Ch. 18 Reading Flashcards
Step 1: Look at the arterial partial pressure of oxygen (PaO2) level, and answer this question: Does the PaO2 level show hypoxemia?
The normal range of PaO2 values for individuals breathing
room air at sea level is 80 to 100 mm Hg.
At any age, a PaO2 lower than 40 mm Hg represents a life-
threatening situation that necessitates immediate action.1,2
Step 2 Look at the pH level, and answer this question: Is the pH level on the acid or alkaline side of 7.40?
The normal pH of arterial blood is 7.35 to 7.45, and the mean is 7.40.
Step 3: Look at the arterial partial pressure of carbon dioxide (PaCO2) level, and answer this question: Does the PaCO2 level show respiratory acidosis, alkalosis, or normalcy?
The normal range for PaCO2 is 35 to 45 mm Hg.
Hypoventilation can result from chronic obstructive pulmonary disease (COPD), oversedation, head trauma, anesthesia, drug overdose, neuromuscular disease, or hypoventilation with mechanical ventilation.
Ventilatory failure results when the PaCO2 level exceeds 50 mm Hg.
Chronic ventilatory failure is defined as a PaCO2 value greater than 50 mm Hg and a pH level greater than 7.30.1
A PaCO2 value that is less than 35 mm Hg defines respiratory alkalosis, which is caused by alveolar hyperventilation.
Step 4: Look at the bicarbonate (HCO3) level, and answer this question: Does the HCO3 level show metabolic acidosis, alkalosis, or normalcy?
The normal range is 22 to 26 mEq/L.
An HCO3 level of less than 22 mEq/L defines metabolic acidosis, which can result from ketoacidosis, lactic acidosis, renal failure, or diarrhea. The cumulative effect is a gain of acids or a loss of base. An HCO3 level that is greater than 26 mEq/L defines metabolic alkalosis, which can result from fluid loss from the upper gastrointestinal tract (vomiting or nasogastric suction), diuretic therapy, severe hypokalemia, alkali administration, or steroid therapy.
Step 5: Look again at the pH level, and answer this question: Does the pH show a compensated or an uncompensated condition?
If the pH level is abnormal (less than 7.35 or greater than 7.45), the PaCO2 value or the HCO3 level, or both, will also be abnormal. This is an uncompensated condition because the body has not had enough time to return the pH to its normal range.
The primary disorder is the abnormality that caused the pH level to shift initially and is determined according to the pH level; the primary disorder is considered the one on whichever side of 7.40 the pH level occurs.
Partial compensation may be present and is evidenced by abnormal pH, PaCO2, and HCO3 levels, indications that the body is attempting to return the pH to its normal range.
Arterial Blood Gasses
is a measure of the amount of oxygen bound to hemoglobin compared with the maximal capability of hemoglobin for binding oxygen.
The hemoglobin level must also be evaluated before a decision on oxygenation status can be made.
Oxygen Saturation
is clinically the easiest formula to calculate because this formula does not call for the computation of the alveolar PO2. Normally, the PaO2/FIO2 ratio is greater than 286; the lower the value, the worse the lung function.
PaO2/FIO2 ratio
is the measurement of exhaled carbon dioxide (CO2) gas and is also known as end-tidal CO2 monitoring.
Causes of decreased PETCO2 (partial pressure of end-tidal CO2) include situations in which carbon dioxide production is decreased, such as hypothermia, cardiac arrest, and pulmonary embolism, or in which alveolar ventilation is
increased, such as hyperventilation.
In the critical care area, continuous capnography is used for assessment and monitoring of the patient’s ventilatory status in various situations, including weaning from mechanical ventilation and undergoing procedural sedation.
The noninvasive measurement of PETCO2 enables assessment of the adequacy of cardiopulmonary resuscitation and endotracheal tube placement. During endotracheal intubation, a low PETCO2 reading indicates that the tube is positioned in the stomach because the amount of carbon dioxide in the esophagus is expected to be low.
All forms can be used in intubated patients, but side-stream and Microstream capnography can also be used in nonintubated patients
Mainstream
Side-stream
Proximal-diverting
Any change in the waveform can indicate a change in the patient’s pulmonary status and warrants further evaluation.
Loss of the waveform may signal loss of effective respirations.
Capnography
measures the carbon dioxide level directly by a sensor in the exhalation port of the ventilator tubing. During exhalation, gas passes over the sensor, and the information is transferred by an electrical cable to the display unit. The display unit produces a waveform, called a capnogram, and a numeric recording (PETCO2). Disadvantages to this form of capnography include the weight of the sensor on the ventilator tubing and possible obstruction of the sensor by secretions and condensation.
Mainstream
the carbon dioxide gas is continuously aspirated through a side port in the ventilator tubing or nasal cannula and is measured and analyzed by a side unit.Disadvantages to this form of capnography include obstruction
of the sampling tube with secretions and slow response time.
Side-stream
is a newer and improved version of side-stream capnography that transports gas a short distance from the airway to a site where the sensor is located, reducing the bulkiness at the airway
Proximal-diverting
is a noninvasive method for monitoring oxygen saturation (SpO2) and is indicated in any situation in which the patient’s oxygenation status requires continuous observation.Physiologic limitations of pulse oximetry include elevated levels of abnormal hemoglobins, the presence of vascular dyes, and poor tissue perfusion.
Because most critically ill patients require some form of oxygen therapy, pulse oximetry is an unreliable method of detecting hypercapnia and should not be used for this purpose.
Technical limitations of pulse oximetry include bright lights, excessive motion, and incorrect placement of the probe. Interventions to limit these problems include using the proper probe in the appropriate spot, applying the probe according to the directions, and ensuring that the area being monitored has adequate perfusion.
Pulse Oximetry
