PE Exam 3 (part 3)

Carbon monoxide (CO) affinity

240 times greater affinity for hemoglobin than oxygen.

Effect of CO on oxyhemoglobin curve

Binds with Hgb and shifts the curve to the left.

PO2 levels in CO poisoning

Patient can be hypoxic even with a normal PO2.

Adult vs. fetal hemoglobin structure

Adult is HgbA; fetal is structurally different as HgbF.

Fetal hemoglobin curve shift

Fetal curve shifted to left facilitates uptake of oxygen by placenta at very low PO2's (27 mmHg)

P-50 Definition

Measurement of PO2 at 50% Hb saturation

P-50 Standard Conditions

37 ºC, PCO2 40 mmHg, pH 7.40

Significance of P-50

Most sensitive indicator of a curve shift by factors other than pH, temperature, and PCO2

Low P-50

Left shift - high affinity

High P-50

Right shift - low affinity

Normal P-50 value

27 mmhg

Normal CaO2 value

16-20 mL/dL

Components of CaO2

consists of O2 bound to hemoglobin and O2 physically dissolved in the plasma.

CaO2 calculation formula

(1.34 × Hb × SaO2) + (0.003 × PaO2)

Greatest factor affecting CaO2

Hemoglobin (Hb)

CaO2 in anemic patients

Low CaO2 leading to tissue hypoxia despite normal PaO2 and SaO2. (The patient has less hemoglobin available to carry oxygen, even though the available hemoglobin is adequately saturated

CaO2 measurement requirement

Requires a co-oximeter to read true results.

CaO2 effect from other gases like CO

CaO2 is low when other gases are attached to Hgb, even if Hgb and SaO2 appear normal.

Polycythemia - PaO2 and SaO2 low, CaO2 normal

Hgb is compensating (Polycythemia does not normally cause low PaO₂ or SaO₂ by itself. Those values may be low if the patient also has a condition causing hypoxemia)

Can a patient have a normal A-aDO2 and still be hypoxic?

Yes, such as a patient that is hypoventilating or has airway obstruction.

Example of normal A-aDO2 with hypoxia

If the PAO2 = 60 mm Hg and the PaO2 = 50 mm Hg, the A-aDO2 = 10 mm Hg.

Significance of a normal A-aDO2

It only tells us that the V/Q matching is optimal.

PvO2 Normal Value

35-45 mmHg

PvO2 Sampling Site

Obtained from the pulmonary artery; reserved for ICU patients.

Significance of PvO2

Good representation of tissue oxygenation and an indicator for hypoxia.

Oxygen Delivery vs. Consumption

Oxygen delivery is a function of cardiac output and CaO2; consumption is a function of tissue extraction.

Arterial vs. Venous Information

Arterial provides information on oxygen supply; venous provides information on the balance of oxygen supply and demand.

PvO2 Determinants

Reflects O2 delivery to tissues and the rate of O2 consumption.

Effect of Cardiac Output on PvO2

Decreased CO with constant extraction decreases PvO2; increased CO increases PvO2.

C(a-v)O2 Normal Value

3.5 - 5.0 vol%

C(a-v)O2 greater than 5.0%

Indicates tissues are taking more oxygen.

C(a-v)O2 less than 3.5%

Indicates tissues are taking less oxygen.

HbCO Normal Value

Less than 3%

Measurement method for HbCO

Co-oximeter

HbCO affinity vs O2

200-250 times more affinity for hemoglobin than oxygen.

Physiological problems caused by HbCO

CO inhalation = tight bond between CO and Hb = shift dissociation curve to the L = decrease unloading of O2 to the tissues which promotes tissue hypoxia

HbCO levels from cigarette smoking and pollution

5-10%

HbCO levels and symptoms from gas fumes in closed spaces

10-40%; causes headache, dyspnea, nausea, tachycardia, and tachypnea.

HbCO levels and symptoms from smoke inhalation or fires

Greater than 40%; causes visual impairment, myocardial disturbances, LOC, and death.

Definition of pH

Measurement of H+ ions in the plasma

Impact of pH unit changes

Changes in pH of 1 unit represent a tenfold change in [H+]

Function of acids

Donate H+ which decrease pH

Function of bases

Removes H+ which increase pH

Alkalemia definition

pH > 7.45

Acidemia definition

pH < 7.35

Optimal pH for body functions

Around a pH of 7.40

CNS effects of acidosis

Depression of the CNS (Lethargic and disoriented to comatose)

Effect of acidosis on the heart

Acidosis reduces cardiac contractility

Effects of alkalosis

Hyperexcitability of CNS and peripheral nerves (muscle spasm)

pH 7.35 to 7.39

Compensated acidosis

pH 7.41 to 7.45

Compensated alkalosis

Effect of a CO2 decrease on pH

Increases pH

Effect of a HCO3 decrease on pH

Decreases pH

PaCO2 normal value

35-45 mmHg.

Significance of PaCO2 in ventilation

It is the most reliable measurement for evaluating effectiveness of ventilation.

PaCO2 and acid-base status

It is a reflection of the respiratory component of the acid-base status.

PaCO2 and metabolic rate

CO2 is a waste product of metabolism; therefore, PaCO2 identifies a degree of ventilation in relation to metabolic rate.

Effect of fever or exercise on PaCO2

Increase metabolic rate (CO2 production) leading to hyperventilation (CO2 elimination).

Hypercapnia or hypercarbia

Greater than 45 mmHg.

Hypocapnia or hypocarbia

Less than 35 mmHg.

Increase in PaCO2

Increase H+ = respiratory acidosis (CNS chemoreceptors respond by increasing RR).

Decrease in PaCO2

Decrease H+ = respiratory alkalosis (CNS chemoreceptors respond by decreasing RR).

Normal minute ventilation (MV) in adults

5-10 L/min

Relationship between PaCO2 and VE

Normal PaCO2 should be accompanied by normal minute ventilation (VE).

Increased VE and hypocapnia

Increased VE results in hypocapnia unless metabolic rate is increased or dead space ventilation reduces lung perfusion.

Reduction in VE and hypercapnia

Reduction in VE below normal results in hypercapnia unless:

-metabolic rate is decreased

-VE reduced, PaCO2 increased

-due to neuromuscular disease or respiratory depressants (morphine).

Normal HCO3 value

22-26 mEq/L (Standard at PaCO2 of 40)

Plasma HCO3- significance

A reflection of the metabolic component of acid-base balance.

Metabolic acidosis and HCO3

Decrease plasma HCO3 and decrease pH

Time required for HCO3- compensation

12-24 hours to occur in response to changes in PaCO2.

Effect of PaCO2 increase on pH and HCO3-

For every 10 mmHg increase in PaCO2 above 40 mmHg, pH decreases by 0.05 and serum HCO3- increases by 1 mEq/L.

Effect of PaCO2 decrease on pH and HCO3-

For every 10 mmHg decrease in PaCO2 below normal, pH increases by 0.10 and serum HCO3- decreases by 1 mEq/L.

Acid-base values at PaCO2 increase to 50 mmHg

pH 7.35, HCO3- 25 mEq/L (representing a 10 mmHg increase above the baseline of 40).

Acid-base values at PaCO2 decrease to 30 mmHg

pH 7.50, HCO3- 23 mEq/L (representing a 10 mmHg decrease below the baseline of 40).