Oxygen Delivery

Global oxygen delivery (Do2) is the total amount of oxygen delivered to the tissues per minute irrespective of the distribution of blood flow. Under resting conditions with normal distribution of cardiac output it is more than adequate to meet the total oxygen requirements of the tissues (Vo2) and ensure that aerobic metabolism is maintained.

Recognition of inadequate global Do2 can be difficult in the early stages because the clinical features are often non-specific. Progressive metabolic acidosis, hyperlactataemia, and falling mixed venous oxygen saturation (SvO2), as well as organ specific features such as oliguria and impaired level of consciousness, suggest inadequate Do2. Serial lactate measurements can indicate both progression of the underlying problem and the response to treatment. Raised lactate levels (>2 mmol/l) may be caused by either increased production or reduced hepatic metabolism. Both mechanisms frequently apply in the critically ill patient since a marked reduction in Do2 produces global tissue ischaemia and impairs liver function.

Table 2.1 illustrates the calculation of Do2 from the oxygen content of arterial blood (Cao2) and cardiac output (Qt) with examples for a normal subject and a patient presenting with hypoxae-mia, anaemia, and a reduced Qt. The effects of providing an increased inspired oxygen concentration, red blood cell transfusion, and increasing cardiac output are shown. This emphasises that: (1) Do2 may be compromised by anaemia, oxygen desaturation, and a low cardiac output, either singly or in combination; (2) global Do2 depends on oxygen saturation rather than partial pressure and there is therefore little extra benefit in increasing Pao2 above 9 kPa since, due to the sig-moid shape of the oxyhaemoglobin dissociation curve, over 90% of haemoglobin (Hb) is already saturated with oxygen at that level. This does not apply to the diffusive component of oxygen transport that does depend on the gradient of oxygen partial pressure.

Although blood transfusion to polycythaemic levels might seem an appropriate way to increase Do2, blood viscosity increases markedly above 100 g/l. This impairs flow and oxygen delivery, particularly in smaller vessels and when the perfusion pressure is reduced, and will therefore exacerbate tissue hypoxia.1 Recent evidence suggests that even the traditionally accepted Hb concentration for critically ill patients of approximately 100 g/l may be too high since an improved outcome was observed if Hb was maintained between 70 and 90 g/l with the exception of patients with coronary artery disease in whom a level of 100 g/l remains appropriate.2 With the appropriate Hb achieved by transfusion, and since the oxygen saturation (Sao2) can usually be maintained above 90% with supplemental oxygen (or if necessary by intubation and mechanical ventilation), cardiac output is the variable that is most often manipulated to achieve the desired global Do2 levels.

Abbreviations: So2, oxygen saturation (%); Po2, oxygen partial pressure (kPa); Pio2, inspired Po2; Peo2, mixed expired Po2; Peco2, mixed expired Pco2; Pao2, alveolar Po2; Pao2, arterial Po2; Sao2, arterial So2; Svo2, mixed venous So2; Qt, cardiac output; Hb, haemoglobin; Cao2, arterial O2 content; Cvo2, mixed venous O2 content; Vo2, oxygen consumption; Vco2, CO2 production; O2R, oxygen return; Do2, oxygen delivery; Vi/e, minute volume, inspiratory/expiratory.

Figure 2.1 Oxygen transport from atmosphere to mitochondria. Values in parentheses for a normal 75 kg individual (BSA 1.7 m2) breathing air (Fio2 0.21) at standard atmospheric pressure (PB 101 kPa). Partial pressures of O2 and CO2 (Po2, Pco2) in kPa; saturation in %; contents (Cao2, Cvo2) in ml/l; Hb in g/l; blood/gas flows (Qt, Vi/e) in l/min. P50 = position of oxygen haemoglobin dissociation curve; it is Po2 at which 50% of haemoglobin is saturated (normally 3.5 kPa). Do2 = oxygen delivery; Vo2 = oxygen consumption, Vco2 = carbon dioxide production; Pio2, Peo2 = inspired and mixed expired Po2; Peco2 = mixed expired Pco2; Pao2 = alveolar Po2.

Figure 2.1 Oxygen transport from atmosphere to mitochondria. Values in parentheses for a normal 75 kg individual (BSA 1.7 m2) breathing air (Fio2 0.21) at standard atmospheric pressure (PB 101 kPa). Partial pressures of O2 and CO2 (Po2, Pco2) in kPa; saturation in %; contents (Cao2, Cvo2) in ml/l; Hb in g/l; blood/gas flows (Qt, Vi/e) in l/min. P50 = position of oxygen haemoglobin dissociation curve; it is Po2 at which 50% of haemoglobin is saturated (normally 3.5 kPa). Do2 = oxygen delivery; Vo2 = oxygen consumption, Vco2 = carbon dioxide production; Pio2, Peo2 = inspired and mixed expired Po2; Peco2 = mixed expired Pco2; Pao2 = alveolar Po2.

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