In summary there are two factors that affect blood pressure and two factors that affect cardiac output (Fig. 11.1). Blood pressure is determined by total peripheral resistance and cardiac output. Total peripheral resistance is defined as the resistance to blood flow created by the peripheral arterial system as well as capillary beds. Cardiac output is determined by a combination of the heart rate and stroke volume. The heart rate is the speed at which the heart is pumping, or the number of contractions per minute. The stroke volume is the amount of blood ejected with one contraction of the heart. Cardiac contractility, preload, and afterload are all factors that can influence the stroke volume. Blood pressure itself may be defined as “the lateral force on the arterial wall” (Love and Harvey 2006). Additionally, blood volume is a major determinant of blood pressure (Durham 2005). Baroreceptors located in the carotid sinus and aortic arch help regulate blood pressure through the sympathetic nervous system control centers. Furthermore, many other factors can impact blood pressure homeostasis, including chemoreceptors, hormonal influences, and the renal sensing systems (Durham 2005; Tefend 2007).
Normal arterial blood pressure values for dogs and cats are indicated in Table 11.1.
Systole represents cardiac contraction, and diastole occurs during cardiac filling. Pulse pressure is a result of the difference between the systolic and diastolic pressures. The mean arterial pressure (MAP) is determined by the following equation:
Diastolic +1/3 Pulse Pressure
(Systolic-Diastolic) = MAP
|Systolic: 110–190 mm Hg||Systolic: 120–170 mm Hg|
|Diastolic: 55–110 mm Hg||Diastolic: 70–120 mm Hg|
Blood pressure monitoring for the critically ill. Proceedings Western Veterinary Conference (Waddell L. 2004).
Blood Pressure Monitoring
There are two techniques available for measuring blood pressure—directly utilizing invasive methods or indirectly via noninvasive methods such as Doppler ultrasonography and DINAMAP (Device for Indirect, Noninvasive, Automatic Mean Arterial Pressure) oscillometric devices (Waddell 2004, Durham 2005). It has been demonstrated that although the heart rate can be reliably and repeatedly predicted there is great variability regarding the indirectly obtained measurements of the diastolic, systolic, and mean arterial pressures as compared to the telemetrically acquired direct blood pressure (Data Sciences International, DSI) readings obtained within a given (3–4 minutes) time frame in a conscious dog. All brands compared in one head-to-head study (using Cardell® 9401 and Dinamap® 1846 SX oscillometric devices) ranged consistently 10% (or as much as 20–30%) lower than the direct, telemetric recordings (Durham 2005; Cowgill 2006; Love and Harvey 2006). Experienced personnel using Doppler (Parks Model 811-BTS) methods to record systolic pressures ranged from 18% to nearly 28% lower than the referenced measurements, demonstrating the lowest level of accuracy. The results of this study found that 1) in normal dogs, “spot” blood pressure measurements may not accurately reflect the patient’s physiologic or pathologic state as accurately as time-averaged blood pressure readings over larger segments of the day; 2) although blood pressure trends were accurately predicted among the indirect oscillometric methods tested, each one provided unique values that were not consistently comparable to the validated standard; and 3) as compared to the referenced measurements, there were wide deviations in recorded blood pressure values when trained and experienced personnel in a clinical setting used oscillometric or Doppler techniques. The apparent inability to accurately gauge blood pressure in veterinary patients using currently available indirect methods led the American College of Veterinary Internal Medicine Consensus Panel to proclaim the following (Cowgill 2006):
… for the diagnosis of systemic hypertension, the indirect device used should be one that is commonly employed or designed for veterinary use, and has been previously validated in conscious animals of the species of interest. However, no indirect device has met these criteria for use in conscious dogs or cats.
These are just a few examples as to why direct arterial blood pressure (DABP) monitoring, proven to be the most accurate method of measuring blood pressure in both human and veterinary patients, is considered the gold standard.
Direct (invasive) blood pressure monitoring
Although a thorough understanding of the regional anatomy and an advanced skill level are necessary to place arterial catheters, their use for DABP monitoring has many advantages when used in critically ill or high-risk anesthetic patients. Preserving blood pressure during anesthesia is imperative to ensure adequate perfusion of tissues and vital organs such as the heart, lungs, kidneys, and brain (McCurnin 1994; Glerum 2005; Love and Harvey 2006). Untreated hypotension can lead to organ failure, shock, and death (Durham 2005; Baetge 2007).
The administration of intravenous crystalloids, colloids, or positive inotropes can be an integral part of maintaining blood pressure in anesthetized patients (Love and Harvey 2006). As such, direct blood pressure monitoring is extremely helpful in assessing the progress of fluid resuscitation, or inotropic or pressor therapy in patients with hypovolemic or septic shock (Waddell 2004). Arterial catheters can also be used for arterial blood gas analysis, considered the gold standard for assessing a patient’s ventilatory function or acid/base status (Marshall 2004; Beal 2006; Tegtmeyer 2006; Terry 2006; Wagner and Ryan 2006).
There are numerous arteries that may be selected as the placement site for an arterial catheter. The dorsal pedal/metatarsal artery is most commonly used and is one of the easiest to maintain on a long-term basis (Beal 2006). Other arteries that are suitable for short-term use in veterinary patients include the radial, brachial, palmar, femoral, auricular, carotid, coccygeal, and sublingual (ventral tongue) arteries. The femoral artery may be used in smaller patients such as cats and small dogs, and the auricular artery (located on midline of the dorsal surface of the pinna) can be utilized in larger dogs with big, pendulous pinnae (e.g., basset hound, beagle) (Waddell 2004; Beal 2006; Love and Harvey 2006; Terry 2006).
Other considerations when selecting the site for an arterial catheter include avoiding compromise of the circulation distal to the placement site (e.g., end arteries or other areas with known deficiencies in collateral circulation) and avoid infected areas or areas that have sustained trauma proximal to the proposed insertion site (Tegtmeyer et al. 2006).
Arterial catheters used for DABP measurements must be connected to a continuous flush system via the shortest possible length of non-compliant extension tubing and to either an aneroid manometer or to an oscilloscope with a strain gauge pressure transducer (Love and Harvey 2006; Wagner et al. 2006; Baetge 2007). Both DABP methods will provide the anesthetist with continuous readings even during extreme physiologic adversities that may be associated with hypotension, vasoconstriction, tachycardia, and arrhythmias (Baetge 2007). Although the less expensive aneroid manometer is capable of recording only the mean arterial pressure value, an oscilloscope monitor displays values for the systolic, diastolic, and mean arterial pressure as well as a series of real-time waveforms (McCurnin 1994; Love and Harvey 2006; Baetge 2007). There is a reported 2–4% level of inaccuracy associated with DABP measurements; 1–2% of the inaccuracy arises from the transducers, and another 1–2% occurs from the amplifier (Waddell 2004).
Not all brands of anesthetic monitors are capable of obtaining DABP measurements. Consult the manufacturer’s instruction manual to find out whether a particular model of anesthetic monitor is capable of performing invasive blood pressure monitoring as well as which type of transducer kit or other equipment may be necessary, and how to correctly assemble and operate it.
DABP waveform interpretation
Understanding the DABP waveform is essential for assessing cardiac function, particularly as it relates to left ventricular ejection. Used in conjunction with an ECG, evaluation of the DABP waveform permits the anesthetist to determine when cardiac arrhythmias may be causing poor pressures or when pulse deficits become detrimental to the patient, thereby allowing timely institution of interventional drugs. For example, a state of poor perfusion exists when cardiac arrhythmias (e.g., premature ventricular contractions) are associated with a dampened waveform appearance in conjunction with an abnormal MAP (Tefend 2003a).
Peak ejection occurs during the highest point on the waveform and is associated with systole (Fig. 11.2). The downstroke of the waveform is associated with a drop in pressure. Midway through the downstroke the dicrotic notch may be visible and indicates closure of the aortic valve. The dicrotic notch also represents the beginning of diastole. The remainder of the waveform downstroke represents the flow of blood into the arterial tree, with the lowest point of the waveform representing diastole. What’s more, it is possible to identify physiologic abnormalities such as vasoconstriction or vasodilation based on the appearance of the DABP waveform (Fig. 11.3) (Muir et al. 2000; Tefend 2003a; Love and Harvey 2006).
A thorough knowledge of the potential complications and limitations associated with arterial catheters can simplify the troubleshooting process and ensure their continued use for DABP monitoring. Some of the most common problems encountered include:
- Waveform dampening or loss: This may indicate actual hypotension or loss of pulse. Assess the patient. It may be associated with air bubbles or blood clots, or excessive blood or kinks present in the catheter or tubing line (Mazzaferro 2004). Additionally, the artery may be in spasm. During arterial spasms the MAP is generally correct even without a good tracing. In all cases, flush the line and catheter after ensuring that the line has not been clamped off. If the catheter has migrated up against the vessel wall, a change of the patient’s position may resolve the problem (Tefend 2003a; Waddell 2004; Love and Harvey 2006).
- The line flows and can be aspirated, but the waveform is not visible: Ensure that the cable is still attached to the monitor and that the stopcock is not in the OFF position toward the patient.
- Sudden change in pressure: Ensure the transducer has not moved and is still at the level of the heart, and that the surgeon is not leaning on the patient line or a major blood vessel. More importantly, a sudden change in pressure can indicate that cardiac arrest has occurred or is imminent: Assess the patient’s pulse and monitor end-tidal carbon dioxide production.
- Inaccurate readings: Lower systolic and higher diastolic values may be present whenever the waveform becomes dampened. Artificially increased pressure readings may occur if the catheter itself obstructs blood flow in smaller arteries (Love and Harvey 2006). Furthermore, inaccurately low readings may be evident in patients with severe peripheral vasoconstriction (e.g., severe hypovolemia or due to higher doses of pressor agents) (Waddell 2004).
- Waveform amplification: Reflections of the waveform from a peripheral catheter may amplify the systolic pressure, resulting in falsely elevated systolic values. Although less common in veterinary medicine, this phenomenon occurs routinely in geriatric human patients when arteries are noncompliant (Waddell 2004).
Do not waste unnecessary time with troubleshooting techniques when there are doubts regarding the accuracy of DABP. Always perform a thorough exam and assessment of the patient first and combine those findings with an indirect blood pressure reading. Although differences will be evident between indirect and DABP measurements, the patient’s clinical management may be affected by large disparities (Tefend 2003a).
Indirect (Noninvasive) Blood Pressure Monitoring
There are three commonly recognized methods used to measure blood pressure indirectly: Doppler ultrasonography, photoplethysmography, and oscillometrically with a DINAMAP device (e.g., Dinamap®, Cardell®, petMAP®) (Acierno and Labato 2004; Durham 2005; Love and Harvey 2006). All indirect methods tend to underestimate the actual blood pressure, and all work best when the MAP is 60–100 mm Hg (Seahorn 2004; Durham 2005; Cowgill 2006; Love and Harvey 2006). However, because they are consistent over a wide range of pressures, they are considered reliable (Durham 2005; Cowgill 2006).
Regardless of the method used, cuff selection is imperative for obtaining the most accurate results. Inappropriate cuff size and placement are the most common sources of errors. The width of the cuff should extend 40% around the circumference of the limb. When the cuff is determined to be too small, the next wider size should be selected. In cats it is acceptable to use a cuff that is only 30% of the circumference of the limb (Fig. 11.4) (Acierno and Labato 2004; Durham 2005; Love and Harvey 2006).
The cuff should be snug but not too tight (Durham 2005). It is acceptable to secure the cuff using a piece of tape to prevent it from becoming dislodged during inflation (Seahorn 2004). Due to the effects of gravity on arterial blood pressure the cuff should ideally be positioned at the level of the heart (Love and Harvey 2006; Duffy 2007). If the cuff is too narrow, too loose, or below heart level, the reading will be falsely high. If the cuff is too wide, too tight, or above heart level, the reading will be falsely low (Durham 2005; Love and Harvey 2006). Additionally, a cuff placed over a joint is less likely to compress the artery, and a hole in the cuff may result in rapid cuff deflation that thwarts machine interpretation (Durham 2005; Duffy 2007). Acceptable cuff locations include the forelimb, tail, and hindlimbs. The areas proximal to the carpus and tarsus work best. The ventral tail is a good choice in cats as well as short-legged dog breeds, such as the bassett hound and dachshund (Durham 2005). In cats the author has had satisfactory results with oscillometric devices by using the distal humeral area, proximal to the elbow.