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© Springer Nature Singapore Pte Ltd. 2021
J.-X. Zhou et al. (eds.)Respiratory Monitoring in Mechanical

1. Gas Exchange

Kun-Ming Cheng1, Linlin Zhang1  , Xiu-Mei Sun1 and Yu-Qing Duan1

Department of Critical Care Medicine, Beijing Tiantan Hospital, Capital Medical University, Beijing, China


1.1 Partial Pressure of Oxygen and Derived Parameters

Respiratory gas exchange refers to the exchange of oxygen and carbon dioxide between alveoli and blood, blood and tissue. The former is lung ventilation, the latter is tissue ventilation. This exchange is a direct diffusion process, which refers to the transfer of gas molecules from the higher pressure side to the lower pressure side. The power of gas diffusion is the difference in gas partial pressure.

1.1.1 The Process of Gas Exchange

Under normal circumstances, the partial pressure of oxygen in the alveoli is higher than that in venous blood, while the partial pressure of carbon dioxide in the alveoli is lower than that in venous blood. Therefore, when the venous blood in the pulmonary artery passes through the pulmonary capillaries, oxygen diffuses from the alveoli to the blood, and carbon dioxide diffuses from the venous blood to the alveoli driven by the partial pressure difference. As a result, the content of oxygen in venous blood increases, the content of carbon dioxide decreases, and venous blood is transformed into arterial blood. The diffusion of oxygen and carbon dioxide between the blood and the alveoli can be balanced in 0.3 s. It usually takes about 0.7 s for blood to be available through the pulmonary capillaries. Therefore, when venous blood passes through the pulmonary capillaries, there is sufficient time to complete the gas exchange. When arterial blood passes through the pulmonary capillaries, tissue cells absorb and utilize oxygen and diffuse the resulting carbon dioxide into arterial blood, turning arterial blood into venous blood (Fig. 1.1) [1].


Fig. 1.1

Schematic of the O2 transport pathway

1.1.2 Partial Pressure of Oxygen and Derived Parameters Transport of Oxygen

Oxygen can be transported in two ways: physical dissolution and chemical combination. Although oxygenated hemoglobin formed by the combination of oxygen and hemoglobin is the major transport mode of oxygen, the physical dissolution of oxygen directly determines the partial pressure of oxygen and affects the content of oxygenated hemoglobin. It also determines the difference of oxygen partial pressure between blood and tissue, which affects the absorption and utilization of oxygen by cells.

Partial Pressure of Oxygen (PO2)

Oxygen partial pressure is an important factor in gas dispersion and transportation. Partial pressure refers to the partial pressure occupied by a certain gas in a mixed gas, or also the partial pressure produced by the physical dissolution of a certain gas in a liquid.

Partial Pressure of Oxygen in Inhaled Gas (PIO2)

At sea level, the oxygen content in the air is 20.9%, the atmospheric pressure is 760 mmHg, and the water vapor pressure is 47 mmHg at 37 °C, so the PIO2 of the human body is 149 mmHg(PIO2 = (760–47)×20.9%).

Alveolar Partial Pressure of Oxygen (PAO2)

It is impossible for a normal person to exhale all the gas in the alveoli. When exhaling, there is always some residual gas in the lungs. When inhaled gas enters the alveoli, it must be mixed with this residual gas, resulting in a decrease in the partial pressure of oxygen. Therefore, in order to estimate the partial pressure of oxygen in the alveoli, the following formula is used:

$$ {\mathrm{PAO}}_2={\mathrm{FiO}}_2\ \left(760\hbox{--} 47\right)-1/R\ \left({\mathrm{PACO}}_2\right) $$
where R is the respiratory exchange ratio, which is equal to 0.8 most of the time [2].

Arterial Partial Pressure of Oxygen (PaO2)

Partial pressure of oxygen in arterial blood refers to the pressure produced by physically dissolved oxygen molecules in plasma. It is an important index to reflect the oxygen situation of the body, and it can be used to judge hypoxia and its degree. However, when using PaO2 for assessment, it is necessary to determine the inspired concentration of oxygen.

Reference value: Under normal circumstances (at sea level), it is equivalent to 80–100 mmHg (10.6 ~ 13.3 kPa). A variety of physiological factors affect PaO2, such as age, body mass index, posture, altitude and so on. A prediction equation of PaO2 for the population of life- long nonsmoking subjects, aged 40–74 years, with normal pulmonary function is given below [3]

$$ {\mathrm{PaO}}_2\left(\mathrm{mmHg}\right)=143.6-\left(0.39\times \mathrm{age}\right)-\left(0.56\times \mathrm{BMI}\right)-\left(0.57\times {\mathrm{PaCO}}_2\right)\pm 5.5 $$

Clinical significance: Under normal circumstances, there is also a right-to-left physiological shunt in the lung, and the uneven distribution of intrapulmonary ventilation blood flow caused by gravity, PaO2 is lower than PAO2.

PaO2 is mainly used to determine whether hypoxia exists, and helpful in evaluating the severity of hypoxia. Those whose PaO2 is lower than the normal value of the same age is called hypoxemia. When the air is inhaled without extra oxygen, if PaO2 <60 mmHg, the respiration may be decompensated, which is the criterion for the diagnosis of respiratory failure. If PaO2 <40 mmHg, severe hypoxia occurs. If PaO2 <20 mmHg, aerobic metabolism cannot be carried out normally, life is difficult to be supported.

Arterial Oxygen Saturation (SaO2)

Arterial oxygen saturation refers to the ratio of the combined oxygen to the maximum amount of oxygen that can be combined in arterial hemoglobin, which reflects the actual combination of arterial oxygen and Hb.

$$ {\mathrm{SaO}}_2=\frac{\mathrm{Actual}\kern0.375em \mathrm{Hb}\kern0.375em \mathrm{combined}\ \mathrm{with}\ {\mathrm{O}}_2}{\mathrm{Total}\kern0.375em \mathrm{Hb}\kern0.375em \mathrm{available}\ \mathrm{for}\ \mathrm{binding}\ \mathrm{with}\ {\mathrm{O}}_2}\% $$

Reference value: 95% ~ 98%

Clinical significance: The determination of SaO2 is helpful for the differentiation of hypoxia. In anoxia, due to lack of oxygen supply, PO2 decreased and SaO2 decreased, which is common in pulmonary ventilation and pulmonary ventilation dysfunction.

If there is hypoxia and SaO2 is normal, it suggests a decrease in effective hemoglobin, such as abnormal hemoglobin disease. Because at this time the effect hemoglobin itself, no longer anoxic conditions, can play the maximum oxygen binding capacity. Therefore, although the partial pressure of oxygen is low, the arterial oxygen saturation does not decrease. Therefore, use of SaO2 to identify whether the body is anoxic has the potential risk of concealing hypoxia. Additionally, there is no obvious change of SpO2 when PaO2 is above 60 mmHg since the oxygen dissociation curve reaching the plateau. At this stage, even if there is a significant change in PaO2, the increase or decrease of SaO2 is very small. Therefore, during mild hypoxia, although PaO2 has decreased significantly, SaO2 may not change significantly [3].


Oxygenation index refers to the ratio of arterial blood oxygen partial pressure to inhaled oxygen concentration.

Reference value: Its normal range is 400 ~ 500 mmHg (53.13 ~ 66.67 kPa). The average normal value is 100/0.21 = 480 mmHg

Clinical significance: PaO2/FiO2 is commonly used for diagnosis of lung injury (acute respiratory distress syndrome (ARDS) and transfusion-related acute lung injury), the assessment of course and therapy in pulmonary disease, and the evaluation of donor lungs and clinical outcome pulmonary transplantation for [4].

To a certain extent, PaO2/FiO2 eliminated the effect of oxygen concentration on PaO2, and reflect the ventilation function of the lung under the condition of oxygen therapy.

PaO2/FiO2 can be used to exclusive categories of ARDS [5]:

Alveolar-Arterial Oxygen Partial Pressure (P(A-a) O2)

Alveolar-arterial oxygen partial pressure difference refers to the difference between alveolar oxygen partial pressure and arterial oxygen partial pressure, also known as the alveolar-arterial oxygen difference. The alveolar-arterial oxygen difference of oxygen is a direct index for evaluating pulmonary gas exchange.

Calculating formula: P(A − a)O2 = PAO2 − PaO2

Reference value: The average value is 8 mmHg (1.07 kPa), ranging from 5 to 10, in healthy people younger than 30 years. It will reach 24 mmHg (3.20 kPa) at the age of 60–80 years. As age increases, P(A-a) O2 rises in a linear manner [6]

Clinical significance: P(A-a) O2 is an index to judge whether the function of gas exchange in the lung is normal or not. There are three main factors that affect P(A-a) O2: anatomical shunt, imbalance of ventilation and perfusion ratio, and diffusion disorder of alveolar-capillary barrier [7].

Therefore, the changes in oxygen concentration, intrapulmonary blood flow rate, prolonged inspiratory time, and positive end-expiratory pressure can result in the change of P(A-a) O2. The greater the difference of P(A-a) O2 is, the greater the difference of oxygen content between arterial blood and alveoli will be, which suggests the impaired diffusion function of the lung. Ventilatory dysfunction often results in an increase of P(A-a) O2 difference, such as chronic obstructive pulmonary disease. In all, the oxygen partial pressure difference of the alveolar artery comprehensively reflects the function of the ventilation and diffusion in the lung [811].

Arterial Oxygen Content (CaO2)

The content of oxygen in arterial blood refers to the sum of the amount of physical solution and chemical binding of oxygen in each artery blood.

Reference value: 8.55 ~ 9.45 mmol/L

Clinical significance: CaO2 is an index to reflect the content of red blood cells and plasma. The decrease of CaO2 may be due to the decrease of hemoglobin, the decrease of blood oxygen saturation, or the decrease of PO2. The normal content of oxygen in arterial blood cannot rule out tissue hypoxia.

1.2 Pulse Oximetry

1.2.1 Introduction

The pulse oximeter is one of the most widely used monitoring techniques in medicine to evaluate human health. Since the mid-1980s, when it began to be used in clinical practice, it has been widely used as a routine measure of arterial hemoglobin saturation in the fields of anesthesia, critical care, emergency, and respiratory medicine, because of its noninvasive, accurate, and real-time monitoring of pulse oxygen saturation (SpO2) [12]. The reliability and accuracy of reflecting real arterial hemoglobin saturation (SaO2) have been reliably verified. This technique contributes to the early diagnosis and treatment of hypoxia, which is lower in incidence and severity than in patients without an oximeter. Pulse oximetry has revolutionized modern medicine with its ability to continuously and transcutaneously monitor the functional oxygen saturation of hemoglobin in arterial blood. Pulse oximetry is so widely prevalent in medical care that it is often regarded as a fifth vital sign.

1.2.2 Principles of Pulse Oximetry

Pulse oximetry is commonly used to assess SpO2 and heart rate, which is made up of an optical spectrometer and a plethysmograph. The working principle of the pulse oximeter is spectral analysis, that is, the unique light absorption characteristics of the solution are used to detect and quantify the components in the solution [13]. There is a significant difference between the absorption spectra of hemoglobin (Hb) and Oxygenated hemoglobin (HbO2) in the range of red to near-infrared wavelengths. In the process of measuring blood oxygen saturation, the working principle of pulse oximetry is mainly divided into two kinds, which is mainly according to the type of light received by the photoelectric sensor, that is, transmitted light and reflected light, that is, light is emitted to the medium, and then reflected and transmitted respectively, propagate the signal, collect the signal through the photoelectric principle, and process the collected signal accordingly, and finally feedback the result. Two circuit schematic diagrams reflect the acquisition principles of these two methods (Fig. 1.2) [14].


Fig. 1.2

Measurement setup for reflection and transmission

In addition, the pulsation of the human arteries can cause changes in blood flow in the test site, which can cause changes in light absorption. Light absorption in non-blood tissues (skin, muscle, bone, etc.) is usually considered constant. Pulse oximetry measurement technology determines blood oxygen saturation by detecting changes in light absorption caused by blood volume fluctuations and eliminating the effects of non-blood tissues [15].

1.2.3 Method of Application

With the wide application of pulse oximetry in clinical practice, the methods of its application have gradually diversified. Although traditional finger-cuff or finger-type pulse oximetry is still commonly used at the bedside, other forms of pulse oximetry have been developed based on clinical requests, such as portable pulse oximetry, nocturnal oximetry, wearable pulse oximetry, hand-held pulse oximetry, and so on. Although the shapes of pulse oxygen monitors are different, the application methods are basically the same.

Although the pulse oximetry is easy to operate, there are still some details that we need to pay attention to. We need to connect the sensor to the appropriate position of the patient’s body, preferably at the same height as the heart. We cannot place sensors on limbs with arterial or intravenous catheters and blood pressure cuffs. Meanwhile, the right adult, child, and newborn status should be set. Use in high humidity environments should be avoided as far as possible. It should be used in non-strong light. If you need to monitor under strong light such as surgical light, bilirubin light, sunlight, etc., cover the probe with an obstacle. It is best to change the measuring position every 3 h to avoid the disturbance of blood circulation caused by long-term wearing on the fixed fingers and affect the measurement accuracy.

Protect sensors and cables from sharp objects.

1.2.4 Clinical Application of Oximetry

Pulse oximetry can be performed by trained personnel in a variety of environments, including (but not limited to) hospitals, clinics, and families. The following is a summary of the application of pulse oximetry in different clinical conditions [16].

  1. 1.

    When the spot SpO2 is checked in ED, primary care, outpatient observation (e.g., rehabilitation, oxygen clinic, preflight assessment, and others), those features or issues should be considered:

    1. 1.1.

      Set long averaging times to minimize motion artifact.


    2. 1.2.

      Pulsatile waveform display useful for checking signal quality.


    3. 1.3.

      Select most appropriate sensor/site (e.g., finger/ear probe).



  2. 2.

    When the nocturnal breathing disorders are detected in the sleep laboratory, those features or issues should be considered:

    1. 2.1.

      Use oximeter in “sleep” mode or with alarms disabled.


    2. 2.2.

      Set averaging time to 3 s or less.


    3. 2.3.

      Set date sampling and storage rate to minimum of 10 Hz.


    4. 2.4.

      Ability to output date in real-time to capture on polysomnography system.



  3. 3.

    When nocturnal breathing disorders are detected in the home setting, those features or issues should be considered:

    1. 3.1.

      Use oximeter in “sleep” mode or with alarms disabled.


    2. 3.2.

      Set averaging time to 3 s or less.


    3. 3.3.

      Set date sampling and storage rate to minimum of 1 Hz.


    4. 3.4.

      Adequate date storage capacity (minimum of 8 h).


    5. 3.5.

      Download/analysis software required for report generation.



  4. 4.

    Adult clinical monitoring in intensive or high-dependency care should consider those features or issues:

    1. 4.1.

      Consider ABG sampling to assess PaCO2, pH, and Hb status.


    2. 4.2.

      Select oximeter with good motion artifact rejection.


    3. 4.3.

      Consider using central sensor site.


    4. 4.4.

      Set alarm levels appropriate for individual patient.



  5. 5.

    Pediatric clinical monitoring in neonatal intensive, high-dependency care should consider those features or issues:

    1. 5.1.

      Select oximeter with good motion artifact rejection.


    2. 5.2.

      Consider using central sensor site.


    3. 5.3.

      Set alarm levels appropriate for individual patient.



  6. 6.

    When performing the screening or titration for supplemental oxygen in the outpatient clinic, domiciliary care, primary care, the pulse oximetry should set long averaging times to minimize motion artifact.


  7. 7.

    When detecting the exercise desaturation in an exercise laboratory or pulmonary rehabilitation, the following features or issues should be considered:

    1. 7.1.

      Consider using central sensor site.


    2. 7.2.

      Select oximeter with good motion artifact rejection.


    3. 7.3.

      Set averaging time at medium to long (balance between motion artifact sensitivity and rapid desaturation detection).



  8. 8.

    When performing noncritical monitoring of hospital ward, the following features or issues should be considered:

    1. 8.1.

      Set long averaging times to minimize motion artifact.


    2. 8.2.

      Set alarm levels appropriate for individual patient.



  9. 9.

    When performing perioperative monitoring of oxygenation in the operating theater or recovery room, the alarm levels appropriate for individual patient should be set.


The use of pulse oximetry has revolutionized the monitoring of respiratory function, especially considering that multiple or continuous measurement results can be obtained quickly and noninvasively.

In general, pulse oxygen measurement plays an important role in the monitoring and treatment of respiratory diseases by detecting hypoxemia, guiding titration of oxygen supplementation and other treatments (such as stopping support ventilation), and reducing the need for blood gas analysis.

The monitoring of blood oxygen saturation during exercise is a standard part of pulmonary function rehabilitation. The easiest way to prove oxygen saturation during exercise is to use a pulse oximeter, which is usually done in the exercise lab during diagnostic tests. During exercise training, it is carried out in a physiotherapeutic environment (especially as part of a lung rehabilitation program).

Pulse oxygen saturation measurement plays a central role in the diagnosis of sleep-related respiratory disorders, especially obstructive sleep apnea (OSA), which is a key component of nocturnal polysomnography. OSA usually shows characteristic repetitive oxygen desaturation and then saturates again at night, so pulse oxygen saturation is increasingly used as a tool for screening OSA in family environments. Family nocturnal pulse oxygen measurement records can be analyzed and calculated [17].

In addition, pulse oximetry also has a unique position in pediatrics. Routine pulse oximetry has been reported as an additional screening test that can potentially improve the detection level of critical congenital heart defects (CCHD). During delivery, fetal pulse oxygen saturation can be measured using a probe inserted through the mother’s vagina [18].

1.2.5 Limitations of Pulse Oximetry and Technological Update

The pulse oximetry cannot determine the adequacy of ventilation. In fact, there is evidence that patients with impaired ventilation may show normal saturation in the presence of life-threatening hypercapnia during oxygen inhalation. In order to correctly explain the results, it is necessary to recognize the inherent limitations of pulse oximeter in different clinical environments [19, 20]. Measurement artifact may occur for a variety of reasons, including motion artifact, nail polish, skin pigmentation, low perfusion state (e.g., low cardiac output, vasoconstriction, or hypothermia), use of intravenous dyes, and anemia [15]. Dyshemoglobin and Multiwavelength Pulse Oximetry

In addition to HbO2 and Hb, adult blood may contain dyshemoglobin: hemoglobin derivatives, which do not work because they do not bind oxygen molecules inversely at the physiological level of PaO2 in the blood, such as methemoglobin (MetHb) and carboxyhemoglobin (COHb). They are common in normal people, but at low concentrations. Functional SaO2 is defined as the percentage of HbO2 in the sum of HbO2 and Hb, while fractional SaO2 is defined as the percentage of HbO2 relative to the total number of four hemoglobin types. In low concentrations of methemoglobin, the difference between the two parameters is not significant, but at a high enough level, both functional and fractional readings may be compromised.

The traditional pulse oximeter uses two wavelengths of light to evaluate blood oxygen saturation, assuming that the only absorbers of these two wavelengths of light in the blood are HbO2 and Hb. Because MetHb and COHb also absorb light within the wavelength range used in pulsed blood oxygen measurement, there will be errors in the measurement of SpO2 in the presence of these hemoglobin. Some manufacturers have developed pulse oximeter with more than two wavelengths to estimate COHb and MetHb values (as well as total hemoglobin concentrations) in the blood [21]. Low Perfusion and Reflection Pulse Oximetry

In a transmissive pulse oximeter, light is detected after it is transmitted through the organ, so it is limited to the fingertips and earlobes. Because of its role in heat transfer, the blood flow to fingertips and earlobes is greater than that required by tissue metabolism, and their pulses have a high signal-to-noise ratio under normal conditions. However, these organs are strongly regulated by the autonomic nervous system, and their arteries contract when the ambient temperature is low or cardiac output is low. However, when the ambient temperature or cardiac output decreasing, vasoconstriction usually happens in these tissues to reduce heat loss or maintain an adequate blood supply to key core organs (heart, brain, and kidney) since they arestrongly regulated by the autonomic nervous system. In this case, the signal is reduced thus reducing the accuracy of the pulse oximetry. The reflective pulse oximetry, that is, the light source and the photodetector are located on the same surface of the skin and can be used in any accessible position, so it has the advantage under the condition of low peripheral perfusion. The main part of the reflex pulse oximeter is the forehead. The reflective finger pulse oximeter has the advantage of low power consumption because it can shorten the distance between the light source and the detector thus reducing the absorption of light. Reflective pulse oximeter is also recommended for use in accessible internal structures such as esophagus, pharynx, and trachea. The researchers claim that measurements in these areas are more reliable when the surrounding perfusion is lower. The low perfusion caused by vasoconstriction leads to the decrease of signal, which is also related to the increase of SpO2.

1.2.6 Future of Pulse Oximetry

The newer pulse oxygen saturation measurement technique can be measured with multiple wavelengths of light. In addition, studies have shown that pulse oximeter signals may be useful for applications other than SpO2. However, the current technology is not yet mature and needs to be further improved. With the improvement of technology, the pulse oximeter may improve the method of using pulse oximetry technology to detect SpCO, SpMet, SpHb, pulsus paradoxus, breathing frequency, and fluid responsiveness is likely to improve in the future [22].

Many reports focus on the use of pulse oximeter as a plethysmogram. Plethysmography analysis of peripheral pulse waves provides relevant hemodynamic information, which has been used in the diagnosis and follow-up of chronic cardiovascular diseases. However, this feature has not been further developed [23].

1.3 Venous Oximetry

1.3.1 Introduction

The imbalance of oxygen delivery (DO2) and oxygen consumption (VO2) can result in circulatory shock. It even develops multi-organ failure and increases morbidity and mortality if tissue hypoxia is unrecognized and uncorrected timely. Although physical examinations, such as vital signs, pulse oximetry, skin mottling, and urinary output, have somewhat distinct value, it is still insufficient to detect tissue hypoxia accurately. Venous oxygen saturation is a physiological parameter that has been introduced as an indirect index to assess the relationship between DO2 and VO2 both in the intensive care unit and in high-risk surgery [24]. The venous oxygen saturation is measured at the exit of any organ or tissue, the pulmonary artery and inferior vena cava or right atrium are easier to access. In 1970, Swan and Ganz developed a pulmonary artery catheter (PAC), also named as Swan and Ganz catheter, that can easily be inserted into the pulmonary artery to collect pulmonary arterial blood at the bedside, where blood is mixed from inferior and superior vena cava [25]. The oxygen content of pulmonary arterial blood is defined as mixed venous oxygen saturation (SvO2), which reflects venous oxygen saturation of the whole body. But due to the high invasiveness of inserting PAC, its application remains under debate and less popular gradually. With the central venous catheter (CVC) of the superior vena cava being applied widely, central venous saturation (ScvO2) has been advocated as a surrogate of SvO2 to measure venous oxygen saturation, but it just contains venous blood from the upper body (brain) [26]. There is still debate about whether ScvO2 can replace SvO2.

1.3.2 Physiology Oxygen Delivery

Oxygen Delivery (DO2, mL/min) is the rate of oxygen being transported from the lungs to the microcirculation per minute, and it is also expressed by oxygen transport and oxygen supply. DO2 is equal to the product of cardiac output (CO) and the oxygen content of arterial blood (CaO2).

$$ {\mathrm{DO}}_2=\mathrm{CO}\times {\mathrm{CaO}}_2 $$


The unit of CO is usually expressed as L/min and CaO2 is mL/dL, so that value of CO should be multiplied by 10 to convert into the unit of dL/min. And CaO2 depends on three parameters: the partial pressure of oxygen of arterial blood (PaO2), the arterial oxygen saturation of hemoglobin (SaO2, %), and hemoglobin concentration (Hgb, g/dL), expressed as

$$ {\mathrm{CaO}}_2=\left(1.34\times \mathrm{Hgb}\times {\mathrm{SaO}}_2\right)+\left(0.0031\times {\mathrm{PaO}}_2\right) $$


So, DO2 can also be calculated by the following equation:

$$ {\mathrm{DO}}_2=\mathrm{CO}\times \left[\left(1.34\times \mathrm{Hgb}\times {\mathrm{SaO}}_2\right)+\left(0.0031\times {\mathrm{PaO}}_2\right)\right] $$

where 1.34 (mL/g) is the Hufner constant and reflects the capacity of the hemoglobin carrying oxygen, but it varies with different species [27]. The 0.0031 is the coefficient of oxygen solubility in blood. The normal value of DO2 is about 1000 mL/min. Oxygen Consumption

Oxygen consumption (VO2, mL/min) is the rate of oxygen being used by the tissues from the blood per minute. It can be calculated by the Fick equation [28]:

$$ {\mathrm{VO}}_2=\mathrm{CO}\ \mathrm{x}\ \left({\mathrm{CaO}}_2-{\mathrm{CvO}}_2\right) $$

where CvO2 (mL/dL) is short of the mixed venous blood oxygen content that is similar to CaO2 and also related to three parameters, the partial pressure of oxygen of venous blood (PaO2), SVO2, and Hgb. It can be calculated by the following equation:

$$ {\mathrm{CaO}}_2=\left(1.34\times \mathrm{Hgb}\times {\mathrm{SvO}}_2\right)+\left(0.0031\times {\mathrm{PvO}}_2\right) $$


So, VO2 can be calculated by the following equation:

$$ {\mathrm{VO}}_2=\mathrm{CO}\left\{\begin{array}{l}\left(1.34\times \mathrm{Hgb}\times {\mathrm{SaO}}_2\right)+\left(0.0031\times {\mathrm{PaO}}_2\right)\\ {}\hbox{--} \left[\begin{array}{l}\left(1.34\times \mathrm{Hgb}\times {\mathrm{SvO}}_2\right)\\ {}+\left(0.0031\times {\mathrm{PvO}}_2\right)\end{array}\right]\end{array}\right\} $$


The normal value of global VO2 in resting status is about 250 mL/min. But it varies with different tissues. For example, splanchna is about 83 mL/min in resting status, skeletal muscle is 57 mL/min, the brain is 52 mL/min, the heart is 34 mL/min, kidney 19 ml/min, and skin 12 mL/min [29]. Oxygen Extraction and Oxygen Extraction Ratio

Oxygen extraction or oxygen extraction rate (O2ER) is equal to the ratio of DO2 and VO2, that represents the percentage of arterial oxygen being used when it passes through the microcirculation:

$$ {\mathrm{O}}_{2\mathrm{ER}}=\left({\mathrm{CaO}}_2-{\mathrm{CvO}}_2\right)/{\mathrm{CaO}}_2 $$ Venous Oxygen Saturation

According to Eq. (1.6), if dissolved oxygen is ignored, the formula can be simplified and converted as follows:

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