Jamie M. Burkitt, DVM, DACVECC
Patients with tissue hypoxia due to cardiovascular compromise are at risk for development of organ dysfunction and death. There is strong evidence in the literature that early identification of and appropriate treatment for cardiovascular compromise in humans significantly reduces cytokine production,1 morbidity, and mortality.2 Therefore, it is important to recognize rapidly cardiovascular instability, whether at initial evaluation or during hospitalization. The following proceedings discuss traditional, newer clinicopathologic, and possible future monitoring tools to aid the recognition of cardiovascular instability in dogs and cats.
Traditional Assessments of Cardiovascular Status
Serial physical examination, performed as often as patient condition dictates, is the cornerstone of traditional cardiovascular monitoring. The six parameters of tissue perfusion--mental status, mucous membrane color, capillary refill time, heart rate, pulse quality, and extremity temperature--are invaluable to recognize changes in patient status. However, it is well known that physical examination alone doesn't always accurately reflect global tissue perfusion in humans, which may contribute to under-resuscitation of patients with occult hypoperfusion. Recent evidence suggests that traditional methods alone (heart rate and blood pressure) may under-represent global tissue oxygen debt in dogs.3 Therefore, additional monitoring is required in animals that are, or at risk for becoming, cardiovascularly unstable.
Direct Systemic Arterial Blood Pressure Monitoring
Systemic arterial blood pressure is most commonly measured in dogs and cats using indirect methods. The Doppler blood pressure method yields only a systolic blood pressure, while oscillometry (Cardell,® Dinamap®) reports mean and diastolic blood pressure readings as well as systolic. Direct measurement of systemic blood pressure from an artery is the gold standard.
Direct arterial pressure monitoring has the advantage of being accurate, and, if desired, continuous. It yields systolic, diastolic, and mean pressures. Recent evidence suggests that indirect methods may underestimate blood pressure in hypotensive dogs.4 Therefore, direct arterial monitoring may be superior in that patient population. Direct monitoring requires arterial catheter placement and connection of the catheter via fluid-filled pressure tubing to a transducer, which transduces the arterial pressure wave into an electrical signal. The signal is sent to a digital display or printed as a strip chart; most display monitors allow for single or dual-channel printing for capture of waveforms for detailed analysis. Full description of components and techniques for assembling and assessing a direct pressure monitoring system is available elsewhere.5 Continuous arterial pressure monitoring is useful in unstable patients, whose status may change rapidly, and the technique is relatively simple to perform in recumbent animals.
Direct arterial pressure waveforms can also be helpful in diagnosing some cardiovascular abnormalities at the bedside. For instance, changes in intrathoracic pressure associated with spontaneous breathing can cause variation in the arterial pressure waveform. This respiratory variation is called pulsus paradoxus, and is seen most commonly in dogs and cats with hypovolemia or cardiac tamponade. Figure 1 is an example of respiratory variation in an arterial pressure waveform in a dog, recorded a single-channel recorder.
Click on the image to see a larger view.
Pulsus paradoxus (respiratory variation in the arterial pressure waveform) in a dog with hypovolemia.
Central Venous Pressure Monitoring
Central venous pressure (CVP) is the pressure in the thoracic vena cava; it approximates right atrial pressure, and is thus used clinically to estimate adequacy of blood volume. Normal CVP is approximately 0-10 cmH20. The CVP waveform is generated by changes in vena caval pressure, which is determined by blood volume, venomotor tone, intrathoracic pressure, and right atrial function. Therefore, though CVP is used to estimate right heart preload, its many determinants make it a very rough estimate, indeed. Trends within an individual are likely more useful than absolute numbers. Central venous pressure waveforms can be used to diagnose specific cardiac abnormalities and to estimate preload continuously. Central venous pressure is most commonly measured via an indwelling jugular catheter, with the catheter tip sitting in the cranial vena cava. The monitoring system is identical to that used for direct arterial pressure monitoring (see above). Figure 2 is a normal CVP waveform in a dog, recorded beneath the its simultaneous ECG on a dual-channel recorder:
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Normal CVP waveform recorded from a dog beneath its simultaneous ECG.
Specific changes in the CVP waveform are indicative of certain cardiac and pericardial diseases.6
Echocardiography can be used to measure chamber size, and Doppler echocardiography has been used to calculate stroke volume and cardiac output. However, one recent investigation found poor agreement between cardiac output calculated using transthoracic Doppler echocardiography and cardiac output calculated using the gold standard thermodilution technique in anesthetized dogs at baseline and after hemorrhage.7 Therefore, clinicians probably should not rely on transthoracic Doppler echocardiography to determine cardiac output until more information is available.
Cardiac Output Monitoring
Tissue perfusion, and therefore tissue oxygenation, may suffer when cardiac output is inadequate. Cardiac output can be calculated using many methods. Thermodilution performed with an indwelling pulmonary arterial catheter is the gold standard, though the method is personnel intensive and requires a cardiac output computer. The technique is feasible in critically ill canine patients.8 Other less invasive methods for cardiac output calculation include the lithium dilution method,9 arterial pulse contour analysis, and ultrasound dilution analysis. Cardiac output monitoring may become standard of care for critically ill veterinary patients in the next decade if these less invasive methods are validated in more species.
Urine Output Measurement
Urine output is used as a marker of visceral tissue perfusion. Urine output for a dog or cat with adequate effective circulating volume on maintenance intravenous fluids should be at least 1 mL urine / kg body weight / hour. An indwelling urinary catheter can be placed and connected to a sterile, closed collection system in the animal at risk for or experiencing cardiovascular instability. Urine output should be measured frequently (every 30-120 minutes) as a "downstream" estimate of visceral tissue perfusion during resuscitation efforts. Because urine production confirms renal perfusion, it may be a more useful monitoring parameter than macrovascular "upstream" parameters like systemic arterial blood pressure.
Clinicopathologic Indicators of Cardiovascular Status
Serum Lactate Concentration
Serum lactate concentration is < 2 mmol/L in normal, healthy dogs and cats. The most common cause for elevated serum lactate concentration is decreased aerobic metabolism associated with inadequate oxygen delivery to tissues, though other (much less common) etiologies exist. When glucose enters the glycolytic pathway, the end product is pyruvate; if the mitochondria are oxygen depleted, the enzymes of the TCA cycle and oxidative phosphorylation will saturate, and pyruvate will be unable to enter the cycle as it normally would. In these anaerobic conditions, pyruvate is converted to lactate. The lactate anion is released from the cell along with a glycolysis-generated proton, creating the lactic acidosis classic for inadequate tissue oxygen delivery. Serum lactate concentration is therefore another valuable "downstream" estimate of tissue perfusion; if the blood pressure seems adequate but the lactate remains elevated, tissue perfusion is most likely still compromised. Serial lactate measurements are helpful to monitor adequacy of tissue oxygenation after initial resuscitation. Other causes of hyperlactatemia do exist (Type B Hyperlactatemia) and should be considered in patients with persistent hyperlactatemia and no other signs of hypoperfusion. Lactated Ringer's solution (a perfectly acceptable fluid for resuscitation in patients with lactic acidosis) contains 28mmol lactate/L, which WILL interfere with serum lactate concentration if the sample is contaminated (i.e., bolus of LRS via a cephalic catheter while a blood sample is taken from a jugular vein). Excessive struggling, slow blood draws, and prolonged venous occlusion will all cause increased lactate concentration unrelated to systemic tissue perfusion. If lactate measurement is unavailable, serial evaluation of pH and base excess can be valuable in its place, as long as no other major cause for acidosis is present (e.g., renal failure, diabetic ketoacidosis).
Venous Oxygen Tension (PvO2)
Venous oxygen tension is similar to serum lactate concentration in its utility and implication--it is a "downstream" measurement of tissue oxygenation adequacy. Normal jugular PvO2 is 45-65mmHg in dogs; the feline range is likely similar. Decreased PvO2 occurs when tissue oxygen extraction increases due to decreased tissue oxygen delivery. When less oxygen is delivered to tissues per unit time, more oxygen is extracted from the plasma, and the venous value drops. Similar to lactate, PvO2 is most valuable when collected from a free-flowing central vein like the jugular or vena cava, to give a global assessment of tissue oxygenation (rather than an indication of pelvic limb paw oxygenation with a saphenous vein sample, for example).
The following methods of cardiovascular monitoring represent technology that may become important in veterinary medicine in the next ten to fifteen years. None is in routine use in veterinary patients at this time. Serum lactate/pyruvate ratios, NADH:NAD+ monitoring, and tonometry all aim to assess tissue perfusion by evaluating end-products of cellular metabolism, while orthogonal polarizing spectral imaging shows functional capillary density on mucosal surfaces in an effort to display "upstream" microvascular function (as opposed to "upstream" macrovascular parameters like systemic blood pressure). Note that these new modalities all aim to see what has, as of yet, been "invisible" to the clinician: what's actually occurring at the cellular level of the tissues.
The ratio of concentrations of serum lactate and pyruvate (L:P) yields more information than serum lactate concentration alone. Elevations in L:P suggest tissue hypoxia whereas normal L:P suggests adequate tissue oxygenation and mitochondrial function, even when serum lactate concentration is elevated. Normal L:P with hyperlactatemia is usually due to Type B Hyperlactatemia--elevated serum lactate concentration for reasons other than tissue hypoxia. Measurement of serum pyruvate concentration is expensive, and is not widely available for veterinary species. If this measurement becomes more widely available, clinicians can use L:P ratios to rule-out Type B Hyperlactatemia (hyperlactatemia in the face of aerobic metabolism) in patients with unexplained, persistently elevated serum lactate concentration.
The ratio of tissue NADH to NAD+ reflects the redox potential of the cell. Normally, NADH produced by the TCA cycle is converted to NAD+ during oxidative phosphorylation. When mitochondria are oxygen depleted, NADH accumulates and NAD+ is not produced; the NADH:NAD+ thus increases. There are devices that measure the NADH:NAD+ from a mucosal surface (such as the rectal mucosa) currently in the experimental phase. Introduction of such a rectal NADH:NAD+ probe into the clinical community could provide continuous, real-time monitoring of splanchnic perfusion.
Tonometry and Tissue Capnometry
Tonometry techniques use the equilibration of gases between a mucosal surface and a gas-filled balloon to estimate tissue perfusion. Poor tissue oxygenation leads to tissue acidosis, which in turn produces CO2 via carbonic anhydrase. The CO2 equilibrates into the balloon, at the tip of a tonometer lying against a mucosal surface (e.g., gastric). More recently, sublingual capnometers have been used to measure tissue PCO2 directly. High tissue PCO2 is consistent with poor perfusion.
Orthogonal Polarizing Spectral Imaging
Orthogonal polarizing spectral imaging (Cytoscan®) is a technology that allows visualization of microvasculature. Sepsis has been associated with loss of functional capillary density due to microangiopathy and deranged arteriolar function. Orthogonal polarizing spectral imaging allows evaluation of functional capillary density. This technology is being used primarily experimentally at this time. Ability to monitor for microvascular dysfunction will become important once we develop treatments for microvascular dysfunction in the future.
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