RUMUS

Kadar oxygen delivery (DO2) sangat ditentukan dari fungsi jantung, hemoglobin dan saturasi oksigen dalam pembuluh darah arteri. PaO2 berpengaruh sedikit sekali bahkan dalam beberapa literatur diabaikan. Oleh karena itu untuk meningkatkan kadar oxygen delivery (DO2) perlu penanganan secara optimal pada penderita cedera kepala, terutama pengelolaan prehospital. Tujuan terpenting pengelolaan prehospital (awal kejadian cedera, tranportasi ke RS ataupun rujukan ke pelayanan bedah saraf) adalah mempertahan jalan nafas dan oksigenasi yang adekuat serta menjaga tekanan darah yang dapat mempertahankan tekanan perfusi otak.

Rob Law, H.Bukwirwa, Physiology of Oxygen Delivery, www.emedicine.com, downloaded May 20th, 2008B.K Siesjo, Mechanism of secondary brain injury, www.emedicine.com, downloaded May 20th, 2008

Oxygen delivery (DO2) adalah jumlah total oksigen yang dialirkan darah ke jaringan setiap menit. Kadar oxygen delivery tergantung dari cardiac output (CO) dan oxygen content of the arterial blood (CaO2). Komponen dari CaO2 adalah oksigen yang berikatan dalam serum (2-3%) yang dapat ditelusuri dengan kadar PaO2 dan oksigen yang berikatan dengan hemoglobin (97-98%) yang dapat ditelusuri dengan SaO2 (saturasi oksigen pada pembuluh darah arteri). Dari definisi ini dapat dijabarkan sebuah rumus : DO2 = CO X (Hb X 1,34 X SaO2) + (PaO2 X 0,0031) Nilai normal oxygen delivery (DO2) adalah 1000 ml O2/menit. Dari rumus diatas dapat dilihat bahwa hemoglobin (Hb) dan saturasi oksigen (SaO2) adalah penentu utama pada pengaliran oksigen dalam darah ke seluruh jaringan tubuh termasuk otak.

8) Simon M, Andrew B, Mark CB. Intensive Care, 2nd ed, Elsevier Churchill Livingstone, 20069) Alex B. Valadka, Bian T.Andrews, Neurotrauma, Thieme Medical Publisher, 2005 10)Lynelle N.B, Mechanical Ventilation and Intensive Respiratory Care, WB Saunders Company, 199512)Rob Law, H.Bukwirwa, Physiology of Oxygen Delivery, www.emedicine.com, downloaded May 20th, 2008

Prevention, early identification, and correction of tissue hypoxiaKey steps in oxygen transport: Uptake in the lungs Carrying capacity of blood Global delivery from lungs to tissue Regional distribution of oxygen delivery Diffusion from capillary to cell Cellular use of oxygenPo2(kPa)

Dry air21.3

Inspired air (humidified)20

Alveolar air14.7

Effect of increasing levels of supplemental oxygen and transfusion in an anaemic hypoxaemic patient showing importance of saturation and haemoglobin concentrationFio2Pao2(kPa)Sao2(%)Hb (g/l)Dissolved O2(ml/l)Cao2(ml/l)Cao2(% exchange)

Air0.21675801.483

35% O20.359.593802.210324

60% O20.616.598803.81107

Transfusion0.616.5981203.816448

Cao2=(haemoglobin (Hb)saturation (Sao2)1.36)+(Pao20.023) ml/l, where 1.36ml is the volume of oxygen carried by 1g of 100% saturated haemoglobin and Pao20.023 is the oxygen dissolved in 100ml of plasma.

Note that the normal extraction fraction for O2[(CaO2 CO2)/CaO2] is 5 mL 20 mL, or 25%; thus, the body normally consumes only 25% of the O2 carried on hemoglobin. When O2 demand exceeds supply, the extraction fraction exceeds 25%. Conversely, if O2 supply exceeds demand, the extraction fraction falls below 25%.

Oxygen StoresThe concept of O2 stores is important in anesthesia. When the normal flux of O2 is interrupted by apnea, existing O2 stores are consumed by cellular metabolism; if stores are depleted, hypoxia and eventual cell death follow. Theoretically, normal O2 stores in adults are about 1500 mL. This amount includes the O2 remaining in the lungs, that bound to hemoglobin (and myoglobin), and that dissolved in body fluids. Unfortunately, the high affinity of hemoglobin for O2 (the affinity of myoglobin is even higher) and the very limited quantity of O2 in solution restrict the availability of these stores. The O2 contained within the lungs at FRC (initial lung volume during apnea), therefore, becomes the most important source of O2. Of that volume, however, probably only 80% is usable.

Carbon Dioxide StoresCarbon dioxide stores in the body are large (approximately 120 L in adults) and primarily in the form of dissolved CO2 and bicarbonate. When an imbalance occurs between production and elimination, establishing a new CO2 equilibrium requires 2030 min (compared with less than 45 min for O2; see above). Carbon dioxide is stored in the rapid-, intermediate-, and slow-equilibrating compartments. Because of the larger capacity of the intermediate and slow compartments, the rate of rise in arterial CO2 tension is generally slower than its fall following acute changes in ventilation.

Ganong WF: Review of Medical Physiology, 20th ed. McGraw-Hill, 2001.

Guyton AC: Textbook of Medical Physiology, 10th ed. W.B. Saunders, 2000.

Nunn JF: Applied Respiratory Physiology, 5th ed. Lumb A (editor). Butterworth-Heinemann, 2000.

West JB: Respiratory PhysiologyThe Essentials, 6th ed. Lippincott, Williams & Wilkins, 2000.