Oxygen therapy and basic ventilator theory (Proceedings)

Oxygen therapy is indicated in the presence of hypoxemia (inadequate oxygenation of tissues within the body. Hypoxemia can result from impaired pulmonary function, hypoventilation, ineffective delivery of oxygen within the body, increased tissue oxygen demand, or inadequate inspired concentrations of oxygen. Examples of these in respective order would be: pneumonia and pulmonary edema, cervical lesions and lower motor neuron diseases, congestive heart failure and shock, sepsis and seizures, and a high-altitude environment.

A valuable technician should be able to recognize dyspnea in any patient. The hallmark of dyspnea, cyanosis, is not recognizable until greater than 5gm/dl of reduced hemoglobin is circulating in the blood. Most patients will have considerable dyspnea before cyanosis can be visualized. Conditions such as carbon monoxide poisoning or severe anemia can prevent or mask cyanosis, so the recognition of other clinical signs is a must for assessing dyspnea. "Air hunger" may present as anxiety, tachypnea, panting, open mouth breathing, excessive respiratory effort, extended head and neck, tachycardia, and aggression and thrashing.

Measurement of partial pressure oxygen concentration of the blood (PaO2) is the gold standard for determining hypoxemia. In order to evaluate pulmonary function an arterial blood sample should be taken from the animal when it is not receiving oxygen supplementation. From arterial blood gas samples, an A-a gradient calculation can give detailed information regarding that patient's pulmonary function. PaO2 concentrations less than 90mm Hg (normally 90-100mm Hg) indicate a need for oxygen supplementation. PaO2 concentrations lower than 60mm Hg would indicate more aggressive intervention such as mechanical ventilation. Other factors including PaCO2 measurement and the level of patient distress or exhaustion are evaluated to determine the delivery method of oxygen supplementation.

If a blood gas analyzer is not available to a clinic, a pulse oximeter can be used to monitor trends in oxygenation. A pulse oximeter does not measure dissolved oxygen particle concentrations but rather reads the percentage of hemoglobin that is saturated with oxygen. There are limitations to the SpO2 measurement from a pulse oximeter and readings should only be used to monitor trends in patient condition. Factors such as body temperature, movement, ambient light, perfusion, and skin pigmentation profoundly affect the accuracy of the machine. When using a pulse oximeter; a technician should assure that the machine is showing a strong signal, a nicely curved waveform, and an accurate heart rate to feel confident in the reading. In general, readings less than 92% (normally 100%) indicate a need for oxygen supplementation. Of course if a patient is anxious, tachycardic, and restless at an SpO2 of 93%, that could still indicate a need for additional oxygen support. All parameters need to be evaluated for each individual patient.

The following are methods to deliver oxygen. An oxygen rich environment means 40% oxygen. Higher concentrations may be indicated. Prolonged exposure at high concentrations can cause oxygen toxicity. Symptoms include pulmonary edema, atelectasis, consolidation, fibrosis, and hemorrhage.

• Mask-for short term needs

*The most important--Do not stress patient while trying to deliver oxygen. If a patient is stressed, the oxygen demands are going to increase. Consider sedation. Requires one-on-one time commitment unless comatose patient. May resort to flow-by oxygen if patient will not accept mask*do not chase patient. Must evaluate patient's response to O2.

• Oxyhood-for short term needs, longer if tolerated and well supervised

Very nice but more expensive version of "oxygen tent" (plastic bag over E. collar). Can reach very high concentrations of oxygen-in excess of 80%. Must have vent holes to eliminate CO2. Highly recommend an oxygen analyzer to take intermittent readings. Be careful of O2 and CO2 concentrations, and overheating. Must evaluate patient's response to O2.

• Nasal catheter-long term needs

Easy to place and usually well tolerated. Use softest red rubber available and largest size possible for nose. Always humidify the oxygen flow, 1L/20# in general. Must evaluate patient's response to O2. If patient is open mouth breathing or panting, may not be raising oxygen saturation. Consider sedation.

• Intratracheal catheter-long term needs

Requires invasive technique and heavy sedation to place. Not commonly performed any longer due to complications such as tracheitis and jet lesions.

• Oxygen cage-long term needs

Expensive. Need to control ambient temperature, humidity, and eliminate CO2. Tend to overheat with larger dogs. Difficult to maintain higher concentrations of O2 with economical flow rates. Access to patient is limited-makes evaluating your patient's response to O2 more difficult.

• Controlled types of ventilation

This includes endotracheal intubation , tracheostomy tube, and PPV (positive pressure ventilation). Endotracheal oxygen usually occurs during anesthesia, arrest, or comatose patients. Tolerance for the endotracheal tube is a limiting factor usually requiring a twilight sleep level of sedation. Tracheostomy tubes are better tolerated by patients, and are frequently used for ventilator patients. Whether using an endotracheal tube or tracheostomy tube, it should have a high volume low pressure cuff to minimize tracheal mucosal damage. Must evaluate patient's response to oxygen.

Principles of Positive Pressure Ventilation

There are three indications for PPV. These include ventilatory failure, pulmonary conditions that have not adequately responded to oxygen treatment, and treatment of intracranial hypertension. Trying to control the respirations of a living being is next to impossible due to spontaneous ventilation (responsiveness to build up of CO2). Unless the patient is comatose, drugs have to be utilized to ensure smooth control over respiration. Commonly, opioids, benzodiazapines, oxybarbiturates, and sometimes paralytics are used in combinations to provide enough sedation and anxiety relief to ventilate the patient.

Several different types of ventilators are commercially available. Anesthesia type ventilators are designed to work in conjunction with an anesthetic machine, and are less expensive. Disadvantages to this type of ventilator for long term care are the inability to deliver less than 100% oxygen, and the inability to humidify the breathing circuit. Delivering the lowest concentration of oxygen to meet the patient's needs is ideal, and will avoid lung damage associated with oxygen toxicity. Most ventilators designed for critical care use have volume, pressure limits, and time all factored into the respiratory cycle. These ventilators have a range from 21% oxygen (room air) to 100% oxygen. Critical care ventilators are obviously much more expensive.

Sterile circuit tubing, filters, water traps, and water chamber must be connected and checked before a patient can be connected to the ventilator. Routinely a sterile rebreathing bag is attached as the initial settings are determined and the system is checked for leaks. The start up settings for the ventilator will be based on the weight of the patient and will usually start out with: 5-15ml/kg tidal volume, 10-15cm H2O peak inspiratory pressure, 1 second inspiratory time, 2 second expiratory time, and 10-30 breaths per minute. Starting a patient on the lowest necessary volume and delivery pressure respiratory cycle will lessen the chance of ventilator induced lung injury. One hundred percent oxygen is initially delivered, and then adjusted down depending on the patient's response. The ventilator circuit is humidified with a heated chamber filled with sterile water.

Monitoring the ventilator patient is a very time consuming challenge. These patients should be instrumented with a central venous catheter, arterial catheter, urinary catheter, ECG stick on patches, temperature probe, and often a naso-gastric tube. With these is place continuous readings of central venous pressures, direct arterial pressures, ECG tracing, body temperature, and urine output can be monitored. The naso-gastric tube can be used for nutrition in a patient's normal GI tract, or can be used to remove fluid build up as a result of poor motility. Of course, a pulse oximeter is attached to the patient to provide constant trends in saturation, and a reading close to 100% should be the goal. Also a capnograph is placed between the CETT or trach tube and the ventilator circuit to monitor expired CO2. Serial blood gas analysis can be obtained from the arterial catheter to monitor pulmonary function. If atelectasis is deemed to be a problem, most critical care ventilators are equipped to provide PEEP (positive end expiratory pressure); which means to provide a small amount of positive airway pressure at the end of expiration to "hold open" smaller airways in the lungs thereby preventing further collapse of lung and inflate areas that may have already consolidated.