Arterial Blood Gas

A Brief Discussion on Blood Gases Often, you will hear someone refer to "blood gas levels" of the patient. These levels are used to determine the patient's acid/base status and blood oxygenation levels, and therefore help determine the extent to which someone may be in pH crisis. One way to measure these levels is by drawing arterial blood and measuring these levels. Arterial blood contains all the information needed to determine the exact pH and the component that is imbalanced. When pH is imbalanced, patients can experience acidosis or alkalosis. Acidosis results from a pH that is too acidic (less than 7.0), while alkalosis results from a pH that is too basic (greater than 7.0). The degree to which someone is either acidotic or alkalotic depends on the presence of two main compounds in the body: HCO3- and CO2. -- HCO3- levels in the blood indicate a response by the kidneys -- CO2 levels in the blood indicate a response from the lungs By watching the degree to which these two compounds shift, we can determine the cause for the patient's distress. Blood gases can be drawn at different times during the crisis to determine whether or not the body is responding to treatment. The various phases of the acid/base problem can be categorized as: · Uncompensated phase - The onset of the problem · Partial compensation phase - Shows which system and to what extent the compensatory mechanism is responding · Full compensation phase - Shows what systems are involved in the correction of the body's pH imbalance

Respiratory Blood Gas Imbalances Respiratory Acidosis: Uncompensated Phase With respiratory acidosis, the uncompensated phase would look something like this: PH

PaCO2

HCO3­

--1. pH is low because of the existing acidosis 2. PaCO2 shows circulating CO2 is high, and as CO2 is related directly to the respiratory system,  this indicates respiratory acidosis. 3. Since there is no HCO3­, there is no response yet from the kidneys to the acidosis. 

Respiratory Acidosis: Partial Compensated Phase With the partial compensated phase, all the values are out of balance and the kidneys begin to  respond. PH

PaCO2

HCO3­

1. pH is still acidotic. 2. PaCO2 is high and shows that the respiratory system is still the reason for the acidosis. 3. The kidneys are starting to respond to the acidosis by increasing the amount of circulating  HCO3­. 

Respiratory Acidosis: Full Compensation Phase With full compensation, the pH returns to a low normal range ­ which was the goal in the first  place, but PaCO2 and HCO3­ levels are still high. PH

PaCO2

---

1. pH has returned to normal. 2. PaCO2 is still high due to hypoventilation. 3. HCO3­ is high to correct the acidosis. 

HCO3­

Respiratory Alkalosis: Uncompensated Phase With respiratory alkalosis, the uncompensated phase would look something like this: PH

PaCO2

HCO3­

---

1. pH is high because of the alkalosis. 2. PaCO2 shows circulating CO2 is low, and as CO2 is related directly to the respiratory system,  this indicates respiratory alkalosis. 3. Since there is no HCO3­, there is no response yet from the kidneys to the alkalosis. 

Respiratory Alkalosis: Partial Compensated Phase During the partial compensated phase, the kidneys begin to respond. PH

PaCO2

HCO3­

1. pH is still alkalotic. 2. Low PaCO2 levels show that the respiratory system is still the reason of the alkalosis.  3. The kidneys are starting to respond to the alkalosis by decreasing the amount of circulating  HCO3­. 

Respiratory Alkalosis: Full Compensation Phase During full compensation, pH returns to a high normal range ­ which was the goal in the first  place, but PaCO2 and HCO3­ levels remain abnormal. PH

---

PaCO2

HCO3­

1. pH has returned to normal. 2. PaCO2 is still low due to hyperventilation. 3. HCO3­ is still low to correct the alkalosis. 

Metabolic Blood Gas Imbalances Metabolic Acidosis: Uncompensated Phase With metabolic acidosis, the uncompensated phase would look something like this: PH

PaCO2

HCO3­

---

1. pH is low because of the acidosis. 2. PaCO2 shows no changes indicating the lungs aren't the reason for the acidosis. 3. HCO3­ levels are already low pointing to the cause of the acidosis ­ metabolic. 

Metabolic Acidosis: Partial Compensated Phase During the partial compensated phase, all the values are out of balance and the lungs begin to  respond by blowing off CO2. PH

PaCO2

HCO3­

1. pH is still acidotic. 2. Low PaCO2 levels show the respiratory system is responding to the acidosis by decreasing the  amount of circulating CO2. 3. HCO3­ levels are still high, indicating this is a metabolic problem. 

Metabolic Acidosis: Full Compensation Phase With full compensation, the pH returns to a low normal range ­ which was the goal in the first  place, but PaCO2 and HCO3­ levels remain abnormal. PH

---

PaCO2

HCO3­

1. pH has returned to normal. 2. PaCO2 is still low due to hyperventilation. 3. HCO3­ is high to correct the acidosis. 

Metabolic Alkalosis: Uncompensated Phase With metabolic alkalosis, the uncompensated phase would look something like this: PH

PaCO2

HCO3­

---

1. pH is high because of the alkalosis. 2. PaCO2 shows no changes, indicating that the lungs aren't the reason for the alkalosis. 3. HCO3­ levels are already high, pointing to the cause of the alkalosis. 

Metabolic Alkalosis: Partial Compensated Phase During the partial compensated phase, all the values are out of balance and the lungs begin to  respond by blowing off CO2. PH

PaCO2

HCO3­

1. pH is still alkalotic. 2. PaCO2 shows the respiratory system is responding to the alkalosis by decreasing the amount  of circulating CO2. 3. HCO3­ levels are still high, indicating this is a metabolic problem. 

Metabolic Alkalosis: Full Compensation Phase With full compensation, the pH returns to a high normal range ­ which was the goal in the first  place, but PaCO2 and HCO3­ levels remain abnormal.

PH

PaCO2

---

1. pH has returned to normal. 2. PaCO2 is still low due to hyperventilation. 3. HCO3­ is high to correct the acidosis. 

HCO3­