Cardiovascular monitoring (Proceedings)

Cardiovascular (ECG, CVP, blood pressure) monitoring is essential to detect patient changes and to direct therapy and supportive care. Veterinary technicians are expected to have the mechanical ability to understand and operate these monitoring systems, the creativity to adapt a single piece of equipment to serve both a 3 pound Chihuahua and a 200 pound mastiff, the technical ability to prepare the patient for monitoring (placement of central IV lines, arterial lines etc..), and the knowledge to interpret and respond to results as they are collected.

Until normal perfusion is restored frequent serial values can be obtained for:

          • Mucous membrane color and capillary refill time

          • Packed cell volume and total solids

          • Urine output

          • ECG

          • Heart rate and pulse equality

          • Respiratory rate and effort

          • Arterial blood pressure

          • Central venous pressure

          • Cardiac output by Swann Gantz Catheter (not practical in most hospital settings)

ECG/heart rate and pulse equality

The ECG is an important monitoring tool for the critical patient, not just for the conduction abnormalities it detects, but for electrolyte abnormalities and changes in heart rate. Coupled with other monitoring devices, the changes in heart rate may give vital diagnostic information. For example, when tachycardia (rapid heart rate) occurs concurrently with hypotension (decreased blood pressure) the patient may be suffering from hypovolemia (lack of sufficient circulating fluids) while patients with tachycardia occurring concurrently with hypertension (increased blood pressure) may be experiencing extreme pain. Auscultation of heart rate and simultaneous palpation of peripheral pulses yields more information that either alone. Pulse quality, and synchronicity with heart rate are major indicators of perfusion.

Arterial blood pressure physiology and measurement

Arterial blood pressure (ABP) describes the force exerted on blood vessel walls, over a cardiac cycle, by the amount of blood being pumped through them. ABP is a summation measurement of cardiac output (the heart's ability to effectively pump the volume of fluid presented to it), vascular resistance (the elasticity of the vessel walls) and blood volume (the quantity of circulating fluid). Changes in any of these factors can have profound effects on ABP.

The overall goal of the cardiovascular system is to provide oxygen and nutrients to all body tissue as well as to remove metabolic waste products. ABP must be normal to insure adequate blood volume is reaching vital organs.

Increased ABP (hypertension) may be caused by hypervolemia, excessive NaCl retention or increased circulating vasopressors such as cathecholamines and can result in edema formation, arteriole damage, hemorrhage and damage to "target organs" such as the eye, kidney, heart and brain. Because ABP is not routinely measured in veterinary patients hypertension is often first detected because of changes in these target organs. Hypertension is corrected by treating the underlying disease or condition and with the use of antihypertensive agents such as diuretics, vasodilators and/or calcium channel blockers.

Decreased ABP (hypotension) may result from hypovolemia poor blood vessel tone or any condition that diminishes cardiac output. After prolonged periods of hypotension the patient is in danger of developing tissue ischemia, brain damage, renal failure, cardiovascular collapse and death. Hypotension can usually be corrected by aggressive fluid volume support, using both crystalloid and colloid solutions, but may require blood pressure supporting (i.e. inotropic and/or vasoactive drugs as well.

Physiology

Blood pressure is the measurement of tension within the arterial vessel walls at varies points in a cardiac cycle. Systolic blood pressure represents the maximal pressure created by ventricular contraction. Diastolic blood pressure represents the minimal pressure at rest just prior to the next contraction. Mean arterial pressure (MAP or MABP) is the average driving pressure over the entire cardiac cycle and is the major determinant of perfusion to most organs MAP is not a mathematical average of systolic and diastolic pressures because the diastolic phase is approximately twice as long as the systolic phase. Therefore, MAP is derived by the following formula:

Normal BP values:

      Systolic: 120-180mmHg

      Diastolic: 60-100mmHg

      Mean: 80-100mmHg

Measurement

      Clinical assessment

Palpation of pulses can give significant information about ABP. Pulses will often wane progressively, starting at the periphery, as ABP drops; therefore, the absence of femoral pulses is likely to be a more serious sign than absence of metatarsal pulses if femoral pulses can still be felt. Abnormally high ABP may be evidenced by very strong or bounding pulses even in the most remote periphery.

At MAP < 60mmHg renal arterial blood flow is restricted and the kidneys are no longer perfused. Patients will often become oliguric and then anuric. Urine production is then an indicator that the MAP is at least 60mmHg. In hypovolemic patients urine output may be less than expected but production of some urine (0.5-1ml/lb/hr minimum) confirms renal perfusion. High blood pressure does not seem to affect urine output.

Weakness, lethargy or inability to rise might be manifestations of low ABP. Poor mucous membrane color and prolonged capillary refill time may indicate insufficient blood flow. Likewise, cold, poorly colored extremities may result from hypotension. Of course, patients with these signs should be monitored for signs of dehydration (hypovolemia) by checking membrane moistness, packed cell volume, total solids, skin turgor etc. Some patients may exhibit diminished bleeding at operative sites.

      Mechanical methods

Blood pressure can be measured by direct or indirect means. Generally, direct methods are more accurate, particularly in hypotensive patients, but because direct measurement requires invasive arterial catheterization, indirect methods, though less accurate are more often used. In either case, clinical judgment should be integrated with blood pressure to provide the most complete assessment.

      Indirect measurement

Indirect blood pressure is measured by inflating an occlusive cuff over an artery and recording the cuff pressure necessary to inhibit and restore blood flow distal to the occlusion. There are two basic types of indirect blood pressure monitors; Doppler ultrasonic blood flow and Dina map oscillation detection.

Doppler Ultrasonic blood flow relies on audio transmission of blood flow and manual correlation to concurrent cuff pressure. First, blood flow to the artery is occluded with an inflated cuff. The cuff is then slowly deflated until blood flow resumes. The motion of the arterial wall as blood flow is restored is transmitted through a transducer into audible tone. A sphygmomanometer is used to read systolic pressure. Diastolic pressure is difficult to ascertain by this method as it is detected by a subtle shift or muffling of audible tone.

Dinamap automated oscillation devices operate by incremental deflation of their cuffs until blood flow is detected. Again, an occlusive cuff is inflated over an artery. As the cuff deflates, arterial wall motion is detected by a microprocessor located in the cuff. Systolic pressure corresponds to the point when the oscillations in the arterial wall begin to increase. MAP corresponds to the peak in oscillations. Diastolic pressure is recorded when the oscillations cease to decrease upon further cuff deflation. Systolic, diastolic and MAP readings are digitally displayed.

Cuff size is a major determinant of results in both techniques. Cuff width should approximate 40% of limb circumference. Cuffs that are too small will yield erroneously high results due to the increased pressure needed to occlude blood flow. Conversely, cuffs that are too large will yield erroneous low blood pressure readings. Studies have shown greater accuracy of results using hind limbs over fore limbs. Some studies show a preference for tail base placement.

Indirect measurement is a simple noninvasive technique to obtain blood pressure readings in most patients who require intermittent monitoring.

Central venous pressure (CVP)

CVP is defined as the luminal pressure of the intrathoracic cranial vena cava which reflects right atrial pressure. The pressure in the right atrium is indicative of the heart's ability to handle the volume of fluid presented to it. Since this ability is dependant on the interaction of cardiac pumping ability, circulating blood volume, vascular tone, and intrathoracic pressure, CVP is considered a measure of the success of the interaction rather than a specific measurement of any individual parameter. Most commonly CVP is monitored for early signs of fluid overload in patients who require blood volume replacement; particularly in patients with heart failure, rapid and/or large changes in blood volume, acute circulatory collapse or impaired renal function.

      Method of measurement

CVP measurement is made using a large bore catheter placed into the anterior vena cava via the jugular vein. A simple water manometer is positioned so that the "zero" mark is level with the right atrium. Fluid in the manometer is allowed to enter the catheter until it has equilibrated with the pressure in the vena cava. This equilibrium point, measured in cm of water, is the CVP. Alternatively, CVP can be constantly monitored and displayed, along with a CVP wave form using a transducer connected to a pressure capable oscilloscope. Note: transducers measure CVP in mm Hg rather that cm H2O. The correlation is: 1 mm Hg = 1.36 cm H2O.

Normal CVP is considered 0 cm to 5 cm H20, 5 cm to 10 cm is considered borderline and greater than 10 cm is a marked increase. Because of the many variables that affect CVP measurement, single readings are not clinically useful; however, if patient positioning and zero point remain constant, changes in CVP of greater than 5 cm H20 are considered significant. While a drop in CVP usually indicates inadequate blood volume replacement or venodilation, a rise in CVP can be influenced by a large number of factors. If mechanical/technical problems can be ruled out, a rise may indicate fluid overload, right heart failure, or increased intrathoracic pressure.