Respiratory/ventilatory monitoring (Proceedings)


Capnographs are used to measure ETCO2. Most capnographs use infrared light absorption to measure CO2 levels. Capnographs allow for continuous monitoring of the patients CO2 levels. ETCO2 is reflective of the patients PaCO2 usually within a 5mmHg gradient, this gradient can be affected by pulmonary perfusion.

ACVA Current Recommendations

          • Circulation

          • Ventilation

          • Oxygenation

          • Central Nervous System

          • Fluid Therapy

          • Temperature

          • Neuromuscular Blockade

          • Record Keeping

          • Personnel

          • Recovery

          • Temperature

     • AAHA requirements

     • At least one of these monitors on all sedated or anesthetized patents (incl – dentals, rads)

          • Electronic respiratory Monitor

          • Pulse Oximeter

          • Blood Pressure Monitor

          • Continuous ECG

          • Esophageal Stethoscope

          • Capnograph

Ventilation = exchange of oxygen and carbon dioxide through inhalation and exhalation

Hyperventilation = ventilation that exceeds metabolic demands

Hypoventilation = ventilation that does not meet metabolic demands


     • Simplest method of monitoring the respiratory system

     • Visualization of the chest wall excursion or breathing bag movement can suggest ventilation

Respiratory Cycle

     • Inhalation - Diaphragm contracts

               • Lungs expand

               • Alveoli open

               • Vacuum created

                         • Air is drawn into lungs

               • Abdominal organs displaced caudally

               • Coordinated outward movement

               • Diffusion

                         • Capillary-alveolar membrane

                              • Oxyhemoglobin

          • Exhalation - Diaphragm relaxes

                  • Thoracic cavity size decreases

                  • Air expelled from lungs

                  • Coordinated inward movement

                  • Carbon dioxide is exchanged at the alveolar level and exhaled

Increased Inspiratory effort may be indicative of:

               • Upper airway

               • Laryngeal paralysis

               • Collapsing trachea

               • Increased expiratory effort

               • Lower airway

               • Asthma

               • COPD

Abdominal Effort

          • Abdominal ventilation occurs when the abdominal muscles, rather than the thoracic muscles, are utilized for respiration.

               • Post-op ventral slot

               • Phrenic nerve

               • Cervical disease

               • Botulism

               • Coon hound paralysis

Respiratory Patterns should be observed if at all possible in the awake patient prior to administering any anesthetic agents:

               • Short/ shallow breaths may indicate:

               • Decreased lung compliance

               • Pneumonia

               • Pleural effusion

               • Pulmonary edema

Restrictive conditions may be caused by:

               • Pneumothorax

               • Pulmonary contusions

               • Hemothorax

               • Diaphragmatic hernia

Slow, deep inhalations may indicate obstructive conditions:

               • Asthma (bronchoconstriction)

               • Laryngeal paralysis

Dyspnea may be observed in a number of respiratory problems and is considered an indication of inadequate ventilation or insufficient amounts of oxygen in circulating blood.

Apnea: temporary cessation of breathing

Tachypnea: increased respiratory rate, above normal parameters

Orthopnea: ability to breathe easily only when standing

Orthopnea is commonly seen in congestive heart failure patients

Apneustic Respiratory Pattern

     • Definition: sustained, gasping inspiration followed by short, inefficient expiration.

     • This respiratory pattern is often associated with lesions in the respiratory center of the brain.


     • Breathing characterized by rhythmic waxing and waning of the depth of respiration; the patient breathes deeply for a short period, then breathes very slightly or stops breathing altogether. The pattern occurs over and over again every 45 seconds to 3 minutes.

     • Disease affecting the respiratory center

Paradoxical Respiration

     • Definition: a type of breathing in which all or part of a lung inflates during inspiration and balloons out during expiration; the opposite of normal chest motion

     • Visually, the chest and abdomen move in opposition to each other.


     • Respiratory rate,

     • lung pathology,

     • tube placement and

     • ventilation

     • *Limitations:

               • Drapes, surgical site, patient size or noise.

               • Breath sounds do not assure adequate ventilation

End Tidal CO2

     • Non-invasive assessment

     • Closely reflects values of alveolar gas and capillary blood

     • Decreases in End Tidal CO2

     • Decreased CO2 production and delivery to the lungs

     • Increased alveolar ventilation

     • Equipment malfunctions

Capnographs are used to measure ETCO2. Most capnographs use infrared light absorption to measure CO2 levels. Capnographs allow for continuous monitoring of the patients CO2 levels. ETCO2 is reflective of the patients PaCO2 usually within a 5mmHg gradient, this gradient can be affected by pulmonary perfusion. Severe hypotension, embolism, shock or hypovolemia, cardiac arrest (if the lung is under-perfused CO2 is not exchanged) can cause significant changes in gas exchange. Capnographs may sample via mainstream or sidestream methods. Capnographs give the anesthetist more than just ETCO2 levels. The graphs often demonstrate a variety of problems that if left untreated can be harmful to the patient


     • Minimal invasive sampling of end-tidal CO2

     • Waveform analysis to diagnose respiratory and equipment abnormalities.

     • Continuous respiratory monitoring (breath to breath)

     • Demonstrates respiratory rate

     • Demonstrates adequacy of ventilation

     • Reflects pulmonary perfusion


     • Sampling malfunctions cause erroneous readings

     • Does not demonstrate oxygenation

     • Readings can be inaccurate depending on pulmonary function

     • Ventilation-Ensuring adequate gas exchange

Effects of general anesthesia:

     • Dose dependent hypoventilation/impaired oxygenation

     • Physiologic dead space increases

     • Extent of effects depends on: age, disease, body condition, duration of anesthesia, positioning and anesthetic technique

     • Decreased CO2 production/delivery

     • Hypothermia

     • Pulmonary hypoperfusion

     • Cardiac arrest

     • Pulmonary embolism

     • Hypotension

     • Increased alveolar ventilation

Hyperventilation- increased respiratory rate or increased tidal volume

Equipment malfunctions

Ventilator disconnect

Complete airway obstructions

Esophageal intubation

Poor sampling

Leak around the endotracheal tube

Exhausted soda lime

Airway obstruction due to positioning

Arterial Blood Gas-Gold Standards

PaCo2 reflects adequacy of ventilation

PaO2 reflects efficiency of ventilation

Causes of hypoventilation

     • Shunt – blood reaches the arterial system without passing through the lungs (reverse PDA)

     • V/Q (ventilation-perfusion) mismatch – (pulmonary thromboembolism)

     • Hypoventilation – (decreased inspiration)

     • Diffusion impairment – physical barrier in place (pulmonary edema, pulmonary contusions)

     • Low inspired oxygen


The measurement of breathing capacity utilizing a spirometer.

Tidal volume is the volume of gas delivered at exhalation and should be at least 10ml/kg of body weight (range 10-20 ml/kg)

Minute volume is the volume of gas delivered at exhalation per minute. (tidal volume x breaths per minute)

It is more accurate when placed closer to the patient vs close to the anesthesia machine

Can be cumbersome due to weight

Small patients may not show precise measurement due to the mechanical resistance of the respirometer

Pulse oximetery

Estimates oxygen saturation of hemoglobin

Mechanics of measurement

Produces light at the wavelength of both oxygenation and deoxygenated hemoglobin

Percentage of oxygenated hemoglobin measured by ratio of infrared and red light transmitted to photo detector in probe

Estimate oxygen saturation of hemoglobin

(PaO2 is oxygenation of plasma)

Pulse rate

Fairly inexpensive


Should be >95%


     • Accuracy +/- 2% - poor below 85%

     • Not a ventilatory monitor – can be normal on increased O2 even if hypoventilating


     • Poor perfusion affecting pulsatile signal

     • Hypothermia

     • Hypotension

     • Use of vasoactive drugs

     • Troubleshooting a Pulse Ox

     • Check for pulse or heart rate!!!

     • Try the probe on yourself

     • Look for causes

               • Oxygen supply (source, obstruction, kink)

               • ET tube in a bronchus

               • PTE

               • Pneumothorax

               • Pulmonary edema

Conditions causing a decrease in SPO2 readings

     • Pneumothorax

     • Pulmonary thromboembolism

     • Hypoperfusion

     • Pulmonary edema

     • Upper airway obstruction

     • Cardiac arrest

Basic metabolic needs

     • Awake dogs – consume 4-7 ml/kg/minute

     • Dogs under anesthesia – 3-14 ml/kg/minute

     • Influenced by:

     • body weight - surface area (smaller patients = higher metabolic rate)

     • Body temperature - metabolic rate decreases with decreased body temperature

Clinical Assessment

You must distinguish hypoxemia from hypoventilation

Hypoxemia = PaO2 < 60 mm Hg

Hypoventilation = PaCO2 > 45 mm Hg

Rules to Live by:

Hypoxemia kills quickly

Hypoventilation kills slowly

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