Exam 3: Lecture 21 - Blood Gas Interpretation Flashcards
What is homeostasis?
-Maintenance of constant conditions through dynamic equilibrium of internal environment of body
What regulates homeostasis?
-Lungs
-Kidneys
-Liver/GI
Individual differences in amount of CO2 and excess H+ produced are influenced by:
-Species
-Diet
-Cellular basal metabolic rate
-Total protein
-Strong ions
-Body temperature
What are the differences in amount of CO2 & excess H+ produced between carnivores & herbivores?
-Carnivores -> produce CO2 & excess H+ precursors
-Herbivores -> produce CO2 & excess HCO3- precursors
What is the gas homeostasis equation?
CO2 + H2O <-> H2CO3 <-> H+ HCO3-
What are the 3 principal mechanisms to buffer H+?
-Chemical (extracellular buffering by bicarb works within seconds; phosphate, hemoglobin & proteins are intracellular buffers that work within 24 hours)
-Respiratory (Chemoreceptors in body monitor changes in [H+] & pCO2 to adjust resp. pattern & works within minutes to hours)
-Renal (increased renal excretion of H+ takes hours to days)
Many approaches to the diagnosis & treatment of acid-base disorders are based on what equation?
Henderson-Hasselbalch Equation
How many primary disturbances are there for acid-base abnormalities?
-4
What do we expect to see with a primary metabolic acidosis?
-Decreased pH
-Decreased HCO3- (primary)
-Decreased CO2
What do we expect to see with a primary metabolic alkalosis?
-Increased pH
-Increased HCO3-
-Increased CO2
What do we expect to see with a primary respiratory acidosis?
-Decreased pH
-Increased HCO3-
-Increased CO2
What do we expect to see with a primary respiratory alkalosis?
-Increased pH
-Decreased HCO3-
-Decreased CO2
What is a mixed disturbance?
-Two separate primary disorders occurring in a patient at one time
What are examples of a mixed disturbance?
-pCO2 & HCO3- changing in opposite directions
-Normal pH w/ abnormal pCO2 and/or HCO3-
-pH change in opposite direction to that predicted for primary disorder
What can disorders do to pH?
-Can have a neutralizing or additive effect on pH
What can a “triple disorder” be caused by?
-Metabolic acidosis, metabolic alkalosis, and either respiratory acidosis or alkalosis
What is one of the “other” methods to diagnose acid-base disorders?
Stewart’s approach determined by “independent variables”:
-PCO2
-Strong ion difference (SID) - Na+, K+, Cl-, Ca2+, Mg2+
-Total concentration of nonvolatile weak acid (Atot)
____ is more informative that H-H equation if mixed acid-base disorders & electrolyte disturbances co-exist
Stewart’s approach
What are causes of a respiratory acidosis?
-Pleural space disease, pneumothorax, severe pulmonary disease
-Upper airway obstruction
-Neurologic disease (central or peripheral)
-Anesthetic drugs & equipment dead space
-Decreased functional residual capacity (pregnancy or full stomach/rumen)
-Malignant hyperthermia
-Cardiopulmonary arrest
What are causes of respiratory alkalosis?
-Pain, fear, anxiety, stress (vet student feels)
-Hypotension, low cardiac output
-Sepsis or SIRS
-Pulmonary thromboembolism
-Overzealous IPPV
-Respiratory disease
-Hypoxemia
-Fever/hypothermia
-Severe anemia
What are causes of metabolic acidosis?
-Vomiting, diarrhea
-Renal loss of HCO3- or retention of H+
-IV nutrition
-Dilutional acidosis
-Ammonium chloride
-Hypomineralocorticism
What are causes of metabolic alkalosis?
-Vomiting due to pyloric obstruction
-Hypocholeremia & hypokalemia
-Furosemide
-Hypermineralocorticism
-Contraction alkalosis
What are some consequences of acidosis?
-Impairs cardiac contractility & response to catecholamines -> decreased cardiac output -> decreased renal & hepatic blood flow
-Ventricular arrhythmias or fibrillation
-Atrial vasodilation & venous constriction -> centralizes blood volume & causes pulmonary congestion
-Shifts oxygen-hemoglobin curve to right initially
What are consequences of alkalosis?
-CNS signs (agitation, disorientation, stupor, coma)
-Seizures or tetany due to hypocalcemia (rare)
-Hypokalemia due to transcellular shifting causes muscle weakness, cardiac arrhythmias, GI motility disturbances, & altered renal function
-Shifts oxygen-hemoglobin curve to the left, which impairs oxygen release from hemoglobin initially
What do we use an arterial vs. venous blood sample for when sampling for acid base?
-Arterial sample used to evaluate respiratory gas exchange
-Venous sample useful in determining acid-base status
Venous blood has a slightly ___ pH & ____ pCO2 than arterial blood due to local tissue metabolism
-slightly lower pH, higher pCO2
How do you know if it is an arteriole sample?
-PaO2 ~80-110 mmHg on room air or ~500 mmHg if on 100% O2
-SaO2 > 88%
-Bright cherry color
-Pulsatile flow if catheter placed & arterial waveform present when attached to pressure transducer
How do you know if it is a venous sample?
-PvO2 ~35-45 mmHg regardless of FiO2
-SvO2 65-75%
-Darker color of red
-No pulsatile flow from catheter & no arterial waveform present when attached to pressure transducer
What is needed to take an acid base sample (steps)?
- Clip & clean site to be sampled
- Dry lithium heparin syringe or heparinize 1-3 mL syringe w/ 22-25 g needle
- Get rid of air bubbles quickly & analyze sample immediately (<10 min) or place rubber stopper on needle & store on ice (up to 1hr)
- Apply pressure to sampling site so hematoma does not form
Where can we obtain an acid base sample in small animals?
-Dorsal pedal artery
-Auricular artery
-Femoral artery
-Caudal artery
-Lingual artery or vein
Where can we get an acid base sample in large animals?
-Facial artery (e.g horses, donkeys)
-Transverse facial artery (horses, donkeys)
-Lateral dorsal metatarsal artery
-Auricular artery (e.g. ruminants)
-Lingual artery
-Femoral artery
-Median artery (e.g. sheep)
Blood gas analyzers directly measure
-pH
-Partial pressures of oxygen (PO2)
-Partial pressure of carbon dioxide (PCO2)
Blood gas analyzers calculate
-HCO3-
-BE
-SaO2
What does the blood gas value pH tell us?
-Reflects overall balance of acid & base producing processes in body & the H+ concentration in extracellular fluid
One unit change in pH causes a ____ in [H+]
10-fold increase or decrease in [H+]
What does the blood gas value PaO2 tell us?
-Oxygen molecules dissolved in the plasma phase of an arterial sample (i.e. not bound to Hb), depends on FiO2 & barometric pressure
What does the blood gas value PaCO2 tell us?
-Reflection of respiratory component of acid-base balance, used to determine if patient is hypocapnic, hypercapnic, or eucapnic
PaCO2 is inversely related to
Alveolar ventilation
Bicarbonate is mainly responsible for regulating ____ & acts as ____
-The pH of body fluids
-Acts as immediate buffer when fixed acids enter blood
Bicarbonate facilitates the transport of ____ from the body tissues to the lungs
CO2
What is total carbon dioxide (TCO2)?
Amount of carbon dioxide gas present in plasma
-85% due to actual bicarbonate
-10% from carbonic acid
-5% CO2 in solution
What is BE?
-Base Excess
-Amount of strong acid or alkali required to titrate 1L of blood to a pH of 7.4 at 37C while partial pressure of CO2 is constant at 40 mmHg
How is the BE in venous or arterial blood samples?
-Value is identical in venous or arterial blood sample
Base excess =
Base deficit =
Base excess = metabolic alkalosis
Base deficit = metabolic acidosis
BE is used to calculate
Bicarbonate therapy
A BE of ___ is mild, ____ is moderate, ___ is severe
Mild = +/- 5
Moderate = +/- 10-15
Severe = > 15
What is SaO2?
-Percentage of all available heme-binding sites saturated w/ oxygen from arterial sample
-Calculated value based on position on the oxygen hemoglobin dissociation curve & PaO2
What are the normal values for arterial blood?
-pH = 7.35-7.45 (7.4)
-PaCO2 = 35-45 mmHg (40 mmHg)
-PaO2 = 80-110 (mmHg) room air (100)
-HCO3- = 15-25 mmol/L carnivore, 20-28 mmol/L herbivore (24 + 1-4)
-BE = 0 +/- 4
-SaO2 95-100%
-Lactate < 2.0 mmol/L
What is the 1st step when determining acid/base status?
Determine if sample is arterial or venous
-SaO2 > 88% = Arterial
-SaO2 < 88% = Mixed sample, Venous, or Pulmonary disease
What is the 2nd step when determining acid/base status?
Determine acid/base status of the patient
-pH = normal, acidemia, or alkalemia
-pCO2 = normal, increased or decreased
-HCO3- = normal, increased or decreased
Is primary problem respiratory or metabolic?
Any compensation occurring?
What is step 3 of determining acid/base?
Assess ventilatory status (PaCO2)
-Hypoventilation = increased PaCO2
-Hyperventilation = decreased PaCO2
-Normal ventilation
What is step 4 of determining acid/base?
Assess how the animal is oxygenating
-Is the patient breathing room air?
-Is the patient on an FiO2 > 0.21?
How do we interpret PaO2:FiO2 ratio?
> 400 = normal pulmonary functino
200-400 = Decreased pulmonary function
<200 = Severe pulmonary dysfunction; ARDS
What is step 5 in determining acid/base status?
-Determine the Anion Gap
-Normal = 12-24 mEq/L (dogs); 13-27 mEq/L (cats)
What can affect the accuracy of an acid/base sample?
-Air bubbles = increased PaO2 & decreased PaCO2
-Excess heparin = decreased pH
-Delay in analysis = decreased PaO2 & pH; increased PaCO2
-Blood clot in sample (hemolysis)
-Syringe type (glass preferred, plastic ok if analyzed within 10 minutes)
-Temperature & barometric pressure (hyperthermia artificially lowers PaO2 & PaCO2, hypothermia artificially elevates PaO2 & PaCO2)
What is hypoxemia?
-Decreased PaO2, SaO2 or hemoglobin content
-Amount of oxygen in the blood (CaO2) determines severity
What is hypoxia?
-General term for impairment of oxygen delivery to tissue (DO2)
-Takes into account cardiac output (CO) & oxygen uptake at tissue level
-Therefore, hypoxemia is one type of hypoxia
What are 5 causes of hypoxemia?
- Ventilation/perfusion (V/Q) mismatch
- Hypoventilation
- Low FiO2
- Right to left shunt
- Diffusion impairment (less common in vet med)
Why would we want to use the oxygen content (CaO2) equation in our patients?
-CaO2 directly reflects the total number of oxygen molecules in arterial blood (both bound & unbound to hemoglobin)
-Want to know your patient has enough hemoglobin to deliver oxygen to the tissues
Which patient is more hypoxemic, and how do we know?
Patient A: Hb= 7 g/dL; SaO2 = 95%; PaO2 = 80 mmHg
Patient B: Hb= 15 g/dL; SaO2 = 85%; PaO2 = 55 mmHg
-Patient A
-Because lower hemoglobin (who’s your daddy)
