Oxygen: Consumption and Delivery PICU Resident Talk Stanford School of Medicine Pediatric Critical Care Medicine June 2010

Learning Objectives After this lesson, the participant will be able to: List the factors that affect oxygen delivery, including blood flow, hemoglobin, and O 2 saturation. List the possible reasons for a low mixed venous O 2 saturation and possible treatments.

Why Is Oxygen Important? Used in cellular respiration – Needed for energy production by cells and tissues GLYCOLYSIS KREB’S CYCLE + ELECTRON TRANSPORT Pyruvate Glucose Oxygen 2 ATP 34 ATP

How is energy produced without oxygen? GLYCOLYSIS KREB’S CYCLE + ELECTRON TRANSPORT Pyruvate Glucose Oxygen 2 ATP 34 ATP Lactate

Measuring Oxygen Oxygen carrying capacity: – amount of oxygen that can be taken up by hemoglobin – Hgb g/dL x 1.34 ml O 2 /g Hgb Oxygen saturation – the proportion of O 2 combined with hemoglobin to the total amount of O 2 that can be taken up by hemoglobin Oxygen content: – the amount of oxygen present in both hemoglobin and dissolved O 2 – CaO 2 = (Hgb g/dL x 1.34 ml O 2 /g Hgb x SaO 2 ) + (0.003 x PaO 2 )

Oxygen Delivery (DO 2 ) “supply” Oxygen Consumption (VO 2 ) “demand” The Oxygen Supply and Demand Balance

Oxygen Delivery Amount of oxygen delivered (DO 2 ) to the body is the product of systemic blood flow / cardiac output (CO) and oxygen content (CaO 2 ) of systemic arterial blood. DO 2 = CO x CaO 2

Poor Oxygen Delivery DO 2 = CO x CaO 2 DO 2 = CO x [(Hgb x 1.34 x SaO 2 ) + (0.003 x PaO 2 )] Stagnant anoxemia (low CO) Anemic anoxemia (low Hgb) Anoxic anoxemia (low S a O 2 )

Oxygen Demand Oxygen requirement determined by metabolic activity. High O 2 requirement: – Heat production (maintain temperature) Fever: 13% ↑ in oxygen consumption for every 1 degree ↑ in temperature* – Physical activity Moderate O 2 requirement: – Cardiac contraction, normal respiration Low O 2 requirement: – Basic cellular function (ionic transport, electrical activity) *DuBois, EF. JAMA 77: , 1921

Oxygen Extraction Oxygen extraction is the proportion of oxygen delivered that is consumed. CaO 2 - CvO 2 Normally about 30% – Delivery is about 500 ml/min/m 2, consumption is about 150 ml/min/m 2 – For normal arterial oxygen saturation of 100%, mixed venous saturation should be 70% Oxygen extraction can be increased to ~70% under conditions of stress

Ways to measure Oxygen supply/demand imbalance Mixed venous saturation Lactate

Mixed Venous Saturation Measured where venous blood is completely mixed Reflects balance of O 2 extraction by tissues and O 2 delivery ↓ SvO 2 = – ↑ O 2 extraction – ↓ O 2 delivery – Both

Lactate Product of anaerobic mitochondrial metabolism which occurs under conditions of inadequate oxygen delivery Cleared by liver so level can be affected by liver failure Lactate can also be elevated by increase in pyruvate production or inhibition of pyruvate metabolism Serial measurements can determine lactate clearance which correlates with mortality

When there is an imbalance you can... Increase oxygen delivery Or Decrease oxygen demand

Ways to ↑ oxygen delivery DO 2 = CO x CaO 2 DO 2 = CO x [(Hgb x 1.34 x SaO 2 ) + (0.003 x PaO 2 )] ↑ HR or SV Transfuse pRBCs Provide O2

Possible ways to ↓ oxygen demand Regulating temperature—keep “cool” Decrease activity—sedation and/or paralysis Decrease work of breathing—NIPPV or intubation with mechanical ventilation Hold enteral feeds

Key Learning Points DO 2 = CO x CaO 2 = (HR x SV) x (Hgb x SaO 2 x 1.34) Reasons for ↓ MVSO 2 ↓ DO 2 – Poor CO => Tx w/ cardiovascular meds – Anemia => Tx w/ transfusion – Hypoxemia => Tx w/ oxygen ↑ VO 2 – Fever => Tx w/ cooling – WOB => Tx w/ mechanical ventilation – Exercise/ Agitation => Tx w/ sedation/ paralysis