Blood Gas & Pleural Effusion

specimens for BG and CAI

BG: heparinized syringe

CAI, MetHgb, carboxyHgb, KWB, NaWB: green top (lithium or sodium heparin)

BG sources

arterial: m/c, most accurate for pO2 & pH

venous: easy to collection, not good for pO2

mixed venous: pulmonary catheter

capillary: babies (=~ arterial)

rejection criteria for BG/CAI?

1. room temp

2. air contamination

3. samples not on ice

RT causes

falsely low pO2, glucose, pH

• RT → glycolysis of rbc, rbc uses O2 & glc → produce lactic acid → acidic pH

falsely high pCO2, lactate, Ca++

• CO2 is inv prop to O2

• Ca++ is inv prop to pH

air contamination causes

f-low pCO2, Ca++

• CO2 inv of O2 → CO2 is acidic but its low so pH is high

• Ca++ low

f-high pO2, pH

• bubble → high O2,

• pH is high, so Ca is low

samples not on ice → ?

Ca++ = pH dependent

each 0.1 unit change in pH → 5% change in Ca++

pH and Ca++ are __

inversely related

high pH: inc ionization, neg charge on albumin → inc in protein bound Ca++, dec in free/ionized Ca++

low pH: dec ionization & neg charge on albumin, dec protein bound Ca++ & inc ionized Ca++

room air is ___? how to convert LPM to FiO2?

21% FiO2

FiO2 = (LPM*4) + 20

heparin is used bc

only anticoag for Ca++, doesn’t lower ionized Ca++

which tubes are rejected?

grey: affect glucose & lactate

EDTA: affects pH, Na, K. Cl, Ca

KWB is usually

ordered for cardiac pt (K+ = muscle contraction)

pH fluid

pleural fluid - the only acceptable fluid

to dx exudative pleural effusion

FiO2

fraction of inspired O2

conc of O2 that pt inhales

pCO2

partial pressure of CO2

resp status of a pt → determine hyper/hypoventilation

PO2

arterial O2 partial pressure

evaluation of oxygenation status → evaluate degree of hypoxemia in pt sample

HCO3

bicarbonate

calculated based on pH & pCO2

metabolic component of ABG

determine non-resp, renal component acid-base disorders

BEvt

base excess - amount of base

needed to bring the pH back to normal

assess metabolic portion of acid-base balance

inc - metabolic alkalosis

dec - “ “ acidosis

ctCO2 (CO2 count)

distinguish respiratory & metabolic acid-base DO when eval’d w pH & CO2

ctO2

• what ?

• purpose?

O2 content of Hgb, vol of O2 actually bound

O2Sat +O2 capacity = amount O2 available to tissues

eval effectiveness of O2 therapy

sO2 (%saturated O2)

ratio of vol O2 carried/max volume of O2 that Hgb can carry

when combined w ctO2 & O2 capacity, it’s useful in determining amount of O2 that’s available to tissues

• effectiveness of O2 therapy

what is used to calculate anion gap?

Na, Cl, HCO3

determines cause of metabolic acidosis

glucose is ___ in diabetes

high

low - insulin overdose

lactic acid

prod by muscle cells, rbc, brain, other tissues during an?aerobic production, usually low in blood

• helps detect hypoxia & other conditions that cause xs production of lactic acid

• lactic acidosis - disrupts pH balances

• symptoms: muscle weakness, rapid breathing, nausea, vomiting, sweating, coma

tHb

total of all measured Hgb fractions

assessment of O2 transport

eval of anemia

O2Hb

oxyhgb

fraction of hgb that is reversibly bound to o2

COHb (carboxyhemoglobin)

fraction of Hb covalently bound to carbon monoxide

• CO is 200X greater affinity than O2 to Hb

MetH

unable to bind O2

HHb

reduced hgb, still capable of binding O2

PF ratio (pO2/FiO2)

PFR = index of efficiency of pulmonary O2 exchange that relates the arterial pO2 to fraction of inspired O2

• amount of O2 that’s going to tissues vs what is in the environment

• to assess severity of hypoxemia

• used as a tool to be able to trend progression of resp failure

PF ratio range

>=400 = normal

<300 = acute resp failure

RapidPoint 500 system principle

analyte of interest in sample + sensor → electrochem signal = amount analyte in sample

what is measured by potentiometry?

pH, Na, K, Cl, Ca, pCO2

measures difference in potential b/t 2 electrodes w/o applied current

what is measured by amperometry

glucose, lactate, pO2

involves application of current of voltage to an electrode then measuring current generated

what is conductance?

readiness of transmission of electric current by conducting substance (opposite of resistance)

co-oximetry measurement tech

measures Hgb & its derivatives

meas light from WB from several wavelengths

detects & quantitates total hgb & its derivatives in sample

pleural fluid pH is used in dx of

exudative pleural effusions

what is pleural fluid

pleura: double-layered serous membrane surrounding lungs, prevents friction when lungs & chest walls slide

what is pleural effusion?

xs fluids in pleura

transudative pleural effusions

from imbalance in hydrostatic & oncotic pressure in normal production of pleural fluid

exudative pleural effusion

from changes w/in pleura ie lymphatic blockage or inc capillary permeability, caused by infn, inflam, or neoplastic dz that don’t originate from pleura ie:

• heart failure, pneumonia, esophageal rupture, TB,

• rhematoid dz, cancer of breast, lung, ovary

---

if pH < 7.3 → always exudative (acidic)

• dx is narrowed down to emphysema, malignancy, rheumatoid pleurisy, lupus, pleuritis, TB, esophageal rupture

• seen in complicated parapneumonic effusions and are exudative in nature, empyema (pus)

if pH is >7.3 (alk), the pleural effusion is

uncomplicated parapneumonic - arises from pneumonia, lung abscess, or bronchiectasis

transudative vs exudative pleural effusion

transudative

• inc hydrostatic P or low plasma oncotic P

• ie CHF, cirrhosis, nephrotic syndrome, PE, hypoalbunemia

• low in protein & LDH

---

exudative

• due to inflam & inc capillary permeability

• pneumonia, cancer, TB, virus, PE, autoimmune

• high in protein & LDH

• dec glucose, inc wbc, inc amylase