Basic Respiratory Mechanics

Basic lung function is designed to exchange oxygen and carbon dioxide. In order to transfer oxygen from atmospheric air to the blood stream three functions must be in place: ventilation, diffusion, and perfusion. Ventilation is the process of air moving into and out of the lungs. Diffusion is the movement of gases between the alveoli and the blood in the capillaries. And perfusion occurs when the cardiovascular system pumps blood throughout the lungs. Where these three things meet are the interface of each tiny alveolus that is wrapped with a capillary bed consisting of both pulmonary artery (carrying oxygen deficient blood) and pulmonary vein (carrying newly oxygen rich blood to the heart). It is the surface area of millions of alveoli that allow for the movement of carbon dioxide out of the blood and oxygen into the blood.

Recognizing Respiratory Distress: Respiratory distress/dyspnea, as defined, is observable respiratory difficulty or physically labored breathing clinically evident by an inability to ventilate and/or oxygenate adequately. Normal breathing should appear natural, free, and easy. Inspiration is normally no longer than one second with moderate expansion of the chest and abdomen occurring simultaneously. Exhalation is normally 2-3 seconds and should not be forced in character. Prolonged inspirations may indicate upper airway disease while prolonged exhalations may indicate lower airway disease. Anxiety ridden or exaggerated breathing efforts may indicate obstructed ventilation and hyperventilation respectively. Physical manifestations of respiratory distress can vary widely from tachypnea and loud panting to slow stridorous whistling attempts to breath. Cats may demonstrate dyspnea by open mouth panting while visually they appear fairly non-distressed. All of these presentations demand fast action to deliver supplemental oxygen.

Gentle handling should be employed in order to decrease anxiety in patients with respiratory distress. Oxygen should be delivered in the least stressful manner possible. Methods of oxygen delivery include; flow by oxygen, oxygen mask, oxygen hood (or e-collar with loose baggy hood), nasal cannulation, oxygen cage, intratracheal catheter, or endotracheal tube. Delivery mode should correlate to the patient's need and should not cause any further anxiety to the patient. Physical examination of the patient can now take place as tolerated by the patient and will include auscultation of upper and lower airways and all lung fields. Cats, however, may require a hands-off approach with oxygen being supplied into an oxygen cage and the physical exam waiting for 5-10 minutes. If the cat's dyspnea is not decreasing in that amount of time some degree of intervention is needed to assess the patient. Appropriate oxygen therapy mode varies depending on the severity of the patient's condition. A patient should never die from respiratory distress. To provide oxygen in times of distress may require sedation (moderate distress) or anesthetic induction (life threatening distress) followed by intubation and positive pressure ventilation. These measures would allow for time to diagnose disease and implement therapeutic measures.

Methods to Assess Oxygenation

Pulse oximetry is a painless, quick, and inexpensive method to measure the percentage of hemoglobin saturated with oxygen (SpO2; units=%) within the blood. It is generally the first objective method of assessing a patient's respiratory status. Pulse oximetry works by spectrophotometry and measures two forms of hemoglobin that circulate in arterial blood; those forms of hemoglobin are oxyhemoglobin (saturated) and deoxyhemoglobin (unsaturated). Carboxyhemoglobin (bound to carbon monoxide), and methemoglobin (oxidized) are two additional forms of hemoglobin that would be measured using a cooximeter. As pulse oximetry does not differentiate carboxyhemoglobin and methemoglobin readings are not considered reliable in patients with smoke inhalation or acetaminophen toxicosis. Pulse oximetry probes transmit wavelengths of red to infrared light waves across vascular beds of tissue and read saturated or unsaturated hemoglobin. The amount of light that is reflected or passed through is evaluated during pulsatile flow. Limitations to pulse oximetry are considerable; tissue perfusion (poor cardiac output or body temperature), icterus, skin pigmentation, skin thickness, mucous membrane dryness, patient movement, and ambient light all may affect readings. Moist mucous membrane areas (tongue, buccal membrane) or thin, light colored, skin areas (point of hock, ear, inter digits) usually work the best. In order to be considered reliable, a pulse oximetry waveform reading should demonstrate an accurate heart rate and stable repeating waveform oscillations. Some units will have a signal strength indicator as a bar graph or indice number. Probe types vary from clip on (gentle clamp) types that transmit light through tissues like the ear, lip, or tongue to reflectance probes that lay against tissue beds such as the inner rectum, prepuce, or under tail base. Be aware that once the probe has been in place for several minutes vascular beds may become compressed and readings may decrease. A normal SpO2 reading for patients breathing room air (21% oxygen) should be at least 96%. Readings of <92% SpO2 require supplemental oxygen delivery and/or ventilatory assistance.

While pulse oximetry is generally the first tool used to assess a patient; arterial blood gas analysis is considered the gold standard in evaluating a patient's ability to oxygenate and ventilate. Arterial blood gas analysis measures the lung's ability to diffuse oxygen into arterial capillary blood (oxygenation) while extracting CO2 from venous capillaries (ventilatory efficiency). Because it also measures PaCO2 (partial pressure of carbon dioxide in arterial blood) it can be used to evaluate for evidence of hypoventilation whereas pulse oximetry only addresses oxygenation status. Blood gas analysis is considered a standard of care now that the units have become more affordable (I-Stat®I-Stat Corporation, East Windsor, NJ: and IRMA SL®Diametrics Medical, St. Paul, MN). Blood gas analysis works by measuring the partial pressure of oxygen (PaO2; units=mmHg) and carbon dioxide (PaCO2; units=mmHg) dissolved in the plasma. It does this by using electrodes to compare a known electrolyte solution to the blood sample across a semi-permeable membrane (permeable to CO2 and 02). Obtaining blood gas samples does require practice and causes some stress to the patient. Measures should be taken to minimize stress because anxiety and struggling will alter results (hyperventilation). Potential sites for sampling are the dorsal pedal, femoral, and auricular arteries. Any artery punctured must be held off for 10 minutes or bandaged to control hemorrhage and preserve the artery. Using the dorsal pedal artery will require less manipulation of patient position , as compared to the femoral artery and is more easily bandaged. Anatomical proximity of the femoral artery and vein makes the possibility of obtaining a venous or mixed venous/arterial sample greater when using the femoral artery. Placement of an arterial catheter will facilitate repeated sampling and serial monitoring. Obtaining arterial gas samples from cats requires extensive experience and is usually not tolerated by a non-anesthetized patient.