Arterial blood gases Flashcards
ABGs - when are they used and what do they measure
- Done in acute care
- Provide a single static picture of patients conditions
○ Gas exchange function of lungs
○ Diagnosis of respiratory failure
○ Acid-base disorder
○ Determine eligibility for long term O2 use
Repeatedly, can show if condition is improving or declining
- Provide a single static picture of patients conditions
Metabolic acidosis vs alkalosis
Metabolic acidosis: increased blood H+ from loss of HCO3 or infusion of a fixed acid
Metabolic alkalosis: decreased blood H+ by excessive loss of fixed acid or infusion of HCO3
Respiratory alkalosis vs acidosis
Respiratory acidosis: increased blood H+ from hypoventilation
○ From increased CO2
Respiratory acidosis: decreased blood H+ from hyperventilation
- Due to decreased CO2
Compensation vs uncompensated
- Uncompensated: pH is abnormal and only 1 parameter (PaCO2 or HCO3 is abnormal)
- Partial compensation: pH is abnormal but both PaCO2 and HCO3 are abnormal
Fully compensated: pH is normal but both PaCO2 and HCO3 are abnormal
- Partial compensation: pH is abnormal but both PaCO2 and HCO3 are abnormal
Type 1 respiratory failure
hypoxemic respiratory failure
a. Low O2 level in arterial blood (PaO2 <55) with no increase in CO2
Common in conditions that damage lung tissue
Type 2 respiratory failure
hypercapnic respiratory failure
a. High CO2 (PaCO2 > 50) usually with low O2 too
Common in conditions that decrease alveolar ventilation
Causes of hypoxemia
impaired oxygenation, low haemoglobin, reduced affinity of haemoglobin for O2
Impaired oxygenation
reduced transfer of O2 from lungs to blood stream identified by low arterial O2
From impaired gas exchange, reduced fraction of inspired O2, hypoventilation or V/Q mismatch
How to assess hypoxemia
- SpO2/SaO2: pulse oximetry (SpO2) and arterial oxygen saturation (SaO2)
a. Do not take into account quantity of haemoglobin (anemia)
b. Normally 95-100%, below 85% is critical
c. Low pH, high temp, increased Co2 can reduce affinity to Hb and increase O2 delivery to tissues - PaO2: partial pressure of oxygen in arterial blood
a. Reflects how well oxygen is able to move from lungs to blood
b. Indicator of V/Q abnormality
Normally 80-100mmHg
FiO2
fraction of inspired oxygen
a. Percent of oxygen a person inhales
If over 21% (atmospheric O2) then its from a therapy
P/F ratio
PaO2/FiO2)
a. Good indicator of gas exchange in patients with supplemental oxygen
b. Normal is around 400-500 (<300 indicated hypoxemia)
Pulse oximetry - what is it and what do you have to be aware of?
uses infrared emission and absorption to measure SpO2
Caution:
○ Does not account for anemia
○ Dye procedures affect transmission
○ Is site adequately perfused?
○ Shock
○ Hypothermia
○ Meds that constrict blood vessels
- Errors from nail polish, poor fitting of sensor and light interference
Indications for O2 therapy
- Hypoxemia
a. Continuous O2 indicated with
i. Resting PaO2 <55mmHg
ii. Resting PaO2 off 55-59 or SaO2 less or equal to 89% with certain CP conditions
b. Discontinuous O2 indicated with
i. PaO2 of less or equal to 55 or SaO2 less or equal to 88% - Increased myocardial work
- Increased work of breathing
- Pulmonary hypertension
Low flow systems
Amount of O2 delivered is affected by breathing pattern (rate and depth)
- Minute ventilation can outpace supply of O2
- Nasal canula
- Face mask
- Oxygen tent
- Partial rebreathing mask
- Non rebreathing mask
High flow system
amount of O2 delivered is not affected by breathing pattern
Venturi mask
Nasal Canula
tube with two hollow prongs that fit into nose and other end attaches to oxygen flow meter
- Increase in flow rate increases FiO2
○ 1L/min increases FiO2 3%
○ Can deliver FiO2 between 24 - 44%
Face/oxygen mask
nose and mouth mask attaching to oxygen source
- Expired air goes out from expiration ports
- Flow rate 5-10 L/min
○ If flow <5L/min then exhaled air accumulates and the high Co2 air is rebreathed
○ Little increase in FiO2 over 8L/min
- Provides FiO2 40-60%
Oxygen tent
loosely attached tent over nose and mouth when patient has face trauma, burns, etc
- Mixture of inspired and expired air
Dificult to determine FiO2
Non rebreathing mask
one way valve that prevents exhalation into bag
- Flow rate >10L/min
- FiO2 60-80% (theoretically 100%)
○ Does not let atmospheric air in so close to 100%
- Risk of suffocation if oxygen source finishes unknowingly
Usually for short term high O2 requirement s (pre-intubation, patient transport
Venturi mask
high flow device that delivers precise measurement of O2
- FiO2 24-50%
- Flow rate 4-15L
○ Precise flow rates are set for each venturi device to achieve set FiO2
Used in COPD patients
Hemodynamic Instability
The state of unstable BP with resultant insufficient blood flow
- Lack of required pressure to keep blood flowing in the circulatory system
- Clinical state indicative of a failing circulatory system
Aka perfusion failure, circulatory failure, hypoperfusion
