Shunt & Deadspace

By Donald R. Elton, MD, FCCP
Lexington Pulmonary and Critical Care

Introduction
Shunt and dead space are terms used to describe extreme conditions where either blood flow or ventilation does not meet the other in the lung as it should for gas exchange to take place. While in their pure sense, the terms refer to the extreme conditions, they can also be used to describe areas or effects where blood flow and ventilation are not properly matched though both may be present to varying extents. Some refer to shunt-effect or dead space-effect to designate the ventilation/perfusion mismatch states that are less extreme than absolute shunt or dead space.

Shunt: that part of the cardiac output that returns to the left heart without the benefit of exposure to ventilated alveoli.

The oxygen content of mixed arterial blood (CaO2) is determined by the content of oxygen in the blood that reached ventilated alveoli (CcO2), the content of oxygen in blood that bypassed ventilated alveoli (CvO2), and the proportion of the two. Thus the formula:

QS/QT = (CcO2 - CaO2) / (CcO2 - CvO2)

Normal shunt fraction (QS/QT) is less than 0.05 (<5%).

Remember CxO2="1.39" x Hb x SxO2 + 0.003 x PxO2

CcO2 is the content of oxygen in pulmonary capillary blood and is estimated by plugging in 100% as the saturation (1.00 for the math) since the PcO2 (pulmonary capillary PO2) can be assumed to be high enough to assure 100% saturation. The PcO2 itself cannot be directly measured so we use PAO2 (from the alveolar air equation) in its place. This means that CcO2="1.39" x Hb + 0.003 x PAO2
Dead space: that part of inspired air that is exhaled without the benefit of exposure to perfused alveoli.
The carbon dioxide content of mixed exhaled gas (PECO2) is determined by the carbon dioxide content of the gas that came in contact with perfused alveoli (PACO2), the content of the gas that did not come in contact with perfused alveoli, and the proportion of the two. It can be assumed that there is no carbon dioxide in inspired air and thus no carbon dioxide in that part of the inspired volume that does not come in contact with perfused alveoli. One can also assume that gas that does contact perfused alveoli will equilibrate to the carbon dioxide content in the perfusing blood PACO2="PcCO2" or arterial blood PcCO2="PaCO2" since arterial carbon dioxide content is not greatly influenced by shunting). Thus the formula:

VD/VT="(PaCO2" PECO2) / (PaCO2 PICO2)

if one assumes that PICO2="0" the formula is simplified to:

VD/VT="(PaCO2" PECO2) / PaCO2

Normal VD/VT is less than 0.33 (<33%).

Example Calculations

Available data:

Hb 10 gm%
PaO2 75
SaO2 97%
Pb 750 mm Hg
PvO2 33
SvO2 65%
PaCO2 45 mm Hg
PECO2 15 mm Hg
FIO2 40%

Calculate the shunt & dead space from the above data.

The dead space is the easiest using

VD/VT="(PaCO2" PECO2) / PaCO2

VD/VT="(45" 15) / 45

VD/VT="30/45"

The shunt equation is more complicated and takes more steps to complete. We start by calculating the contents to plug into the shunt equation:

QS/QT="(CcO2" CaO2) / (CcO2 CvO2)

 arterial content:

CaO2="Hb" x SaO2 x 1.39 + 0.003 x PaO2

CaO2="10" x .97 x 1.39 + 0.003 x 75

CaO2="13.48" + 0.225="13.705" Vol %

mixed venous content:

CvO2="Hb" x SvO2 x 1.39 + 0.003 x PvO2

CvO2="10" x .65 x 1.39 + 0.003 x 33

CvO2="9.035" + 0.099="9.134" Vol %

Before we can calculate the CcO2, we must calculate the PAO2:

PIO2="FIO2" x (PB 47)

PIO2="0.4" x 713="285.2" mm Hg

PAO2="PIO2" PaCO2 x 1.25

PAO2="285.2" 45 x 1.25

PAO2="285.2" 56.25="228.95" mm Hg

pulmonary capillary content:

(CcO2): CcO2="Hb" x 1.39 + 0.003 x PAO2

CcO2="10" x 1.39 + 0.003 x 229

CcO2="13.9" + 0.687="14.59" Vol %

We can now fill in the final shunt equation:

QS/QT="(CcO2" CaO2) / (CcO2 CvO2)

QS/QT="(14.6" 13.7) / (14.6 9.1) QS/QT="0.9" / 5.5="0.16" %

Example Cases
Joe Crisco: A 64 year old male presents to the emergency room with chest pain, shortness of breath, and nausea. Initially he only had pain upon heavy exertion but over the last week he's had episodes at rest. Today he has had the pain continuously for the past hour. On physical examination he is acutely dyspneic and cyanotic. His lung exam reveals crackles up to the shoulders bilaterally and his neck veins are distended. A blood gas is drawn on 60% oxygen by mask and reveals a pH of 7.34, a PaCO2 of 33, and a PaO2 44.

1. What is wrong with this patient?

2. Why is he hypoxic?

3. What can be done therapeutically?

Cathy Crush: A 32 year old woman is in the hospital recovering from a fractured hip which she suffered in a motor vehicle accident. At 3 am one morning she develops sharp right sided chest pain and becomes acutely short of breath. She's never had these symptoms before. On physical examination she appears anxious and tachypneic. Her lung examination reveals a pleural friction rub heard over the right lateral chest. A chest x-ray is essentially normal and an electrocardiogram is significant for right atrial enlargement and tachycardia. A blood gas on a 40% oxygen mask shows a pH of 7.35, a PaCO2 of 33, and a PaO2 58.

1. What is wrong with this patient?

2. Why is she hypoxic?

3. What can be done therapeutically?

Winston Salem: This 49 year old man had his right lung removed because of lung cancer 6 months ago. He presents today for a routine follow-up visit and room air blood gasses are drawn revealing a pH of 7.42, a PaCO2 of 35, and a PaO2 88.

1. Why are this man's blood gasses normal?