Basics of monitoring equipment

Abstract

Monitoring equipment has become reliable and affordable for use in general veterinary practice. This article provides a guide to technology, troubleshooting, and obtaining quality data using 4 non-invasive techniques that are commonly used in practice. Pulse oximetry estimates oxygen saturation of hemoglobin in arterial blood, capnography measures the carbon dioxide content of inspired and expired gas, and either Doppler shift detection or oscillometry can be used to measure blood pressure. These useful non-invasive techniques all rely on adequate perfusion of the tissues for optimum function.

Introduction

Although a vigilant staff member can ensure a suitable depth of anesthesia in a patient using a “hands-on” approach, monitoring equipment can provide objective data to aid decision-making and anesthetic management. Four non-invasive techniques will be discussed and readers will be given a guide to the technology that is involved, troubleshooting, and obtaining quality data with these techniques. For all the methods discussed, adequate tissue perfusion is required to obtain reliable measurements. If the monitor cannot provide an accurate measurement, or it malfunctions or displays an error message, it may indicate tissue perfusion is compromised, and the animal’s physical condition should be checked by the attendant before dismissing the monitor as problematic. If more than one monitor simultaneously malfunctions, a potential cardiopulmonary collapse may be imminent or has occurred.

Pulse oximetry

Pulse oximetry continuously estimates the average saturation of hemoglobin (Hb) with oxygen (SpO₂) in pulsatile arterial blood (1). The pulse oximeter does not display the SpO₂ of venous blood because the veins do not have pulsatile flow. The ability of oxygen molecules to bind to Hb is related to the partial pressure of oxygen dissolved in plasma (pO₂) and this relationship is described by the oxygen Hb dissociation curve (Figure 1). The pO₂ of arterial blood (PaO₂) is dependent on the concentration of inspired oxygen, adequate lung ventilation and perfusion, and the ability of oxygen to diffuse across the alveolar/capillary junction. The PaO₂ in a healthy conscious mammal breathing air (21% oxygen) is normally in the range of 85 to 110 mmHg (11.3 to 14.7 kPa) and corresponds to an oxygen saturation value of 98% to 100% on the Hb dissociation curve and pulse oximeter (Figure 1). Hypoxemia is present when there is a PaO₂ < 60 mmHg (8 kPa) and this corresponds to an SpO₂ value of < 90% on the pulse oximeter (Figure 1). Normally, venous blood has a PvO₂ of 40 mmHg (5.3 kPa) and a corresponding oxygen saturation of 75% (Figure 1). There is some species variation with shape of the curve, and the curve can slightly shift to the left or right depending on whether the blood is passing through tissue beds (warmer, lower pH) or is contained within the pulmonary circulation (cooler, higher pH). Most pulse oximeters are programmed with the oxygen Hb dissociation curve for humans, and therefore may not be accurate in species with different curve characteristics, for example with avian blood.

Figure 1.

The oxygen hemoglobin dissociation curve (white line) illustrating the relationship of oxygen partial pressure in the plasma and oxygen saturation of hemoglobin. The saturation of hemoglobin is indicated by black lines for normal arterial blood (21% inspired oxygen; PaO₂ 100 mmHg), mixed venous blood (PvO₂ 40 mmHg) and the hemoglobin saturation when arterial blood is considered to be hypoxemic (PaO₂ 60 mmHg).

A healthy mammal should normally have a PaO₂ of approximately 5 times the inspired oxygen concentration (in percent) (2–4). For example, an animal breathing 100% O₂ should have a PaO₂ of approximately 500 mmHg (66.7 kPa). PaO₂ values greater than 110 mmHg (14.7 kPa) cannot produce an oxygen saturation value higher than 100%. The only method to measure pO₂ in an arterial or venous blood sample is to use a wet-chemistry analyzer or point-of-care dry-chemistry cartridges. It is worth noting that if SpO₂ is 98% to 100%, the PaO₂ could lie anywhere between 100 mmHg (13.3 kPa) and approximately 500 mmHg (66.7 kPa) depending on efficiency of oxygen uptake within the lungs. If the animal is breathing 100% oxygen and the PaO₂ is only 100 mmHg (13.3 kPa), this would indicate a severe reduction in oxygen uptake. If the problem is not resolved and the animal then breathes air, there will be a decrease in PaO₂ to hypoxemic levels. Therefore in some cases, knowing the value of PaO₂ can be important for case management and a blood gas analyzer is then used alongside a pulse oximeter.

The pulse oximeter has 2 light-emitting diodes (LEDs) which emit red (660 nm) and infrared (range: 905 to 950 nm) light and a photosensor positioned opposite the LED (transmittance probe). As the light passes through tissues, a certain amount is absorbed and less light is detected. The amount of light absorbed depends on the type of Hb and the wavelength of light. The LED cycles between switching each of the diodes on and off, and both are switched off many times a second in order to compare the light absorption at different periods of the cardiac cycle, and between the 2 different light wavelengths. De-oxyhemoglobin will absorb a different amount of each wavelength compared to oxyhemoglobin. The software algorithms calculate the percentage oxygen saturation of pulsatile arterial Hb by removing the background absorption from the tissue bed and non-pulsatile venous blood. The device takes about 5 to 10 s (several cardiac cycles) to search for a useful signal before it will provide a measurement and signal quality (strength) can be displayed using light bar indicators or by displaying the waveform (plethysmograph) (Figures 2, 3). Otherwise, the monitor may display error messages such as “artifact” or “poor signal.” The saturation measured is linked to an audible tone which decreases in frequency as the saturation falls. This audible signal can provide an alert for those not able to observe the visual signal.

Figure 2.

Photograph illustrating a pulse oximeter transmittance probe and the light-emitting diodes (left) and placement on the paw of a cat (right). The right lower photograph shows a reflectance probe.

Figure 3.

A pulse oximeter (Masimo, Irvine, California, USA) displaying the plethysmograph, oxygen hemoglobin saturation, pulse rate, and perfusion index (PI). The PI in the lower illustration indicates abnormally low PI and therefore questionable saturation. The plethysmograph in the lower illustration also does not have a defined waveform.

The ear lobe transmission probes used for humans are useful for animals, whereas finger transmission probes can be more difficult to use (5). Ear lobe probes can be placed where well-perfused vascular tissue with a similar thickness to a human ear lobe can be sandwiched between the LED and photosensor. For anesthetized small animals the most commonly used sites include the tongue, lips, paws, vulva, and prepuce. This tissue thickness may be found in the center of a cat’s tongue or the lateral edge of a large dog’s tongue. With transmission probes, ensure the LED and photosensor are positioned directly opposite each other so there is no distortion of the signal. Ensure the probe is not slipping off the tissue (Figure 2) because the photosensor in older first generation pulse oximeters can detect ambient light and produce erroneous measurements (approximately 85%) (1,5). Place the photosensor on the upper side of the tongue and the LED on the lower side to prevent the receiver detecting overhead artificial lighting. Placing wet gauze swabs between probe and tongue may improve signal quality by enhancing tissue perfusion, preserving correct alignment of the LED/photosensor or by shielding the photosensor from ambient light (6). Reflectance probes (Figure 2) can be placed on the hard palate, underside of the tail, within the rectum or esophagus or where a Doppler probe is often positioned on the front or back leg. The LED and photosensor are on the same plane and the LED sends light through vascular tissue and is reflected from a bony background to the photosensor.

Signal quality and strength can be shown by bar-indicators or by displaying the waveform. The ability of the software to isolate the signal of interest (arterial blood) from background tissue absorption of light depends on quality of arterial blood flow. Inability to detect a good signal quality can indicate the presence of severe hypotension and/or hypovolemia, or peripheral vasoconstriction which could arise from use of alpha₂ agonist drugs or from hypothermia (7). Bradycardia, tachycardia, and cardiac arrhythmias can also interfere with signal quality. Anemia (packed cell volume < 10%) causes under-estimation of the true Hb saturation by an average of 5.4% in dogs, but may alert the clinician earlier to problems of oxygenation and oxygen delivery (8). The ability to detect a strong signal can also depend on the model and species in which each model is used, and may be due to the site of probe placement (9). Older pulse oximeters tend to become more inaccurate at Hb oxygen saturation < 70% (7). Second and third generation pulse oximeters compensate for motion artifact and perfusion states, respectively, but these may not be any better than first generation pulse oximeters in dogs (10). Dark pigmented areas also appear to interfere with signal quality in animals (5). The pulse oximeter cannot be used during cardiopulmonary resuscitation (CPR) when there is poor pulsatile flow even with good CPR technique.

The pulse oximeter cannot differentiate abnormal types of Hb [carboxyhemoglobin and methemoglobin (MetHb)] from normal de-oxyhemoglobin and oxyhemoglobin (11). Carbon monoxide poisoning forms carboxyhemoglobin and this can interfere with the ability of conventional pulse oximeters to accurately measure oxyhemoglobin saturation. The oxidation of ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) results in MetHb and Fe³⁺ is incapable of carrying oxygen: it is normally present in concentrations of less than 1% in arterial blood. MetHb also shifts the O₂ dissociation curve of the Fe²⁺ form of Hb to the left (increasing O₂ affinity for Hb, decreasing O₂ release) and further affecting O₂ delivery to tissues. High concentrations of MetHb give blood a bluish or chocolate brown color. Pulse oximeters are available which use multiple wavelengths of light; these can detect these different types of Hb and have many other useful functions (http://www.masimo.com/Rainbow/about.htm). This model may be useful for monitoring animals with smoke inhalation (carboxyhemoglobin) or animals which have received an overdose of drugs (acetaminophen, prilocaine, benzocaine, sulfonamides, metoclopramide) or ingested toxins (onions, garlic, mothballs) which can promote formation of MetHb, especially in cats. It can also assess the signal strength and quality and relate it to the amount of tissue perfusion; this is called the perfusion index (PI) by the manufacturer (Figure 3) and has been found to be higher in dogs sedated with vasodilatory acepromazine compared to dexmedetomidine which produces peripheral vasoconstriction (12). An increase in PI has been used to confirm successful blockade of the sciatic nerve by placing the probe on the lower extremity of the blocked hindlimb in dogs (13). The unit can also provide the plethysmograph variability index (PVI) which measures the variability within the pulse oximeter waveform during the inspiratory and expiratory phases of mechanical ventilation. Positive pressure lung ventilation can decrease venous return (and therefore cardiac output) and this effect can be severe in individuals with inadequate circulating volume. A high degree of variability indicates fluid therapy may be warranted to increase venous return and therefore cardiac output. Currently, the PVI is under investigation in anesthetized, mechanically ventilated, and volume-depleted dogs with promising results (14). This device can also estimate hemoglobin concentration and has been found to be useful in humans, but is not as reliable in anesthetized dogs (12).

The color of mucous membranes can be used to assess oxygenation by an observer, but the ability to recognize the blue color of de-oxygenated blood depends on the person’s ability to see and appreciate the color change, and on ambient lighting conditions. Furthermore, once the mucous membranes have an observable blue coloration, the animal is already severely hypoxemic. In general practice, pulse oximetry is useful to guide oxygen therapy and to monitor oxygenation during anesthesia. Hypoxemia is most likely to occur during heavy sedation, anesthetic induction, and during the recovery period when animals are often breathing an inspired oxygen concentration of 21% (air) instead of a higher inspired O₂ concentration (4,15). The pulse oximeter is useful for animals in respiratory distress and indicates when O₂ therapy is urgently required.

Capnography

Diverting or sidestream capnography continuously samples gases for carbon dioxide (CO₂) concentration in respiratory gases at the point where the endotracheal tube is attached to the breathing system during all phases of the respiratory cycle (4,16,17). A sampling port and tubing are used to aspirate the gas (50 to 200 mL/min) to the sampling cell housed within a monitor positioned away from the animal. The device uses infrared light absorption technology to measure the amount of CO₂ present in the sampling cell. The sampling line for sidestream capnography may be available in different lengths and long lines can delay the response time with a low sampling flow rate. Longer lines are often used where the monitor has to be placed far away from the patient. Most units have high sampling flow rates which cause inaccurate end-tidal CO_{2 (}ETCO₂) values with very small patients and high respiratory rates, and only a few units can allow the sampling flow rate to be lowered. Microstream devices using laser technology are now available which use sampling flow rates of 50 mL/min (http://www.medtronic.com/covidien/products/capnography). Water vapor can interfere with the measurement of CO₂ concentration and filters or water traps are used with sidestream devices (16). Sidestream capnographs should be connected to a waste gas scavenging system because the sampled gases also contain anesthetic agents, or should return the gases to the breathing system. Calibration gases may be required for some sidestream devices and others require calibration using ambient air before use (http://www.midmarkanimalhealth.com/products/monitoring). The infra-red analyzer may also be placed directly within the breathing system (non-diverting or mainstream capnography) (16). Mainstream devices use heat to prevent water vapor from forming, but these can still be prone to malfunction because of water condensation (16,17). These devices are bulky and can cause disconnections or place excessive traction on an insecure endotracheal tube. Mainstream units have a rapid response time, require no or few consumables, and do not remove gases from the breathing system. One mainstream capnograph was found to be slightly more accurate in measuring PaCO₂, compared to a sidestream capnograph in mechanically ventilated dogs, although both units were clinically useful (17). Measurements can be displayed graphically or by lighting diode bars (Figure 4). Nitrous oxide can cause overestimation of the CO₂ concentration and newer models automatically correct for the presence of this gas in the breathing system (18). On some older models, this correction needs to be done manually.

Figure 4.

A handheld sidestream capnograph and pulse oximeter device (http://www.viamed.co.uk).

Inspired gas should be free of CO₂ and expired gases should have a high CO₂ concentration (4). The ETCO₂ is the point at which the last portion of alveolar gas is exhaled and just before the animal begins inspiration (Figure 5). The measurement of ETCO₂ represents the arterial partial pressure of CO₂ (PaCO₂). The ETCO₂ may be lower than PaCO₂ by 1 to 3 mmHg (0.1 to 0.4 kPa) in healthy anesthetized dogs (16,19) and cats. Capnographs can display the pCO₂ (mmHg or kPa) or can be programmed to display the percentage by volume of CO₂ compared to the entire gas sample. To derive the pCO₂ from percent values, divide the percentage by 100 and multiply the result by ambient atmospheric pressure. For example, 5% CO₂ is equivalent to 38 mmHg (5.1 kPa) at sea level.

Figure 5.

Diagram illustrating a normal capnogram and carbon dioxide level during inspiration and expiration. ET — the point at which the capnogram measures the End-Tidal carbon dioxide level.

Normal PaCO₂ ranges from 35 to 45 mmHg (4.7 to 6.0 kPa) and the respiratory center in the pons/medulla mainly controls lung ventilation by detecting changes in cellular pH related to PaCO₂ (16). This area of the brain is depressed during anesthesia and hypoventilation will produce a higher than normal PaCO₂ (Figure 6). Hypercapnia [> 50 mmHg (7.3 kPa)] should be treated with mechanical lung ventilation or by continual hand ventilation if reducing depth of anesthesia is not an option. If mechanical ventilation is routinely applied, it is important that blood pressure is also assessed because mechanical ventilation can reduce venous return to the heart, especially in animals with circulating volume deficits (20). Some debilitated animals such as those with increased intracranial pressure may require strict control of lung ventilation and PaCO₂.

Figure 6.

Stylized capnograms indicating hypoventilation (6a), rebreathing of carbon dioxide and capnogram from using inadequate fresh gas flow and a Bain breathing system (6b), sudden loss of lung perfusion (6c), cardiogenic oscillations (normal artifact) and capnogram illustrating oscillations and hypercapnia (6d), spontaneous breathing in a cat (6e). These abnormalities usually develop over several respiratory cycles.

Severely low pulmonary perfusion can reduce delivery of CO₂ to the alveoli and result in a low ETCO₂ measurement, despite concurrent hypercapnia. A sudden drop in ETCO₂ of more than 15 mmHg (2 kPa) may indicate impending cardiac arrest because the cardiac output from the right side of the heart may have suddenly decreased lung perfusion. Conversely, during CPR, the capnograph can reveal the effectiveness of CPR attempts by indirectly assessing pulmonary perfusion and delivery of CO₂ to the alveoli. An ETCO₂ of > 10 mmHg (1.6 kPa) indicates good technique, but this value cannot compare with normal physiology (16,21).

Severe hypoventilation, apnea, or accidental endobronchial intubation can cause hypercapnia [ETCO₂ > 80 mmHg (10.7 kPa)], which can also result from breathing equipment failure. If the breathing system or an incompetent expiratory valve is not efficiently removing exhaled CO₂ to the scavenging system, the animal will rebreathe previously exhaled CO₂. An indication of breathing system failure to remove CO₂ will be observed as a high inspired CO₂ concentration (Figure 6). The high inspired CO₂ concentration will immediately direct the practitioner to fix the problem (low fresh gas flows, faulty one-way valves, or exhausted CO₂ absorber). If the sampling point is placed within an area of large apparatus dead space, the inspired CO₂ will also be higher compared to an arrangement with less apparatus dead space. Inspired levels of > 10 mmHg (1.3 kPa) require intervention to lower the inspired CO₂, whatever the cause (16).

The capnograph can also be used as an apnea alarm if expired CO₂ is not detected by the device for > 30 s. This may also indicate a disconnection of the sampling port from the breathing system, but beware of disconnections in which the sampling connector remains attached to the endotracheal tube. In this case, spontaneous breathing will still produce CO₂ and produce a display on the capnograph. However, a vigilant observer will also notice the breathing system reservoir bag is deflated and may smell anesthetic agent before the animal awakes from anesthesia.

When using a facemask applied to the animal, a catheter attached to the sidestream sampling line inserted into the facemask with the distal end by the nares can be used to measure CO₂. Watching the capnograph can also aid the accurate placement of an endotracheal tube for example, tracheal intubation of rabbits.

In awake animals receiving oxygen therapy using bilateral nasal lines, ETCO₂ can be measured using one of the nasal lines (22). Flow of oxygen from the contralateral nasal line will not interfere with accuracy, but the ETCO₂ measurement is not reliable if the dog is panting. This system is also prone to water condensation blocking the sampling line.

Measurement error can be introduced by placing the gas sampling point close to a rapid egress of fresh gas from the breathing system resulting in dilution of the sampled gas (16). This is common with use of non-rebreathing systems, especially the Bain system, in cats and small dogs. A more reliable measurement can be obtained by placing a well-sealed and long intravenous catheter through the sampling port so the distal end of the catheter resides in the region of the proximal portion of the endotracheal tube and away from the fresh gas outlet (Figure 7).

Figure 7.

Photograph illustrating the use of an intravenous catheter placed in the capnograph sampling port to ensure gas is sampled within the proximal part of the endotracheal tube and not close to exit of fresh gas from the breathing system.

A capnograph provides more information on lung ventilation, but observation of chest excursion and adequate breathing bag movement is still important. Signs of severe hypercapnia are tachycardia, deep red mucous membrane color, bounding pulse quality, and incisional oozing of blood. An increase in respiratory rate may be observed where there is breathing system failure. These signs may be more difficult to appreciate in cats or debilitated animals and the observer must determine the cause of hypercapnia. A capnograph is useful for rapid detection of severe hypercapnia and can clearly indicate the origin of hypercapnia such as from the animal (apnea, hypoventilation) or equipment failure (breathing system malfunction) (16).

Volumetric capnography is an extension of this technology and incorporates the measurement of expired volumes using a pneumotachograph, with simultaneous measurement of exhaled CO₂ concentration. This device can provide information on dead space within the lung which may be important in pulmonary disease and long-term ventilator management of critically ill cats and dogs in the future (23).

Non-invasive blood pressure measurement

The 2 main non-invasive techniques for measuring systemic blood pressure use either an ultrasonic probe or oscillometrics (24). Both rely on adequate perfusion of an extremity, usually a limb or the tail, where the cuff is placed. There have been many studies to test the ability of indirect techniques, and various models of oscillometer, to accurately reflect arterial blood pressure (24). Recommendations for accuracy and reliability have been developed to identify suitable techniques and oscillometers to be used in awake cats and dogs (25). The accuracy of indirect blood pressure measurement can depend on many factors and include: Doppler probe and cuff position (26), oscillometry cuff position (26–28), cuff width:limb or tail circumference ratio (29,30), cuff shape (31), body position (28,32), site of arterial catheter used for comparison (33,34), and method used for statistical analysis (35).

Averaging the measurements obtained from 3 to 5 separate readings can increase the reliability of the blood pressure measurement (28,36–41). Trends and serial measurements may be more important than obtaining 1 measurement, and other clinical findings also need to correlate with the data the device provides (42). Non-invasive blood pressure measuring devices can perform poorly in the presence of severe hypotension, and the clinician’s ability to assess cardiac output, peripheral pulse quality, and perfusion using a thorough physical examination still remains important (43). Loss of palpable metatarsal pulses has an 85% chance of detection of severe hypotension in cats, but these can still be present even if systolic pressure is < 90 mmHg (44). The Doppler technique may be more useful in detecting hypotension compared to an oscillometer (43,45,46).

The cuff requires good contact with the limb and excess or matted hair can interfere with obtaining a good quality signal. Placing the cuff over bony structures such as directly over the carpus does not produce a reliable signal for oscillometry. For every 10 cm the cuff is positioned above the heart, the pressure measurement obtained should be increased by 7.36 mmHg, and for every 10 cm the cuff is below the heart, the measurement should be reduced by 7.36 mmHg. Accuracy of the measurement depends on correct cuff width:limb circumference. If the cuff is too wide, it will require less cuff pressure to occlude arterial perfusion because there is more cuff surface area pressing on the limb and underestimation of true blood pressure can occur. If the cuff is too narrow, a higher cuff pressure is required to occlude perfusion and over-estimation will occur (24).

The Doppler technique uses a probe with a piezoelectric crystal ultrasound wave transmitter and receiver (Figure 8) (24). When the concave portion of the probe is correctly positioned over a peripheral artery (Figure 8) the ultrasound signal is reflected from red blood cells moving through the lumen of the artery. The reflected wavelength changes according to the different velocities of the cells; this is called a Doppler shift. The different reflected wavelengths are converted to audible frequencies and the characteristic “whooshing” sound. If anemia or peripheral vasoconstriction is present, the quality of the signal can be poor. Deterioration in sound quality may be clinically useful in identifying hypotension (43).

Figure 8.

Placement of a Doppler probe (left) and cuff on the forelimb of a dog (right).

Ensure there is no air present between the probe and skin with a generous amount of ultrasound contact gel or the ultrasound signal will not be conducted to the body. Without moving the probe too much to prevent the loss of gel contact, the probe should be firmly attached to the body with sticky tape or flexible tape (Vetrap bandaging tape; 3M Animal Care Products, St. Paul, Minnesota, USA). Gels for lubricating endotracheal tubes should not be used as they can damage the probe.

Recommendations for the cuff width for the Doppler technique have been reported to be 36% (33), 40% (40), and up to 55% (41) of the limb circumference, placed proximal to the probe and level with the heart. The cuff is attached to an aneroid manometer and the cuff inflated to occlude the underlying artery causing Doppler unit sounds to disappear (Figure 8). By slowly deflating the cuff, the pressure at which the first sounds can be heard to return is assumed to represent the systolic arterial pressure (SAP). There is continual debate as to whether this returning sound more accurately represents the mean arterial pressure (MAP) in cats (47), Doppler pressure + 14 mmHg is SAP (33), or is the SAP (48). Assuming the value is SAP provides a safety factor and prompts corrective treatment for hypotension earlier, compared to using the value to represent MAP. Using a change in Doppler sound to detect diastolic arterial pressure (DAP) is not accurate (47–49).

A Doppler probe can have other uses besides providing a value for SAP and heart rate. It provides a continuous audible indication of blood flow and a functional heartbeat. The probe can be placed on the eye during CPR, and detection of moving red cells through the retinal artery can indicate good technique. The Doppler unit can also be used for small or young dogs and cats, and exotic animals by placing the probe directly over the heart to provide an audible indication of blood flow through the heart chambers (Figure 9).

Figure 9.

Doppler probe used to monitor blood flow through the heart of anesthetized fish.

A pulse oximeter probe can be used in place of a Doppler probe (50). The cuff is inflated and a measurement can be obtained just as the waveform display is lost. As the cuff pressure is reduced, the waveform is displayed once more and the pressure measured at this point. The 2 measurements can be averaged to obtain the systolic blood pressure. Accuracy is similar to the Doppler but this instrument can be more difficult to use.

Oscillometric devices available nowadays are more accurate compared to units available 15 to 20 y ago and measure SAP, MAP, DAP, and heart rate (24). A cuff width of 40% (29) or between 40% and 60% (51) of the circumference of a limb or the tail has been recommended with oscillometry. The device automatically inflates the cuff to a pressure higher than SAP and gradually deflates the cuff in steps (Figure 10) (24). Arterial blood flow will produce a slight increase in volume of the limb and increase the cuff pressure with each pulsatile flow. When the device detects these small oscillations in cuff pressure it will display the average pressure as equivalent to SAP. As the cuff pressure decreases further, the pressure oscillations become larger until they peak in difference, and this point is assumed to be equivalent to MAP. The oscillations diminish as the cuff pressure is decreased further until the oscillations are not detected. At this point, the cuff pressure is taken as the equivalent to DAP (Figure 10). Because the cuff pressure changes from the pulsatile blood flow are small, the quality of the signal can be affected by movement or handling of the cuff while the instrument is taking a measurement.

Figure 10.

Diagram of cuff pressure oscillations (in black) with automatic deflation of the cuff (cuff pressure in gray) during 1 cycle of an oscillometer. The point at which systolic, mean and diastolic systemic arterial pressures are measured, is indicated.

High definition oscillometry uses advanced software algorithms to increase accuracy (http://www.vethdo.com/) (34,42). The device currently available can produce reasonable results in cats and small dogs, and signal quality can be displayed on a laptop, but this is not essential (Figure 11). The observer can then decide whether to accept or reject the measurements obtained by examining the graphic display. This can be useful in conscious animals in which movement and signal quality may be a problem. Comparison of the oscillometer with direct peripheral arterial pressure revealed this device was reasonably accurate, better used for trends, and may not be relied upon in anesthetized dogs with severe hemodynamic instability (42). Another device (petMAP + II) also displays signal quality using proprietary software and is designed for use in cats and dogs with reasonable results (http://www.petmap.com/). These devices can also combine blood pressure measurement with pulse oximetry (Figure 12). This device was found to more accurately represent direct arterial pressure compared to using Doppler based measurements in anesthetized and conscious dogs (38). Another device (Sun Tech Vet 20) designed for dogs and cats is also available, but at the time of writing, it had not been validated in cats and dogs although it appears to give reasonable results in clinical practice (https://www.suntechmed.com/bp-products/veterinarian-blood-pressure-monitor) (Figure 12). Graphically displaying decreasing cuff pressure without analysis of the oscillation pressure changes does not provide signal quality.

Figure 11.

A commercial high definition oscillometer (MemoDiagnostics: http://www.vethdo.com/) and the manufacturer’s software signal quality display on computer (top left). Display top right indicates there is movement artifact and the results are not acceptable. Display (bottom) illustrates good signal quality and the results are acceptable.

Figure 12.

Oscillometers from Suntech (https://www.suntechmed.com) and the petMAP (http://www.petmap.com) with pulse oximetry.

The oscillometer can have difficulty finding the signal in low perfusion states, in the presence of cardiac arrhythmias and may have problems during hypertension. The device will fully deflate the cuff and display an error signal or a message if the search for the oscillations takes too long. The automatic deflation is to prevent the device depriving the distal extremity of vital blood flow. With hypertensive states, the device may continue to reset the peak cuff inflation pressure to higher levels in an attempt to find the oscillating pressure changes and may reach the safety cut-off time-point and stop the search completely. Equipment design safety features also make it impossible to accidentally connect an oscillometer cuff inflation tubing to a fluid therapy line and have the device pump air into the venous circulation.

In conclusion, these devices are useful adjuncts to monitoring of anesthesia. When considering the various units that are available, it is advisable to negotiate a trial period and/or demonstration of the device. There are also multimodal devices available which combine these several modalities into one physiological monitor. This has the advantage of using less space, but the disadvantage of losing all the modalities if repairs are required. CVJ