Cardiopulmonary monitors: what to use and what does it mean? (Proceedings)


Monitoring the patient adequately is the basis of safe clinical anesthesia.

Objectives of the presentation:

  • To describe the function of commonly available monitors.

  • To describe the data obtained, normal values, and significance of values outside normal range.

  • To discuss examples of anesthetized patients to illustrate evaluation of cardiopulmonary status.

  • To indicate appropriate treatment for different abnormalities.

  • To emphasis the importance of monitoring from induction to the recovery period.

Monitoring the patient adequately is the basis of safe clinical anesthesia. No one method is completely adequate. Common recommendations are to monitor several systems and to monitor several parameters of each system and make frequent comparisons of serial recordings.

Central Nervous System (CNS)

Eyeball movements can be a useful guide to depth of anesthesia with thiopental, propofol, and/or inhalation anesthesia. In light anesthesia the eyeball is centrally placed in the orbit. As anesthetic depth increases, the muscles surrounding the eyeball relax at different rates and the eye rotates exposing the sclera or white of the eye. In deep anesthesia all the muscles become relaxed and the eyeball returns to the central position. Most surgical procedures can be performed when the eyeball is rotated in the orbit. If the eyeball is rotated and you can see white of the eye, then severe CNS depression is not present.

Ketamine maintains muscle tone, thus the eyeball is usually centrally placed in the orbit. With the addition of Domitor, or as the blood levels of ketamine decrease, the eyeball will rotate.

Measurement of alveolar anesthetic gas concentration using an anesthetic gas analyzer. This monitor is expensive and at present is only available for use in large practices or universities. The monitor is able to fairly accurately determine the concentration of inhalation agent in the patient thus providing information on depth of anesthesia. I shall use this information in case discussions to more accurately explain patient assessment and treatment.

An anesthetic gas analyzer measures inspired and expired anesthetic gas concentrations by aspirating gas from a T-port interposed between the endotracheal tube and the delivery circuit. The inspired concentration may be the same as the concentration leaving the vaporizer or it may be less, especially in a circle circuit with low oxygen flow and a large dog. Typically the inspired concentration is about 0.2% higher than the expired concentration. End-expiration (end-tidal) concentration approximates alveolar and brain concentrations. Depth of inhalation anesthesia can be regulated by adjusting the vaporizer to maintain patient's end-tidal % at 1.2-1.5 xMAC.

Less than MAC will be adequate for maintenance of anesthesia when other agents are administered that provide moderate to severe sedation or good analgesia, for example, premedication with large doses of Domitor or Telazol, or continuous infusions of morphine or fentanyl, or nerve blocks of the surgical site with bupivacaine. The definition of MAC is the Minimum Alveolar Concentration (%, percent) of the anesthetic that will prevent response to a standard painful stimulus (e.g. tail clamp) in 50% of patients. MAC is a different value for each anesthetic agent. MAC remains constant from animal to animal within a species but may vary between species (cats always requires more than dogs) (MAC does decrease in old patients). MAC is unaffected by long anesthesia.

MAC values

Cardiovascular System

Heart rate can be monitored from an ECG, from a stethoscope, esophageal stethoscope, pulse oximeter, or blood pressure monitor. Simultaneous comparison of heart rate (ECG or stethoscope) with pulse rate (palpation or blood pressure monitor) allows the anesthetist to pick up some dysrhythmias. During maintenance of anesthesia the heart rate should not decrease below 55 beats/min in the dog, 80/min in the cat, except for animals premedicated with medetomidine.

Peripheral arterial pulse: Palpation is not a reliable guide to arterial pressure. An arterial pulse can often be palpated when pressure is so low that it is life-threatening.

Mucous membrane color: It is important to look at the gums, not the tongue. The tongue may remain a pink color long after the animal is dead, especially in cats. In general, pink mucous membranes indicate good peripheral perfusion and adequate O2ation. A healthy pink color indicates more vasodilation than a pale pink color. When vasoconstriction is present, the mucous membranes are pale, even grey or white. Mucous membranes can look an off-muddy pink color when a dog is hypoxemic.

Capillary refill time (CRT): This procedure will give some information about tissue perfusion. CRT should be 1 second. Prolonged time is caused by low cardiac output and/or blood pressure.

Arterial blood pressure: Blood pressure can be measured indirectly (no skin penetration) or directly (catheter or needle in artery). Anesthetized patients should have a mean pressure (MAP) of 65 mm Hg or higher. MAP = one-third (systolic - diastolic) + diastolic pressure.

Noninvasive measurements of blood pressure

Falsely high values are obtained when the cuff is too small, is applied too loosely, has slipped down over the carpus or hock, and when the measuring point is below the level of the heart (leg hanging off the table, table in head-down tilt). Falsely low values are obtained when the cuff is too large, when the cuff is above heart level, or when the cuff is deflated too quickly.

Normal values for systolic/diastolic and (mean) pressures quoted by the Veterinary Blood Pressure Society 2002 for awake healthy animals are for dogs 133/75 (94) mm Hg and for cats 124/85 (98) mm Hg. Whereas the systolic and diastolic pressures vary from publication to publication, mean arterial blood pressure (MAP) values are often 90-100 mm Hg. Hypotension is generally defined as a MAP of less than 65 mm Hg. MAP of 55 is seriously life threatening. Although noninvasive methods of blood pressure measurement are not always accurate, these methods accurately predict hypotension 80% of the time. Consequently, a low pressure obtained by such means is an indicator for treatment.

Doppler ultrasound NIBP

Oscillometric NIBP

Invasive/Direct measurement of arterial blood pressure

This involves insertion of a catheter in a peripheral artery and connecting it by saline-filled tubing to an electrical pressure transducer that transmits the pressure waveform to a monitor. Stiff tubing specific for blood pressure monitoring produces the best recording, however, cheap venous extension tubing is a practical alternative and mean pressures remain accurate. The transducer must be placed level with the heart (spine or sternum with patient in lateral recumbency; mid axilla or thoracic inlet with patient in dorsal recumbency) for the pressure to be accurate. The dorsal pedal artery located below the hock on the dorsomedial surface of the metatarsus is the most popular for catheterization because the artery is a reasonable size, the catheter is easily secured, and trauma and hematoma formation produce limited injury. Catheters can also be placed in an auricular artery, lingual artery, or coccygeal artery. The femoral artery is easily catheterized but the catheter needs to be sutured for security and is easily dislodged when the patient is moved or positioned on the table, and 3-min pressure is essential when the catheter is removed to avoid hematoma formation. Thrombosis of the artery can be minimized by using small catheters, avoiding multiple sticks and local trauma, and reducing the duration of catheterization. Dilute heparin saline (4 units/ml) should be used for multiple flushing of the artery as heparinization of the patient can occur, especially in cats and small dogs. Continuous flushing devices are available that deliver 3 or 6 ml/h of heparinized saline.

Invasive/direct measurement

Electrocardiogram (ECG)

The ECG provides information on heart rate and rhythm. Its main use in anesthesia is for early recognition of ectopic beats or dysrhythmias which could lead to cardiac arrest. The ECG is not an indicator of blood pressure or tissue perfusion. A normal ECG can be present in the absence of an effective cardiac output or blood pressure.

Central venous pressure measurement (CVP)

A long catheter is inserted into a jugular vein until the tip of the catheter is within the thorax. The pressure is measured either with an electrical pressure transducer in the same way that arterial pressure is measured or by connection by LRS-filled tubing to a plastic manometer that is graduated in centimeters, with the level of the heart being the reference for zero. It is proposed that CVP pressures are essentially equivalent to right atrial pressures and are a reflection of right ventricular preload. The waveform is influenced by heart rate, intrathoracic pressure, valvular insufficiency and cardiac disease. Movement of the waveform with breathing should be seen and serves as a confirmation that the tip of the catheter is in the thorax. Normal range for CVP is 0 to 5 cm H2O (4 mmHg) and a negative value indicates hypovolemia. Pressures over 10 cm H2O are indicative of either fluid overload or right heart failure.

Cardiac output measurement

Several techniques are available for measurement of cardiac output – thermodilution, Lithium dilution, carbon dioxide rebreathing method, and transesophageal echocardiography.


The mean arterial pressure in awake, healthy dogs and cats is 90-100 mm Hg (systolic pressure 135-160 mm Hg and diastolic pressure 65-80 mm Hg). Autoregulatory mechanisms of blood flow in specific organ circulations are inactive when mean arterial pressure decreases below 70 mm Hg. Mean arterial pressure of 65 mm Hg is hypotension (newborns and young pediatrics are exceptions) and 55 mm Hg is life threatening. Indicators of low blood pressure or poor tissue perfusion in anesthetized animals would be oozing blood or dry tissues at the operative size, prolonged CRT, peripheral pulse may feel weak on palpation or it may feel good if the pulse pressure is wide due to vasodilation, and heart rate can be normal, slow, or fast. An increase in heart rate is a normal response to hypotension in conscious animals but is blunted or abolished by anesthetic agents. Sometimes decreased myocardial perfusion shows on the ECG as a tall T wave and ST segment slur.

Treatment of hypotension should be directed at the cause. Hypotension, mean pressure less than 65 mm Hg, may be the result of one or several factors. Mechanical causes of hypotension, such as a high circuit pressure from a closed pop-off valve or compression of the caudal vena cava from an enlarged spleen or mass, should be ruled out. Treatment of bradycardia or dysrhythmias with the appropriate drug is indicated when arterial pressure is low. Symptomatic treatment may involve decreasing anesthetic administration. With a few exceptions, usual intraoperative fluid rate is 10 ml/kg/h for the first 3 hours, then half that rate, with additional fluid to compensate for blood loss. If hypovolemia is the possible cause of hypotension in healthy animals, an additional infusion of 20 ml/kg (one time) of LRS may achieve an adequate cardiac output and blood pressure. Further infusion of LRS without improvement in cardiovascular function may not be worthwhile and an alternative cause of low pressure should be determined. Infusion of LRS over 60 ml/kg to cats will produce pulmonary edema. Cardiovascular support can be provided by infusion of dopamine or dobutamine or injection of ephedrine. Expansion of vascular volume with hypertonic saline or hetastarch may be indicated.

Blood loss

  • Patients with fractures may have had considerable blood loss before anesthesia.

  • Patients with a PCV less than 20% are not usually anesthetized without blood transfusion. Note that the PCV will decrease several % on induction of anesthesia due to splenic sequestration and subsequently increase ~4% during recovery from anesthesia.

  • HR and MAP may not change during acute loss during anesthesia until the volume of blood loss is severe. Cardiac output decreases to a greater extent than MAP.

  • Blood loss can be measured by weighing gauzes in grams and subtracting the weight of dry gauzes. Use 1 gram weight = 1 ml blood.

  • Blood loss is initially treated by decreasing the vaporizer setting and infusing LRS at 3 x lost volume.

  • Circulating blood volume of dogs is 86 ml/kg and in cats is 56 ml/kg.

  • Loss of 20% blood volume during anesthesia seriously compromizes cardiovascular function. Blood transfusion may be needed at that point.

Monitoring Ventilation

Carbon dioxide elimination

Elimination is dependent on alveolar ventilation (RR + TV). A decrease in ventilation from normal (hypoventilation) results in an increase in arterial blood carbon dioxide concentration (PaCO2, hypercarbia, hypercapnia). Carbon dioxide in plasma combines with water to form carbonic acid, hence hypercarbia is called respiratory acidosis. Normal PaCO2 in dogs and cats is 32-40 mm Hg. An increase of 10 mm Hg above normal is mild hypoventilation and an increase of 20 is severe. The definition of significant hypoventilation is PaCO2 > 50 mm Hg.

In the normal conscious animal, any change in metabolic status or respiratory status evokes a reaction in the other to maintain the balance between bicarbonate ions and carbon dioxide at a 20:1 ratio so the pH remains normal. Respiratory adjustment can occur immediately whereas metabolic change, which is effected through the kidneys, takes many hours. Thus hypercarbia developing because of anesthetic agent depression will not be compensated quickly enough and results in acidosis during anesthesia.

Ventilation adjusts to metabolic changes only in awake animals, according to the Henderson-Hasselbalch equation: pH = pK + [HCO3] renal {20

0.03pCO2 respiratory {1

Recognition of hypoventilation

Assessment of adequacy of ventilation can be difficult without specific monitors. Observation of a shallow thoracic excursion during inspiration may indicate hypoventilation. Animals with neurologic disease may have altered breathing patterns. Close observation may reveal diaphragmatic breathing without normal thoracic movement. Inadequate ventilation (hypoventilation, hypercarbia, hypercapnia) is common in sick patients during anesthesia and is expected in some animals (overweight, abdominal mass, prone/ head-down position, old age, pulmonary disease. Hypoventilation is likely to be present even in healthy anesthetized Bulldogs and Pugs, overweight dogs and cats, animals that are prone and hanging off the end of the table, when the operating table is put in a head down tilt, and when the abdomen open with a retractor for an exploratory laparotomy. Hypoventilation is probable when the animal has rapid shallow breathing or breathing at a respiratory rate of less than 6 breaths/min. Significant hypoventilation is present when an end tidal CO2 monitor (capnography) reads greater than 50 mm Hg or a blood gas analysis confirms a PaCO2 greater than 55 mm Hg. Hypercapnia can cause hypertension, tachycardia, or hypotension.


Capnography measures carbon dioxide that is expelled from the animal's lungs (Vetrosonics Tidal Wave, Datex, Surgivet, Cardell). There are 2 types of capnographs: one that measures the CO2 in-line at the endotracheal tube adapter and one that continually aspirates gas from the anesthetic circuit and measures the CO2 at a distance. The in-line measuring monitor is necessary for use with non-rebreathing circuits and for the smallest dogs and cats. Measurement of CO2 concentration or pressure at the end of exhalation (called end-tidal, ETCO2) can usually be a reliable indicator of adequacy of ventilation. Arterial blood concentration is approximately 6 mm Hg above the ETCO2. Thus an anesthetized dog or cat breathing well should have an ETCO2 in the low 30s. Ideally your monitor should display a CO2 waveform (see image above).

CO2 waveform

A high ETCO2 means that PaCO2 is high. No ETCO2 means apnea, esophageal intubation, or disconnected endotracheal tube. A sudden decrease in ETCO2 means blood pressure has seriously decreased. Sometimes the inspired CO2 concentration increases from a normal zero to 7-14 mm Hg. This indicates that the patient is rebreathing carbon dioxide. A common cause is a stuck one-way valve open or closed, but check to see if the sodalime needs changing.

Troubleshooting the capnogram

Capnogram at right shows rebreathing because the wave should decrease to zero on inspiration. Correction of the one-way valve function was followed immediately by a decrease in the INCO2 from 12 to 0.


Capnogram at left shows hypoventilation because the end-tidal CO2 is 57 mmHg, despite a RR of 8.


Significance of hypercarbia Hypercarbia causes 1) vasodilation which increases intracranial pressure, 2) sympathetic stimulation which may cause tachycardia, 3) vasoconstriction in the vessels of the mesentery (splanchnic) which decreases hepatic blood flow, and 4) acidosis which decreases cardiac contractility and decreases organ function.

Treatment of hypoventilation Use controlled ventilation (artificial ventilation, IPPV) and lighten depth of anesthesia if possible. Normal PaCO2 can be achieved with the following guidelines. Close the pop-off valve and squeeze the bag to an inspiratory pressure of 15-18 cm H2O for tiny dogs and cats, or to 20 cm H2O for bigger dogs at a RR of 12 breaths /min. Large dogs and dogs in prone position may need inflation pressures of 25-30 cm H2O but the maximum pressure should not exceed 40 cm H2O. Animals with pneumonia or damaged lungs from an automobile accident (HBC) may suffer alveolar rupture by pressures that normally would be safe. In such cases, the peak inspiratory pressure should be kept low at about 25 cm H2O. Tidal volume can be set more precisely when a ventilator is used. Normoventilation is achieved at 15 ml/kg /breath and 12 /min or 20 ml/kg at 10 breaths/min. The increased pressure generated in the hoses during inspiration will also distend the hoses, resulting in a smaller tidal volume delivered than planned. As always, the patients' chest should be observed for satisfactory distension and the ventilator bellows capacity increased if needed. The time taken to achieve one inspiration (inspiratory time) should be about 1.5 sec and always be shorter than time allowed for exhalation and the pause before the next inspiration (I:E ratio 1.2 to 1:4).


It is important to remember that arterial oxygenation depends not only on ventilation but also cardiac output and blood pressure. Hypoxemia (inadequate arterial oxygenation) is defined as a PaO2 < 60 mm Hg and a hemoglobin saturation (SpO2) < 90%.

Recognition of hypoxemia

Hypoxemia can be predicted to occur in some patients and with some procedures. For example: Animals anesthetized with injectable drugs breathing room air, brachycephalic breeds during induction and recovery from anesthesia, overweight conformation that limits diaphragm movement, seniors that have lung changes and decreased reflexes associated with increasing age, head/neck or torso bandages that are too tight, patients with thoracic pathology or recovering from thoracotomy, and during recovery from long duration anesthesia, especially after receiving multiple doses or continuous infusions of opioids.

Mucous membrane color in extreme cases may be blue (cyanosis) but usually the color is only 'off color' or muddy pink. Sometimes, not always, the HR, BP, and RR increase. In sedated or awake patients a facial grimace may develop (open mouth, withdrawn lips, excessive nostril movement).

Pulse oximeter is a non invasive method of measurement of hemoglobin saturation. Normal hemoglobin saturation is > 90%. Is the reading reliable? First check that the signal strength is good and allow at least 30 sec to elapse after attachment of probe before reading. The pulse rate must be accurate so check the value against an alternative source of HR. The probe should be screened from light by covering it with wet gauze. In an anesthetized animal, a decrease in SpO2 to less than 90% when the value had been stable at 97% is suspicious but first the position of the probe should be moved to ensure that it was not a malfunction due to local compression of capillaries. Note that the pulse oximeter provides no information about adequacy of ventilation and no information about adequacy of oxygen delivery to tissues.

Oxygenation of arterial blood (PaO2) can be measured directly by a blood gas machine using a blood sample collected anaerobically. The patient's body temperature will be needed for accurate values.

Blood gas analysis

At sea level PO2 of arterial blood is 90-110 mm Hg when breathing air (21% oxygen). Venous blood PO2 is 40 mm Hg

Barometric pressure is 760 mm Hg

Water vapor in lungs is 47 mm Hg

Air is 21% oxygen

(760 - 47) x .21 = 150 mm Hg

PaCO2 is 40 mm Hg

150 - 40 = 110 mm Hg is maximum that PaO2 can be under these conditions

An anesthetized animal on the anesthesia machine is breathing 90-100% inspired oxygen. The PaO2 should be up to 5 times higher, 440-500 mm Hg.

It is important to remember that uptake of oxygen into pulmonary capillaries depends on both ventilation and perfusion. Tissue oxygenation depends on

  • Hemoglobin content

  • Hemoglobin oxygen saturation (determined by the PaO2)

  • Oxygen transport (cardiac output)

Causes of hypoxemia

  • Low inspired oxygen concentration

  • Hypoventilation if severe and breathing air

  • Diffusion impairment

  • Ventilation - perfusion mismatch

  • Shunting of blood through the lungs

Treatment of hypoxemia

The animal should be given 100% oxygen to breathe. If the patient is on inhalation anesthesia, check anesthesia machine and circuit (failure O2 supply, kinked hoses, closed pop-off valve, endobronchial intubation, esophageal intubation) and consider controlled ventilation. A change in body position e.g. dorsal to lateral, may be necessary. Apply a stethoscope to each side of the chest and listen for air flow during inhalation. Management of hypoxemia during recovery from anesthesia will depend on the situation. Check for tight bandages. A dog that has excessive respiratory depression from anesthetic agents may develop hypoxia when removed from the anesthesia machine and allowed to breathe air. Consider partial opioid reversal.

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