Blood Gas Interpretation

Lecture 21

Define homeostasis.

the maintenance of constant conditions through dynamic equilibrium of the ineternal environment of the body

Homestasis is regulated by which organs/body systems?

lungs

kidneys

liver/GI

Which type of animals produce carbon dioxide and excess H+ precursors?

carnivores

Which type of animals produce carbon dioxide and excess HCO3- precursors?

herbivores

What are the three principal mechanisms to buffer H+?

chemical (bicarbonate, phosphate, hemoglobin and proteins)

respiratory system

renal system

What is the Henderson Hasselbalch equation?

pH = pKa + (log [HCO3-] / [0.0301 * pCO2])

What are the four primary acid base disturbances?

metabolic acidosis

metabolic alkalosis

respiratory acidosis

respiratory alkalosis

What abnormality causes metabolic acidosis?

decreased HCO3-

What abnormality causes metabolic alkalosis?

increased HCO3-

What abnormality causes respiratory acidosis?

increased CO2

What abnormality causes respiratory alkalosis?

decreased CO2

What compensatory response should be expected for cases of metabolic acidosis?

decreased CO2

What compensatory response should be expected for cases of metabolic alkalosis?

increased CO2

What compensatory response should be expected for cases of respiratory acidosis?

increased HCO3-

What compensatory response should be expected for cases of respiratory alkalosis?

decreased HCO3-

How should you approach acid base abnormalities?

What is the pH? —> acidosis or alkalosis

What is causing the abnormality? —> CO2 = respiratory, HCO3- = metabolic

What are consequences and how do they affect treatment plan?

Look at entire blood gas

If CO2 is causing the acid base disturbance, what systemic is responsible?

respiratory

If HCO3- is causing the acid base disturbance, what systemic is responsible?

metabolic

What are mixed distubances?

two separate disorders occurring in a patient at one time

What may cause respiratory acidosis (things that increase CO2)?

pleural space disease, pneumothorax, severe pulmonary disease

upper airway obstrucion

neurologic disease

anesthetic drugs and equipment dead space

decreased functional residual capcity due to pregnancy ot full stomach/rumen

hyperthermia

respiratory arrest

What may cause respiratory alkalosis (things that decrease CO2)?

pain, fear, anxiety, stress

hypotension, low cardiac output

pulmonary thromboembolism

overzealous IPPV

respiratory disease

hypoxemia, decreased O2 delivery

fever/hyperthermia (hyperventilating)

What may cause metabolic acidosis (things that decrease HCO3-)?

vomiting, diarrhea

increase in other acids due to disease (lactic, uremic, phosphoric)

renal loss of HCO3- or retention of H+

dilutional acidosis

What may cause metabolic alkalosis (things that increase HCO3-)?

vomiting or sequestration of acids

hypochloremia and hypokalemia

What are consequences of acidosis?

impairs cardiac contractility and response to catecholamines

ventricular arrythmias or fibrillation

arterial vasodilation and venous constriction

shift oxygen-hemoglobin curve to the right initially

insulin resistance that imapirs uptake of glucose

increased iCa2+

CNS depression and coma

osteodystrophy and hypercalicuria

What are consequences of alkalosis?

CNS signs (agitation, disorientiation, stupor, coma)

seizures or tetany due to hypocalcemia

hypokalemia —> muscle weakness, cardiac arrythmias, GI motility disturbances, and altered renal function

shift oxygen-hemoglobin curve to the left, which impairs oxygen release from hemoglobin initially

Why do we need to know about blood gas analysis?

tool for monitoring respiratory and acid base status in a patient

What type of blood gas sample is used to evaluate the respiratory gas exchange?

arterial

What type of blood gas sample is used to determine metabolic status?

venous

What are the characteristics of an arterial sample?

PaO2 80-110 mmHg on room air or 500 mmHg if on 100% O2

SaO2 > 88%

bright cherry red color

pulsatile flow

What are the characteristics of a venous sample?

PvO2 35-45 mmHg regardless of FiO2

SvO2 65-75%

darker color red

no pulsatile flow

Where can you get an arterial sample for blood gas in small animals?

dorsal pedal artery

auricular artery

femoral artery

caudal (tail) artery

lingual artery

Where can you get an arterial sample for blood gas in large animals?

facial artery

transverse facial artery

lateral dorsal metatarsal artery

auricular artery

lingual artery

Explain the steps to obtain a sample for blood gas.

clip and clean site

dry lithium heparin syringe or heparinize a 1-3mL syringe with a 22-25 gauge needle by pulling up liquid sodium heparin the putting it back in the bottle. Aspirate air and rapidly expel a few times before filling with 1mL of blood.

get rid of any air bubbles quickly and analyze sample immediately or place rubber stopper on needle and store on ice up to 1 hr

apply pressure to sampling site so a hematoma does not form

What do blood gas analyzers directly measure?

pH

partial pressures of oxygen (PO2)

partial pressure of carbon dioxide (PCO2)

What do blood gas analyzers measure by calculation?

HCO3-

Base Excess (BE)

SaO2

What blood gas value is the H+ concentration in the extracellular fluid?

pH

What is the equation for pH?

log [1/[H+]]

What blood gas value is the amount of oxygen molecules dissolved in the plasma of a sample (i.e. not bound to hemoglobin), depends on FiO2?

PO2

What blood gas value is the reflection of the respiratory component of acid-base balance, used to determine if patient is hypocapnic, hypercapnic, or eucapnic; is inversely related to alveolar ventilation?

PCO2

What is mainly responsible for regulating the pH of the body fluids, faciliates the transfer of carbon dioxide from the body tissues to the lungs, and is an assessment of the metabolic component of acid-base status?

bicarbonate (HCO3-)

What value represents the amount of carbon dioxide gas present in the plasma?

total carbon dioxide

What percent of TCO2 is due to actual bicarbonate?

85%

What percent of TCO2 is from carbonic acid?

10%

What percent of TCO2 is carbon dioxide in solution?

5%

What is the amount of strong acid or base required to titrate 1L of blood to a pH of 7.4 at 37C while the partial pressure of CO2 is constant at 40mmHg?

base excess

What value is used to calculate bicarbonate therapy?

base excess

Base excess indicates metabolic alkalosis or metabolic acidosis?

alkalosis

Base deficit indicates metabolic alkalosis or metabolic acidosis?

acidosis

What is considered mild base excess?

±5

What is considered moderate base excess?

±10-15

What is considered severe base excess?

>15

What value is the percentage of all available heem-binding sites saturated with oxygen from an arterial sample is a calculated value based on the position on the oxygen hemoglobin dissociation curve and PaO2?

SaO2

What is the normal value for arterial blood pH?

7.35-7.45

What is the normal value for arterial blood PaCO2?

35-45 mmHg

What is the normal value for arterial blood paO2?

80-110 mmHg (room air)

What is the normal value for arterial blood HCO3-?

15-25 mmol/L (carnivore)

20-28 mmol/L (herbivore)

What is the normal value for arterial blood base excess?

0 ± 4 mmol/L

What is the normal value for arterial blood SaO2?

95-100%

What is the normal value for arterial blood lactate?

< 2.0 mmol/L

What may effect sample accuracy?

air bubbles

excess heparin

delay in analysis

blood clot in sample; hemolysis; collecting from catheter

syringe type

temperature and barometric pressure

What is defined as decreased PaO2, SaO2, or hemoglobin content; the amount of oxygen in the blood determines the severity?

hypoexmia

What is defined as the general term for impairment of oxygen delivery to tissue; takes into account cardiac output and oxygen uptake at tissue level?

hypoxia

What are causes of lung related hypoxemia?

ventilation/perfusion (V/Q) mismatch

hypoventilation

Low FiO2

right to left shunt

diffusion impairment

What is the most common cause of hypoxia?

V/Q mismatch