OXYGEN THERAPYOXYGEN THERAPYDora M Alvarez MDDora M Alvarez MD

20012001

Oxygen Delivery SystemsA-a GradientOxygen TransportOxygen Deliver to Tissues

ABC of Critical CareABC of Critical Care

Basic Principle:The normal cellular function and survival

depends upon a continuous supply of oxygen.

Continuos monitoring is aimed to prevent tissue hypoxia by early detection and treatment of abnormalities leading to hypoxia

Inspired oxygen from the

environment moves across the

alveolar-capillary membrane into

the blood and is then transported

to the tissues.

INDICATORS OF OXYGENATIONINDICATORS OF OXYGENATION

Arterial Oxygen Tension (PaO2)Pulse Oximetry basic principles are:

– light absorption (different wavelengths)– Plephysmography

Mechanisms of HypoxiaMechanisms of Hypoxia

Pathophysiologic Mechanism leading to hypoxia– Hypoventilation– V/Q Mismatch– R to L shunt– Diffusion impairment– Reduced inspired oxygen tension

The alveolar-arterial oxygen (A-a)

(A-a) gradient is a useful measure of the efficiency of oxygenation.

It compares the diffusion of oxygen from the patient's alveoli to his or her pulmonary capillaries with diffusion in an idealized model of the lung without ventilation/perfusion inequalities or cyclical variations in ventilation or circulation.

A-a GradientA-a Gradient

Calculation of this value incorporates a measure of alveolar ventilation (alveolar CO2, approximated as arterial CO2), therefore it is unaffected by hyper- or hypoventilation

The A-a gradient can be calculated The A-a gradient can be calculated from the following formula:from the following formula:

  A-a gradient = FiO2 x (pAtm-pH2O) - (paCO2/R) +

[paCO2 x FiO2 x (1-R)/R] - paO2,

where pAtm = 760 mmHg x exp( -altitude in

meters/7000 ) [3],

and pH2O = 47 mmHg x exp( (Temperature in

centigrade-37)/18.4 ) [4].

 

Arterial pCO2 in mmHg: 40

Arterial pO2 in mmHg: 90

Percent of inspired O2 (%): 21

(Fraction of inspired O2: 0.21 )

Respiratory quotient: 0.8

Patient's temperature in °F: 98.6 (or in °C: 37.0 )

Approximate elevation in feet: 0 (or in meters: 0 )

 

A-a gradient in mmHg: 10

• Normal A-a gradient values have not been well established, but • Tend to increase with age • Are slightly higher on 100 percent oxygen than on room air.

Oxygen DeliverOxygen Deliver

This delivery system sustains aerobic cellular metabolism throughout the body.

The Arterial Oxygen Content (CaO2)

The sum of the quantity of oxygen bound to hemoglobin

plus the amount of free oxygen dissolved in the blood,

according to the equation is

CaO2 = (1.34 x Hb concentration x SaO2) + (0.0031 x PaO2)

where:

PaO2 is the partial pressure of oxygen in the arterial blood

SaO2 is the arterial oxyhemoglobin saturation

Hb-Oxygen DisociationHb-Oxygen Disociation

.O2 Sat

PaO2

P50

CaO2 = (1.34 x Hb concentration x SaO2) + (0.0031 x PaO2)

= (1.34 x 15 x 0.98 % ) + (0.0031 x 90)

19.7 + 0.28

= ~ 20 mL of O2 /dL of blood

Arterial Oxygen ContentArterial Oxygen Content

Mixed Venous Oxygen Mixed Venous Oxygen Content Content

CaO2 = (1.34 x Hb concentration x SvO2) + (0.0031 x PvO2)where:

• PvO2 is the partial pressure of oxygen in the mixed venous blood

• SvO2 is the mixed venous oxyhemoglobin saturation

= (1.34 x 15 x 0.7 % ) + (0.0031 x 40)

14 + 0.124

= ~ 14.1 mL of O2 /dL of blood

The normal mixed venous oxygen content is ~ 15 mL O2/dL.

Oxygen DeliveryOxygen Delivery

Oxygen delivery (DO2) is the amount of oxygen transported from the lungs to the microcirculation. Oxygen delivery depends upon the cardiac output (Q) and CaO2:

DO2 (mL/min) = Q x CaO2 The normal oxygen delivery is approximately 1000 mL/min. If the calculation is done using cardiac index rather than cardiac output, then DO2 normalized

to body surface area is ~ 500 mL/min/m2. 

Oxygen ConsumptionOxygen Consumption

Oxygen consumption (VO2)

Alternatively, oxygen consumption can be indirectly calculated from the Fick equation:

  VO2 = Q x (CaO2 - CvO2) ~ 250 mL/min (1/4 of Oxygen deliver)

Oxygen Toxicity