HypoxiaZhihua Gao

Zhejiang University

Review of respirationMeasurements of O2

Hypoxia definition classification, etiology, mechanism

resultant changes in the body prevention and treatment

Overview

1. O2 intake

2. Hb carrying O2

3. O2 transport in circulation

4. O2 utilization in the tissue

Respiration-a process of gas exchange

External respiration

O2 dependence

Large amount of O2 consumption ---- 250ml/min (360L/day)

Small amount of O2 storage ---- 1.5L (sustain life only six minutes)

Continuous hypoxia

>6 min--life-threatening

PO2 (partial pressure of oxygen) 血氧分压

Oxygen tension 血氧张力 the tension produced by oxygen molecules

physically dissolved in the blood Arterial (PaO2): 100 mmHg (13.3kPa)

Venous (PvO2): 40 mmHg (5.33kPa)

Influence factors (IFs): Pressure of O2 in the air （空气氧分压） External respiratory function （外呼吸功能） Shunt of blood ( 血液分流 )

Measurements of O2 (1)

CO2max (oxygen binding capacity) ，最大血氧容量

Maximal amount of oxygen that can be potentially bound by the haemoglobin (Hb), a reflection of the ability of Hb carrying O2.

Under ideal condition, the binding capacity is ~1.34 ml/g Hb

IFs:

Quantity of Hb ( 血红蛋白的量 )

Quality of Hb or affinity of Hb ( 血红蛋白的质与亲和力 )

Measurements of O2 (2)

CO2: (oxygen content), 实际血氧含量 the actual O2 content in the blood, including the

oxygen bound to the Hb and O2 dissolved in the blood. Arterial CO2 (CaO2): 19 ml/dl

Venous CO2 (CvO2): 14 ml/dl

Differences of CO2 between arterial and venous (Da-vCO2): 5 ml/dl

IFs: PaO2

CO2 max-Hb quantity and quality

Measurements of O2 (3)

SO2 (oxygen saturation) ， 血氧饱和度 the percentage of oxygen-bound form

(oxyhemoglobin) in total Hb. Arterial (SaO2): 97-99% Venous (SvO2): 75%

IFs: PO2

pH, temperature CO2, DPG

Measurements of O2 (4)

A curve reflecting the relationship between the PO2 and SO2

S (sigmoid) shape

SO2 , SDC left shift Hb affinity SO2 , SDC right shift Hb affinity

O2 saturation dissociation curve (SDC)

Right shift

Left shift

P50: the PO2 @ 50% SO2

Reflecting the flexibility of Hb carrying O2

SDC right shift , P50↑

SDC left shift, P50↓

P50-a value indicating the felexibilty of Hb

Hypoxia

Defintion:

A pathological process in which O2 supply to tissues is

inadequate to meet the demand of cells, or utilization of

O2 is insufficient in cells despite adequate O2 supply,

leading to changes in functions, metabolisms and

structures of cells and tissues in the body.

Inadequate supply of O2 ( 氧供不足 )

Insufficient utilization of O2 （用氧障碍）

1. O2 intake

2. Hb carrying O2

3. O2 transport in circulation

4. O2 utilization in the tissue

Classification of hypoxia

1. Hypotonic hypoxia

2. Hemic hypoxia

3. Circulatory hypoxia

4. Histogenoushypoxia

The most common type of hypoxia

Causes:

O2 supply Tibetan Plateau, mineral wells

Dysfunction in external respiration, a.k.a respiratory hypoxia Bronchial obstruction

Respiratory muscle paralysis

Venous-to-arterial shunts Congenital cardiac defects

Hypotonic/hypoxic hypoxia( 低张性缺氧 )

Features:

minor: PaO2 ↓ ; CO2 N

Severe:PaO2 ↓ ; CO2max N, CaO2 ↓ ; SaO2↓D(a-v)O2 ↓or N

Central cyanosisRespiratory compensation

Hypotonic/hypoxic hypoxia

Deoxyhemoglobin >5g/dl (central cyanosis)

Patients with hypoxia can show cyanosis, but cynosis does not mean hypoxia.

Hypoxia patients may not have cyanosis.

Enterogenous cyanosis ( 肠源性紫绀 ):

Cyanosis ( 发绀或紫绀 )

Hypoxia caused by abnormalities in hemoglobin (Hb)

Amount of Hb ↓ Altered binding capacity of Hb

Causes: Anemia CO poisoning Methemoglobinemia Higher affinity of Hb to O2

Hemic/isotonic hypoxia （血液性缺氧）

20 60 100 120

PaO2 , mmHg

300

200

150

Ca

O2,

ml/

l

100

100

anemiaHb = 10

normalHb = 15

100

Sa

O2,

%

polycythemiaHb = 20

Hb concentration &CaO2 relationship

CO poisoning

CO binds to Hb to form

COHb,

which cannot take up

O2.

Affinity of Hb to CO is

~210 times higher than

O2, dissociation of CO

from Hb is much slower

than O2.

Methemoglobinemia ( 高铁血红蛋白血症 )

正常血红蛋白的铁主要以二价形式存在，亚硝酸盐，过氧酸盐、磺胺衍生物等氧化剂可使血红蛋白中的二价铁氧化成三价铁，形成高铁血红蛋白血症 （ Methemoglobinemia ， HbFe3+OH ）。

Central cyanosis ( 紫绀 ) v.s. Enterogenous cyanosis ( 肠源性紫绀 ):

食用大量含亚硝酸盐的腌菜后，硝酸盐颈肠道细菌还原为亚硝酸盐，吸收入血后，使血红蛋白的二价铁氧化为三价铁，导致高铁血红蛋白血症。

当血液中 HbFe3+OH>1.5g/dl, 皮肤，粘膜可出现青紫，为肠源性紫绀。

Features:Normal PaO2

CO2 max ↓ and CaO2↓→ O2

delivery to tissue ↓CO2 max N but affinity ↑→ O2

release to tissue ↓

Hemic/isotonic hypoxia

Inadequate blood flow resulting in insufficient oxygenation of the tissue

Causes: General circulatory dysfunction-shock,

heart failure Local circulatory dysfunction-stenosis,

occlusion, thrombosis

Mechanisms---tissue perfusion ↓ ischemia hypoxia （缺血性缺氧） congestive hypoxia （充血性缺氧）

Circulatory/hypokinetic hypoxia （循环性缺氧）

Features: a. PaO2: N PvO2↓

b. CaO2: N CvO2↓

c. CO2max: N

d. SaO2: N

e. (Da-vCO2)↑

f. Peripheral cyanosis ( 外周性紫绀 )

g. Respiratory compensation

Histogenous/dysoxidative hypoxia ( 组织性缺氧 )

Causes: Cell poisoning

cyanide poisoning→histotoxic hypoxia

Mitochondria injury radiation ; ROS

Inadequate synthesis of biological oxidation coenzyme deficiency of vitamin B2 or PP

Mechanisms:

Aberrant biological oxidation or oxidative phosphorylation → deficiency in oxygen consumption →ATP↓.

Features: a. PaO2 = N, PvO2↑

b. CaO2 = N, CvO2↑

c. CO2max = N

d. SaO2 = N

e. (CaO2-CvO2)↓

f. No cyanosis

g. No respiratory compensation

Features of different types of hypoxia Type PaO2 SaO2 CO2 max CaO2 D(a-v)CO2

Hypotonic hypoxia ↓ ↓ N ↓ ↓ or N

Hemic hypoxia N ↓ or N ↓ or N ↓ or N ↓

Circulatory hypoxia N N N N ↑

Histogenic hypoxia N N N N ↓

Notes: ↓— decrease; ↑ — increase; N — normal.

Hypotonic hypoxia: PaO2

Hemic hypoxia: Normal PaO2, altered Hb quantity and quality, CO2max or

Circulatory hypoxia: Normal PaO2, Hb quantity and quality, blood flow

Histogenous hypoxia: normal O2 supply to the tissue, O2 utilization

Features of different types of hypoxia

Metabolic and functional changes

Respiratory system

Circulatory system

Hematologic system

Central nervous system

Tissues and cells

Compensatory reaction-hyperventilation ( 过度通气 )

PaO2 <8Kpa or 60 mmHg → chemoreceptors↑→respiratory rate and depth↑→ hypoxic ventilation reaction (HVR).

Consequences:respiratory surface↑, O2 diffusion↑, PaO2&SaO2↑

More fresh air into the lung, PaO2 ↑ ， PCO2↓

blood volume returning to the heart↑

Respiratory system

Injury manifestation

High-altitude pulmonary edema

Respiratory failure

Severe hypoxia (PaO2 <30 mmHg)→ inhibition of respiratory center→ slow and periodic or irregular breathing → stop of breathing.

Circulatory system

Compensatory responses

Cardiac output ↑ : tachycardia （心动过速） → arrhythmia → myocardial contractility↑

Pulmonary vasoconstriction → pulmonary arterial hypertension → right heart failure

Redistribution of blood → ensure enough blood to heart and brain

Capillary hyperplasia

Injury manifestation

Pulmonary hypertension Decreased diastolic and systolic

myocardial function Arrhythmia Decrease in Venous blood return to

the heart

Hemic system

Compensatory responses Rightward shift of oxyhemoglobin dissociation

curve

Increase of red blood cell and erythropoietin(EPO)

Injury manifestation

Blood viscosity ↑

Brain is highly sensitive to hypoxia.

Blood redistribution to the brain

PaO<28 mmHg ， mental disorder

Cerebral edema

Central nervous system

Cell

Compensation: increased ability to use O2

anaerobic glycolysis ↑

increase of myoglobin

Injury manifestation cellular membrane injury

mitochondria impairment

lysosome breakage

Factors involved in tolerance to hypoxia

Oxygen consumption rate Brain → oxygen consumption rate↑→

tolerance↓

Skin → oxygen consumption rate↓→ tolerance↑

Compensatory ability of the body

Prevention and treatment

Inhalation of oxygen ( 氧吸入 )

Efficiency: Hypotonic hypoxia — the best Histogenous hypoxia —the worst

When the patient inhaled high pressure of oxygen(PO2 is 2-3 times atmosphere) ,a series of toxic signs and symptoms was appeared, this condition is termed as oxygen toxication.

Oxygen toxication: 1. Pulmonary oxygen toxication 2. Cerebral oxygen toxication

The mechanisms of oxygen toxicity:

Reactive oxygen species or oxygen free radicals .

掌握各项血氧检测指标的意义。掌握缺氧概念，分类与特征。掌握紫绀，肠源性紫绀的概念。熟悉呼吸的基本过程。熟悉机体对缺氧的代偿反应。熟悉缺氧治疗的原则。了解氧中毒概念。

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