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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
Rhonchi	Course, low-pitched rumbling	Airflow over secretions or narrowing of large airways
Wheeze	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 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 the percent of hemoglobin that is fully saturated. 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 FIO_2: 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 CO_2, 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 and, therefore, great care must be taken when predicting the PaCO₂. PetCO₂ should not take the place of arterial blood gases during ventilator weaning (Hess, 1996).

Arterial Blood Gases (ABG)

Arterial blood gas interpretation could be a whole separate program in itself. Just the basics will be covered 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.

pH

		Normal pH of body fluids = 7.35-7.45
		Acid  capable of releasing hydrogen ( H+) in a solution (increased H+ = increased acidity = decreased pH)
		Base  capable of accepting H+ in a solution (decreased H+ = increased alkalinity = increased pH)
		pH < 7.35 = acidosis
		pH > 7.45 = alkalosis

PaCO₂

		PaCO2 is the partial pressure of dissolved CO2 in blood.
		Most is excreted by the lungs, although some is excreted by the kidneys as HCO3.
		Normal = 35-45 mmHg
		PaCO2 is directly related to rate and depth of respiration. It’s a direct indicator of the effectiveness of ventilation.
		As PaCO2 rises, the blood becomes more acidic and pH drops.
		As PaCO2 decreases, the blood becomes more alkaline and pH rises.
		If a change in PaCO2 is the primary alteration, then a respiratory problem exists.

HCO₃

		Bicarbonate (HCO3) is the primary buffer in the body and is able to take up and release H+.
		Normal = 22-26 mmHg
		As HCO3 rises, the blood becomes more alkaline and pH increases.
		As HCO3 drops, the blood becomes more acidic and pH decreases.
		If a change in HCO3 is the primary alteration, then a metabolic problem exists.

CO₂

		Considered a measure of bicarbonate concentration; includes total of bicarbonate and carbonic acid.
		Normal = 23-27 mEq/L
		Generally not used in deference to HCO3 measurement.

Base Excess/Deficit

		Measures excess amount of acid or base present in blood. This is independent of changes in PaCO2, therefore, it’s a measure of metabolic acid-base balance.
		Increased HCO3 = base excess (alkalosis)
		Decreased HCO3 = base deficit (acidosis)

PaO₂

		The amount of oxygen dissolved in plasma (about 3% of total; the other 97% is bound to hemoglobin).
		Normal is 80-100 mmHg in healthy young people breathing room air at sea level; this decreases with age and altitude.
		PaO2 > 60 mmHg is considered acceptablein critically ill, mechanically ventilated adults.
		Adequacy of PaO2 must be weighed against potential oxygen toxicity.

 

 

Take some time to figure out the ABG results below. Determine which imbalance each result indicates: respiratory acidosis, metabolic acidosis, respiratory alkalosis, or metabolic alkalosis. Click on the red square to reveal the answers. If your browser does not support Java Click Here to reveal the answers.

		Answers:
1. pH 7.30, PaCO2 40, HCO3 18		Metabolic acidosis  (pH     , PaCO2 ok, HCO3   )
2. pH 7.48, PaCO2 30, HCO3 24		Respiratory alkalosis (pH    , PaCO2    , HCO3 ok)
3. pH 7.25, PaCO2 54, HCO3 26		Respiratory acidosis (pH   , PaCO2   , HCO3 ok)
4. pH 7.50, PaCO2 42, HCO3 33		Metabolic alkalosis (pH   , PaCO2 ok, HCO3 )