Assessment Criteria

The respiratory status of patients who are mechanically ventilated must be frequently assessed. The following are some of the most important assessment categories.

Breath Sounds

Breath sounds should be assessed at least every four hours while patients are mechanically ventilated, and more frequently as needed. Both the anterior and posterior chest should be auscultated bilaterally. A good time to do this is when repositioning the patient every two hours, since you will have another staff member who can assist in holding the patient on his or her side while you listen to the back. The following is a review of abnormal breath sounds.

Lung Sound Description Cause

Crackles (rales)

Popping or crackling sound

Fluid in small airways/alveoli or collapsed airways snapping open on inspiration


Course, low-pitched rumbling

Airflow over secretions or narrowing of large airways


High-pitched squeak or whistling

Airflow through narrowed small airways

Pleural friction rub

Creaking, leathery, course sound

Inflamed pleural surfaces rubbing together

Spontaneous Respiratory Rate and Tidal Volume

Even if a patient is mechanically ventilated, the spontaneous respiratory rate and tidal volume (the volume of air exhaled after a normal resting inhalation) can give some important clues about respiratory function. For example, if the spontaneous tidal volume is low the patient may not do well with weaning attempts. If the respiratory rate is high, particularly with weaning modes, it may indicate that the patient isn’t tolerating the mode, needs suctioning, or that he or she is anxious or trying to communicate. This topic will be further addressed when we discuss weaning.

Pulse Oximetry

Usually patients who are mechanically ventilated have a continuous pulse oximeter to measure oxygen saturation (SpO2). The machine detects how much oxygen is bound to all hemoglobin and reports the value as percent saturated hemoglobin. Pulse oximetry is a useful monitoring tool, but it provides minimal indication of the patient’s ventilatory or acid-base status. Thus, it should not take the place of arterial blood gases. Readings can also be affected by abnormal hemoglobins, vascular dyes, and poor perfusion. The pulse oximeter can’t differentiate between normal and abnormal hemoglobins; thus, a patient with carbon monoxide poisoning could have a pulse oximetry reading of 100%, but may not be adequately oxygenated. However, pulse oximetry can be a helpful guide when titrating FIO2. In general, a SpO2 of 92% in white patients, and 95% in black patients indicates adequate oxygenation (PaO2 > 60 mmHg).

(Capnography) End Tidal CO2

Capnography, also called end tidal CO2, is CO2 measured at the end of exhalation. It’s usually measured via the exhalation port on the ventilator tubing; the gas is analyzed by a sensor and the data are transferred to a display where a waveform (capnogram) is created, along with a number that closely approximates the PaCO2. In a hemodynamically stable patient with a normal ventilation/perfusion relationship, the end tidal CO2 (also called PetCO2) is generally 1-5 mmHg less than the PaCO2. The reliability of this number is decreased in patients with abnormal cardiopulmonary function.

The most useful function of end tidal CO2 measurement is to confirm ETT placement in the lungs. There are disposable devices available that produce a color change when CO2 is detected; these are often used to confirm placement after intubation. (If the ETT is in the esophagus, little CO2 will be detected unless the patient consumed a carbonated beverage a short time before the intubation.) Some ICUs routinely monitor the PetCO2 in adult, mechanically ventilated patients, but this practice has not been supported by research. There is wide variability in the relationship between PaCO2 and PetCO2 among patients; therefore, great care must be taken when predicting the PaCO2. PetCO2 should not take the place of arterial blood gases during ventilator weaning.

Arterial Blood Gases (ABG)

Arterial blood gas interpretation could be a whole separate program in itself; therefore, we’ll just cover the basics here.

The following are the components of the ABG that are the most crucial for you, the nurse, to know. Follow along for a review of basic chemistry.





Base Excess/Deficit


Arterial Blood Bases (ABG) Result Imbalance Type

pH 7.30, PaCO2 40, HCO3 18

Metabolic acidosis (pH \/, PaCO2 ok, HCO3 \/)

pH 7.48, PaCO2 30, HCO3 24

Respiratory alkalosis (pH /\, PaCO2 \/, HCO3 ok)

pH 7.25, PaCO2 54, HCO3 26

Respiratory acidosis (pH \/, PaCO2 /\, HCO3 ok)

pH 7.50, PaCO2 42, HCO3 33

Metabolic alkalosis (pH /\, PaCO2 ok, HCO3 /\)