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The Physiologic Based Approach to Hypoxemic Respiratory Failure
Dr. Yasser Elsayed, MD, PhD
Associate Professor
Integrated Hemodynamics Program
International POCUSNEO
University of Manitoba
Objectives
This presentation is aiming to demonstrate :
• Basic pathophysiology of hypoxemia
• Management of critically ill infants at risk of hypoxemia orwith pulmonary hypertension
Important Definitions
• Hypoxemia:
SpO2< 80%, or PaO2 < 50 mmHg
• Hypoxia:
Low oxygen delivery (Low SpO2, blood flow, or Hb)
< 18 of oxygen ml/kg/min
• Hyperoxemia
SpO2 higher than the upper limit of target saturation,
or PaO2 > 90 mmHg (infant on oxygen)
• Hyperoxia:
High oxygen delivery and PaO2 at the tissues (infant on oxygen)
PHYSIOLOGY OF:VENTILATION PERFUSION MISMATCH AND R-L SHUNT
Decreasing VentilationDecreasing Perfusion
VA/Q=1/0
VA/Q=0/1
VA/Q=0.8
Pulmonary hypertension (PH)Massive pulmonary embolism ShockBPD with PH Lung collapse
Pneumonia BPD with no PHLung congestion
Ventilation –Perfusion Mismatch and Right to Left shunt
0.6 0.4 0.21.8 1.4 1.2
SevereV-Q mismatchR-L shunt is 50% (assumed example)
INOO2
R
L
75%
100%
75%
87%
0.21 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Oxy
hae
mo
glo
bin
satu
rati
on
%7
5
80
8
5
9
0
9
5
10
0
FIO2
1020
30
40
Oxygen reduction test
0.21 0.35 0.45 0.55 0.65
Oxy
hae
mo
glo
bin
satu
rati
on
%7
5
80
8
5
9
0
9
5
97
FIO2
0.8
0.4
0.2
0.15
Oxygen reduction test
PHYSIOLOGY OF PULMONARY HYPERTENSIONAND FACTORS AFFECTING PVR
High Pulmonary hypertension
BP α F (CO) X PVRPPHN
Pulmonary Congestion
Circulatory Changes During Transition
16Rudolph AM. Circulation. 1970;61:343-359.
Types of Pulmonary Hypertension in Neonates
18
1 Hypoxic Vasoconstriction1
2 Abnormal Vasoconstriction (Idiopathic)1
eg. Premature Closure of the Ductus Arteriosus, Alveolar Capillary Dysplasia
3 Pulmonary Vascular Hypoplasia1-3
eg. Congenital Diaphragmatic Hernia, Oligohydramnios, Potter Sequence
4 Excessive Pulmonary Blood Flow1
eg. Congenital Heart Defects With Left-to-Right Shunting
5 Pulmonary Venous Hypertension1
eg. Congenital Heart Defects With Left Atrial Hypertension, Mitral Valve Disease, Aortic Valve Disease
1 Lakshminrusimha S. Clin Perinatol. 2012;39(3):655-683. 2 Bohn D. Am J Respir Crit Care Med. 2002;166:911-915. 3 Gubler MC. Pediatr Nephrol. 2014;29:51-59.
PVR
PVR
PVR
FLOW V-Q mismatch
Pathological Effects of High PVR on the Newborn’s Heart
When pulmonary vascular resistance (PVR) does not decrease appropriately during the transition from a fetus to a newborn, a condition called persistent pulmonary hypertension of the newborn, or PPHN, results.1,2
191 Levin DL, et al. Pediatrics. 1975;56:58-64.2 Lakshminrusimha S. Clin Perinatol. 2012;39:655-683.
Intracardiac Shunting
Ventricular Dysfunction
a. RV dilation and decreased functionb. Decreased LV preload and
contractility c. Bowing of interventricular septum
into the LV
1
High PVR during PPHN can cause:1,2
2
Reduced Oxygen Delivery to Tissues.
High PVR
Elevated PVR Promotes Shunting Through the Intracardiac Fetal Channels
High PVR promotes shunting of deoxygenated blood through the fetal channels, DA and FO, reducing the oxygen saturation of the blood feeding the body.1,2
201 Levin DL, et al. Pediatrics. 1975;56;58-64.2 Lakshminrusimha S. Clin Perinatol. 2012;39:655-683.
Shunting Through the FO
Shunting Through the DA
Deoxygenated Blood to Body
Elevated PVR Affects Heart FunctionHigh PVR affects heart function in several ways:1,2
211 Levin DL, et al. Pediatrics. 1975;56;58-64.2 Lakshminrusimha S. Clin Perinatol. 2012;39:655-683.
Decreased Contractility
Increase Pooling of Blood in the RV and the RA:
Decreased RA Emptying
RV Dilation and Decreased Contractility
Elevated PVR Causes Distention of the Right Ventricle That Affects Left Ventricular Output
22Bronicki RA, et al. Pediatr Crit Care Med. 2016;17:S182-S193.Animation adapted from Constantino J, et al. Am J Physiol Heart Circ Physiol. 2013;305:H1265-H1273.
High PVR causes a bowing of the interventricular septum into the LV which can:
• Decrease the ability of the LV to contract appropriately (reduces oxygen delivery)
• Decrease the LV volume and the LV stroke volume (reduce oxygen delivery)
Distention of the Right Ventricle Obstructs Blood Flow From the Left Ventricle
231 Levin DL, et al. Pediatrics. 1975;56;58-64.2 Lakshminrusimha S. Clin Perinatol. 2012;39:655-683.
Bowing of interventricular septum into the left ventricle becomes an impediment to the LV outflow tract (reduces oxygen delivery).
Effect of lung volume on resistance
FRC
In the CINRGI Study, Clinical Evidence of
PPHN Was Defined as One of the Following1:
a The attending physician must attribute the desaturation events to PPHN and not to changes in lung disease or
ventilator strategy.
SaO2=arterial oxygen saturation.
1. Clark RH, et al. N Engl J Med. 2000;342:469-474. 2. INOMAX [package insert]. Hampton, NJ: Ikaria, Inc; 2013.]
• Differential oxygenation in
preductal and postductal areas
(ie, 5% difference in preductal
and postductal saturations by
pulse oximetry or arterial blood
gases)
A
Differential Oxygenation
Preductal
Postductal • Marked clinical lability in
oxygenation despite optimized
treatment of the neonate’s lung
disease
• Marked clinical lability is defined as
more than 2 desaturation (SaO2
<85%) events occurring within a
12-hour perioda
B
>2 Desaturation Events
in 12 hours
2
1
3
Integration
Diffusion limitation
V/Q mismatch
Pulmonary BF
Effect on MV on PBF
Evidence of Hypoxia
Step 1
Step 2
Step 3
Evaluate for the mechanism of
hypoxemia
Formulated medical recommendation
Effect on systemic circulation
Case scenario
• Baby girl born at 26 weeks, PMA is 35 weeks
• With VAP, FIO2 :0.9-1 to maintain saturation between 90-95 % for last 10 days before assessment of oxygen physiology
• HFJV with MAP of 14
Pulmonary hypertension was wrong clinical assumption
Gradual oxygen reduction with pulse oximetry and NIRS
Time (minutes) FIO2 SpO2 % Cerebral oxygen Saturation (NIRS)Normal 60-80%)
Fractional oxygen extractionNormal (0.15-0.33)
0 0.85 93 88 0.05
5 0.83 93 88 0.05
10 0.8 92 84 0.08
15 0.78 92 81 0.12
20 0.75 91 80 0.12
25 0.7 89 77 0.13
30 0.6 88 77 0.13
35 0.55 78 66 0.15
Software assessment of V-Q mismatch and R-L shunt