enters the artery and that the integrity of the arterial line is regularly breached for arterial blood gas samples, and in removing the cannulae, there is potentially a significant personal risk to the nurse of a blood splash injury to the most susceptible of mucosal sites namely the eyes and mouth. For this reason nurses must wear protective measures such as goggles and masks to protect themselves or run the risk of being infected with any infectious diseases a patient may have.
Conclusion
In conclusion, nurses have a crucial role to play in the maintenance, accuracy, protection of patients from complications and interpretation of arterial blood pressure monitoring. The nurse must ensure the pressure bag remains pressurised to 300mmHg with adequate amounts of non-heparinised normal saline; that the transducer remains free of blood clots and air bubbles and is accurately zeroed and that the tubing between the transducer and cannulae is not too long, that it is stiff, non-pliant and free of debris. The cannulae must also remain a primary focus of nursing care to prevent infection, haemorrhage, unintentional dislodgement and distal perfusion beyond the cannulae. There is a vital role in correlating the arterial blood pressure monitored trends to the patient condition and other available data and acting on this information appropriately. Finally the nurse has a responsibility to protect themselves from possible exposure to infectious diseases when drawing arterial blood gas samples and the removal of the arterial cannulae.
Intra-arterial monitoring is a useful tool for monitoring immediate changes in a pateint’s blood pressure values. The waveform offers valuable diagnostic information which is frequently used in critical care settings to titrate and manipulate various treatment modalities. It is a tool which must be cared for by competent, knowledgable nurses so the greatest benefits for patients can be realised and achieved.
REFERENCES
Ahrens, T (1994). Ask the Experts. Critical Care Nurse,14 (6), 98-99.
Anderson, L.E. (Ed.). (1998). Mosby’s medical, nursing, and allied health dictionary. (5th Ed.). St. Louis: Mosby.
Campbell, B (1997). Arterial waveforms: Monitoring changes in configuration. Heart and Lung, 26 (3), 205-215.
Chulay, M (1995). Ask the experts. Critical Care Nurse, 15 (2), 108.
Chulay, M., & Holland, S (1996). Ask the experts. Critical Care Nurse, 16 (6), 103-107.
Chulay, M., & Holland, S (1997). Ask the experts. Critical Care Nurse, 17 (3), 14-16.
Gamby, A., & Bennett, J (1995). A feasibility study of the use of non-heparinised 0.9% sodium chloride for transduced arterial and venous lines. Intensive and Critical Care Nursing, 11 (3), 148 – 150.
Daily, E., & Schroeder, J. (1995). Techniques in bedside hemodynamic monitoring. (5th ed). St Louis: Mosby.
Darovic, G., Vanriper, J., & Vanriper, S. (1995). Arterial pressure monitoring. In Darovic, G. (Ed.), Hemodynamic Monitoring: Invasive and noninvasive clinical application. (pp.177-210). Philadelphia: W.B. Saunders Company.
Darovic, G., & Vanriper, S. (1995). Fluid filled monitoring systems. In Darovic, G., (Ed.), Hemodynamic monitoring: Invasice and noninvasive clinical application. (pp.149-175). Philadelphia: J.B.Lippincott Company.
Gavenstein, G., Paulus, J., & Paulus, D. (1987). Clinical monitoring practice. (2nd ed.). Philadelphia: J.B.Lippincott Compnay.
Hudak, C. (1998). Assessment: Cardiovascular system. In Hudak, C., Gallo, B., & Benz, J. (Eds.), Critical Care Nursing: A holistic approach. (5th ed.). (pp.124-135). Philadelphia: J.B. Lippincott Company.
Lough, M. (1987). Introduction to hemodynamic monitoring. The Nursing Clinics of North America. 22 (1), 89-103.