Critical Care Monitoring. Assessing the Adequacy of Tissue Oxygenation. Tissue Oxygenation - Step 1. Tissue Oxygenation

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1 Critical Care Monitoring 1 Assessing the Adequacy of Tissue oxygenation is the end-product of many complex steps 2 - Step 1 Oxygen must be made available to alveoli 3 1

2 - Step 2 Oxygen must cross the alveolarcapillary membrane 4 - Step 3 Oxygen must load into blood and be transported 5 - Step 4 Tissues must uptake oxygen 6 2

3 - Step 5 Tissue utilize oxygen 7 Problem in assessment Tissue oxygenation cannot be directly measured Use indirect assessment techniques VD, shunt calculations (distribution of ventilation) AaDO2, a/a (ability to cross AC membrane) PaO2, CaO2, QT (transport) _ C(a-v)O2 (O2 extraction by tissues) 8 Clinical assessment 9 3

4 Step 1 Purpose of ventilation Ideal for ventilation to match perfusion 10 V/Q match is not the case Matching does not occur causing variations that affect tissue s ability to Receive adequate O2 Remove CO2 11 V/Q mismatch is any condition that ventilation or blood flow through the lungs 12 4

5 V/Q Relationships Abnormal Deadspace ventilation (VD) Deadspace effect Intrapulmonary shunt (QS/QT) Shunt effect (Venous admixture) 13 Deadspace Ventilation Equals the portion of inspired volume that does not participate in gas exchange 14 Deadspace Ventilation (total = anatomical + alveolar) 15 5

6 Anatomical Deadspace Normal = Ventilation in conducting airways (down to respiratory bronchioles) 16 Anatomical Deadspace 17 Anatomical Deadspace 18 6

7 Alveolar Deadspace Theoretically doesn t exist except in disease Defined as the volume of gas in the alveoli that are ventilated but not perfused 19 Alveolar Deadspace The classic deadspaceproducing disease = 20 Deadspace Effect V/Q mismatch when ventilation is in excess of perfusion Ventilation > Perfusion 21 7

8 Deadspace Effect ie, anything that ventilation and/or lung perfusion 22 Assessing Deadspace Measured in 2 ways: 23 Assessing Deadspace Normal VD/VT = Spontaneous breathing: On vent: 24 8

9 Intrapulmonary Shunt Defined as the portion of the cardiac output that returns to the left heart without being oxygenated Perfusion w/o Ventilation 25 Intrapulmonary Shunt total = anatomic + capillary (physiologic) Normal = 26 Anatomic Shunt Is the major portion of total shunt 2-5% of cardiac output Normal blood flow through: 27 9

10 Increased in: Anatomic Shunt 28 Capillary Shunt True shunt Small amount is normal Perfusion of non-ventilated alveoli 29 Capillary Shunt Causes of capillary shunt: 30 10

11 Capillary Shunt Capillary shunt is refractory to oxygen therapy Alveoli are unable to ventilate Blood passing by good alveoli cannot carry more O2 once it is fully saturated Treatment is 31 Shunt Effect (Venous Admixture) V/Q mismatch when perfusion is in excess of ventilation Perfusion > Ventilation 32 Shunt Effect (Venous Admixture) Responsive to O2 therapy Caused by: 33 11

12 Shunt Effect (Venous Admixture) ie, anything that ventilation and/or perfusion 34 Assessing Shunt Classic Shunt Equation 35 Assessing Shunt Clinical Shunt Equation 36 12

13 Assessing Shunt Shunt equation is useful tool: To evaluate effectiveness of O2 therapy To differentiate shunt or V/Q imbalance as cause of hypoxemia 37 Crossing AC Membrane AaDO2 Step 2 = PAO2-PaO2 = P(A-a)O2 Normal Room air 100% O2 Normal lungs readily transfer O2 Transfer hindered in pulmonary disease 38 Crossing AC Membrane AaDO2 Diffusion across AC membrane is dependant on: 39 13

14 Crossing AC Membrane Causes of AaDO2 Surface area of AC membrane 40 Crossing AC Membrane Causes of AaDO2 thickness of AC membrane 41 Normal Crossing AC Membrane Causes of AaDO

15 Crossing AC Membrane Remember!! Hypoxemia caused by: Crossing AC Membrane Estimate Shunt Rough estimate of % shunt from AaDO2: Every 20 mmhg AaDO2 = 1% shunt (at FiO2 10) Example: 45 15

16 Use AaDO2 to: Crossing AC Membrane AaDO2 Summary 46 Crossing AC Membrane a/a Ratio = PaO2 PAO2 = % of alveolar O2 arterial blood Normals Also assesses diffusion across AC membrane 47 Crossing AC Membrane a/a Ratio Better assessment than AaDO2 since AaDO2 changes with different FiO2 s 48 16

17 Step 3 O2 has crossed alveolar-capillary membrane and must be: 49 Uptake by the Blood Oxygen carried in the blood 2 ways: 50 Uptake by the Blood Dissolved (per 100 ml blood) 51 17

18 Uptake by the Blood Combined with hemoglobin (per 100 ml blood) 52 Uptake by the Blood Total amount carried in arterial blood (per 100 ml blood or vol%) = (134 x Hgb x SaO2) + (PaO2 x 003) 53 Oxyhemoglobin Dissociation Curve Cardiopulmonary Anatomy & Physiology Des Jardins pp

19 Transport by the Blood Heart must pump oxygenated blood to the tissues Normal cardiac function is essential for adequate cardiac output QT is the amount of blood pumped by the ventricles in 1 minute QT = 55 Transport by the Blood QT = stroke volume x heart rate (QT = SV x HR) 56 Transport by the Blood Fick Equation 57 19

20 Transport by the Blood Cardiac output varies according to Tissue oxygen demand Amount of oxygen carried in the blood 58 Transport by the Blood Effect of O2 demand on QT As VO2 - QT must QT is accompanied by Causes of demand 59 Transport by the Blood Effect of CaO2 on QT with normal O2 demand, a in CaO2 Duh! hypoxia Causes of CaO

21 Transport by the Blood Total O2 delivery to tissues (DO2) DO2 = QT x CaO2 x Transport by the Blood Total O2 delivery 62 Tissue O2 Uptake = VO2 = O2 extraction Normal = ml/min ( ) _ Difference between CaO2 and CvO2 = O2 extracted but only by _ that organ drained so must use CvO2 Step

22 Tissue O2 Uptake Total O2 extracted by body tissues (per 100 ml blood) CaO2 - CvO2 = C(a-v)O2 Normal = 64 Tissue O2 Uptake Total O2 extracted by body tissues (VO2) C(a-v)O2 x QT x Fick Equation Tissue O2 Uptake 66 22

23 Tissue O2 Uptake From Fick Equation can see that with a steady VO2, the QT and C(a-v)O2 are inversely proportional, ie as QT C(a-v)O2 67 Tissue O2 Uptake Essential relationship QT O2 extraction PvO2 SvO2 CvO2 C(a-v)O2 68 Tissue O2 Utilization Hindered by Cyanide poisoning Amobarbital (Amytal) overdose Step 5 = 69 23