Anestesiologia UIS Hipoxemia perioperatoria Dr. Raúl Vásquez

La anestesia general se asocia con hipoxemia Historia La anestesia general se asocia con hipoxemia

Oxygenation and mechanisms of hypoxemia Fisiologia Oxigenación Entrega O2 (DO2) Consumo O2 (VO2)  • Oxygenation is the process of oxygen diffusing passively from the alveolus to the pulmonary capillary, where it binds to hemoglobin in red blood cells or dissolves into the plasma. Insufficient oxygenation is termed hypoxemia.  • Oxygen delivery is the rate of oxygen transport from the lungs to the peripheral tissues.  • Oxygen consumption is the rate at which oxygen is removed from the blood for use by the tissues. Metabolismo celular aerobico Oxygenation and mechanisms of hypoxemia

Monitoring Respiratory Function Definicion Disminucion de la entrega de oxigeno de la atmosfera a la sangre arterial (oxigenación insuficiente) Hipoxemia Disminucion de la entrega de oxigeno de la sangre arterial a los tejidos Hipoxia Monitoring Respiratory Function

Hipoxemia perioperatoria Incidencia Hipoxemia perioperatoria The incidence of hypoxemia during surgery: evidence from two Institutions Duration of Hypoxemic Episodes at Hospitals A & B. The incidence and maximum duration of intraoperative hypoxemic episodes. Episodes are grouped by maximum duration per patient for both hypoxemic (SpO2\90) and severely hypoxemic episodes (SpO2 B 85). The percentage of patients experiencing two consecutive minutes or longer of hypoxemia and severe hypoxemia was 6.8% and 3.5%, respectively During the intraoperative period, 6.8% of patients had a hypoxemic event, and 3.5% of patients had a severely hypoxemic event of two consecutive minutes or longer. Seventy percent of the hypoxemic episodes occurred during either induction or emergence— time periods that represent 21% of the total intraoperative time. Can J Anesth/J Can Anesth (2010) 57:888–897

Hipoxemia perioperatoria Incidencia Hipoxemia perioperatoria 1 : 15 Hipoxemia por dos minutos 1 : 64 Hipoxemia ≥ 5 minutos ˃3 millones pacientes hipoxemia ≥ 5 min During the intraoperative period, 6.8% of patients had a hypoxemic event, and 3.5% of patients had a severely hypoxemic event of two consecutive minutes or longer. Seventy percent of the hypoxemic episodes occurred during either induction or emergence— time periods that represent 21% of the total intraoperative time. 234 Millones Cx Año Can J Anesth/J Can Anesth (2010) 57:888–897

Hipoxemia perioperatoria Efectos Deletereos Hipoxemia perioperatoria Altera cicatrización, integridad anatomosis y resistencia a infección Arch Surg 1997; 132: 991-6. Arch Surg 1997; 132: 997-1005. N Engl J Med 2000; 342: 161-7 Translocacion bacteriana GI – Sepsis Arch Surg 1996; 131: 57-62 Disfunción cognitiva – delirio Am J Med 1981; 79:1247-54. Br J Anaesth 1994; 72: 286-90 ↓↑GC, precipita arritmias, hipertensión arterial, Taquicardia, isquemia Anesthesiology 1999; 91: 1246-52. Br Heart J 1993; 69: 3-5 The thresholds for the duration and severity of hypoxemia that are likely to affect clinical outcomes are unknown, but the levels of hypoxemia reported in this study may have a clinical impact. Although the effect of oxygen saturation levels on surgical patient morbidity has been studied in several clinical trials, little has been published on the impact of transient hypoxemic events on surgical outcomes Reduced cerebral oxygen saturation levels have been correlated with higher postoperative complication rates in thoracic surgery.27 Perioperative administration of supplemental oxygen has been shown to reduce the incidence of surgical-wound infections,28,29 to improve immune function,30 and to decrease the incidence of postoperative nausea and vomiting.31 Furthermore, hypoxemia has been demonstrated in a variety of animal models to have detrimental effects on almost every end organ.32 At the cellular level, hypoxia has been shown to lead to acute heart failure,33 pulmonary hypertension,34 and acute renal failure.35 Hypoxia-induced changes in neural tissue have been associated with decreased cognitive function.36 Additionally, numerous studies have demonstrated that even modest fluctuations in oxygen delivery may lead to cognitive dysfunction.37 Decreased cognitive function with moderate levels of hypoxaemia has also been demonstrated. Delirium, which is troublesome in itself, can lead patients to remove nasogastric tubes, surgical drains and intravascular devices. Hypoxaemia can be an important factor, and oxygen therapy very beneficial The effect of hypoxaemia on the heart is variable, but cardiac output may be reduced and arrhythmias precipitated. Some studies in the post-operative period have confirmed a temporal relationship between hypoxaemia and adverse cardiovascular responses, including hypertension, tachycardia, myocardial ischaemia, cardiac arrhythmias and increased production of catecholamines. Rosenberg-Adamsen et al (1999) found that using supplemental oxygen, between days 1 and 4 after abdominal surgery significantly reduced patients’ heart rates. Tachycardia is a known risk factor for myocardial ischaemia

Hipoxemia perioperatoria Medidas Oxigenacion Hipoxemia perioperatoria Examen físico Cianosis franca 5 g/dl deoxihemoglobina SaO2 67% GASA gold standard PaO2 SaO2 “Quinto signo vital” Proyecto global pulso oximetria OMS 100000 pulso oximetros The presence of cyanosis or dark blood on the operating field were the typical earliest warnings to the anesthesiologist to herald oxygen desaturation. Because anesthetics attenuate many of the normal cardiorespiratory responses to hypoxia, other subtle warnings, such as tachycardia and tachypnea, are blunted, and mental-status changes (e.g., restlessness, somnolence) may be missed because of baseline unconsciousness of the patient Pulse oximetry

Hipoxemia perioperatoria Oximetría de Pulso Hipoxemia perioperatoria A pulse oximeter placed on the toe may delay the time until hypoxia is detected by 57 to 63 seconds when compared with the finger or ear, respectively Monitoring Respiratory Function

Hipoxemia perioperatoria Pulso Oximetria Hipoxemia perioperatoria Depicted here is the oxyhemoglobin dissociation curve for normal adult hemoglobin (Hemoglobin A, solid line). Note that hemoglobin is 50 percent saturated with oxygen at a partial pressure of 27 mm Hg (ie, the P50 is 27 mm Hg) and is 100 percent saturated at a pO2 of approximately 100 mm Hg. Depicted here are curves that are "left-shifted" (blue line, representing increased oxygen affinity) and "right-shifted" (red line, decreased oxygen affinity). The effect of right- or left-shifting of the curve is most pronounced at low oxygen partial pressures. In the examples shown, the right-shifted curve means that hemoglobin can deliver approximately 70 percent of its attached oxygen at a pO2 of 27 mm Hg. In contrast, the left-shifted hemoglobin can deliver only about 35 percent of its attached oxygen at this pO2. A high proportion of fetal hemoglobin, which has high oxygen affinity, shifts this curve to the left in newborns. Pulse oximetry

Hipoxemia perioperatoria Oximetria de Pulso Hipoxemia perioperatoria Aumenta deteccion hipoxemia 20 veces Dx temprano Intubación endobronquial Hipoventilacion Pacientes monitorizado 50% menos eventos isquemia miocárdica Global Pulse Oximetry Project

Hipoxemia perioperatoria Oximetría de Pulso Hipoxemia perioperatoria La monitorización perioperatoria con oximetría de pulso no resulta en mejores resultados, efectividad o eficacia. No reduce transferencia a UCI ni mortalidad y es incierto si existe beneficio real en Cx Cardiotoracica Oximetría de pulso para la monitorización perioperatoria 2008

Fuentes comunes de artefacto Oximetría de Pulso Mala Colocacion Artefacto Movimiento Luz Ambiental Radiación Electromagnetica Improper probe placement — Due to partial detachment of the probe, light from only one of the two light-emitting diodes may pass through the tissue, resulting in either a falsely elevated or depressed reading [11,33]. A similar problem can occur in infants and small children, because the small size of fingers or other tissues may result in differences in the path length of one light source compared to that of the other. These problems can be minimized by ensuring that the probe is properly attached with the light sources and detectors opposite each other in a nontangential path [34]. Placement of the sensor on the same extremity as a blood pressure cuff or arterial line can cause erroneous readings and should be avoided [35]. The choice of probe site may also affect accuracy; finger probes appear more accurate than forehead, nose, or earlobe probes during low perfusion states [12]. Transesophageal probes have been developed, and provide data that appear to be less influenced by changes in patient temperature, mean arterial pressure, or peripheral vasoconstriction than probes placed at other sites [36,37]. Motion artifact — A poor signal-to-noise ratio will cause signal artifact [1,7]. This most commonly results from motion due to shivering, seizure activity, pressure on the sensor, or transport of the patient by ambulance or helicopter. Newer pulse oximeters appear to be less influenced by motion artifact [38,39]. Many pulse oximeters display a waveform extrapolated from the arterial pulse signal. Artifactual changes or a diminished arterial pulse may be evident in this waveform, and repositioning the probe may improve the signal.Ambient light — Intense daylight, fluorescent, incandescent, xenon, and infrared light sources have been reported to cause spurious pulse oximetry readings [2]. In such cases, the oximeter will often give a falsely low reading of 85 percent, the saturation at which the ratio of red to infrared light is one. Falsely elevated readings due to ambient light of normal intensity have also been reported, but are rare, particularly among newer devices [11,40].Electromagnetic radiation — Radio frequency emissions from magnetic resonance imaging (MRI) scanners may interfere with pulse oximetry. In addition, second- and third-degree burns beneath pulse oximeter probes have been reported in patients undergoing MRI studies [13]. This complication is believed to result from the generation of electrical skin currents beneath the looped pulse oximeter cables, which act as an antenna. (See "Principles of magnetic resonance imaging", section on 'Precautions'.) Other sources of electromagnetic radiation, such as cellular phones and electrocautery devices, can also interfere with pulse oximeters [15,34]. Fuentes comunes de artefacto Pulse oximetry

Oximetría de Pulso Errores relacionados Con el Paciente Hg Anormales Hipoperfusion Hipotermia Congestion Venosa Pigmentacion Piel Colorantes Vitales Esmalte Anemia Abnormal hemoglobins — Abnormal hemoglobins or hemoglobin variants may interfere with pulse oximetry if their absorption properties are similar to those of oxyhemoglobin or deoxyhemoglobin. (See "Structure and function of normal human hemoglobins".) Carboxyhemoglobin — Carboxyhemoglobin absorbs approximately the same amount of 660 nm light as does oxyhemoglobin. Thus, the pulse oximetry reading represents an inexact summation of oxyhemoglobin and carboxyhemoglobin (figure 3) [1,11,12,41-43]. In cases of carbon monoxide poisoning or in chronic, heavy smokers, a falsely reassuring pulse oximetry reading may mask life-threatening arterial desaturation. A pulse oximeter that uses eight wavelengths of light and can measure both methemoglobin and carboxyhemoglobin has also been developed and is being adopted by fire services around the country for evaluation of firefighters and patients at the site of a fire [44]. Until this or similar devices are validated, co-oximetry will be required to accurately measure the oxyhemoglobin level whenever carboxyhemoglobinemia is suspected. Co-oximeters use four rather than two wavelengths of light to detect oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin; however, they require a sample of arterial whole blood [8,22]. (See "Carbon monoxide poisoning".)Methemoglobin — Methemoglobin absorbs at both 660 and 940 nm [12]. Up to a methemoglobin level of 20 percent, SaO2 drops by about one-half of the methemoglobin percentage. At higher methemoglobin levels, SaO2 tends toward 85 percent regardless of the true percentage of oxyhemoglobin (figure 4) [1,13,45,46]. (See "Clinical features, diagnosis, and treatment of methemoglobinemia".)Sickle hemoglobin — Sickle hemoglobin generally produces pulse oximeter readings similar to normal hemoglobin, but cases of falsely elevated and falsely low readings have been reported [21,47]. Patients with sickle cell disease are at risk of hypoxemia caused by a number of pulmonary complications, which are discussed separately. (See "Pulmonary complications of sickle cell disease".)Fetal hemoglobin — Fetal hemoglobin gives pulse oximetry readings clinically indistinguishable from those of adult hemoglobin [7]. (See "Structure and function of normal human hemoglobins".) Hypoperfusion — Pulse oximetry readings can be falsely low due to signal failure in the setting of hemodynamic instability or poor limb perfusion from extremity elevation or vasoconstriction [48-50]. In adults, the accuracy of standard pulse oximeters decreases dramatically when systolic blood pressure falls below 80 mmHg, generally resulting in underestimation of the actual arterial oxygen saturation [51]. Measures that may improve the signal in these settings include vigorous rubbing of the affected extremity, application of heat, or the use of topical vasodilators such as nitroglycerin paste or oil of wintergreen (methyl salicylate) [2,3,21]. Forehead sensors may also be more accurate than other pulse oximetry probes in hypoperfused patients [52]. Hypothermia — Hypothermia may interfere with pulse oximetry because of the associated peripheral vasoconstriction. This can contribute to a delay in the recognition of acute hypoxemia. particularly if finger probes are used [53]. Hypothermic patients should be monitored using either an ear or forehead probe, which are less likely to delay recognition of acute desaturation. (See "Accidental hypothermia in adults".) Anemia — In vitro and animal studies suggest that pulse oximetry readings may be affected by profoundly decreased hemoglobin concentration [21]. In vivo, low hemoglobin concentrations appear to cause falsely low readings when the SaO2 is below 80 percent [13]. However, this effect is not clinically significant until the hemoglobin level is less than 5 g/dL [49,54]. Venous congestion — Venous congestion due to tricuspid valve incompetence or cardiomyopathy may yield falsely low SaO2 readings, due to generation of venous pulsations. This results from the instrument treating less oxygenated, pulsatile venous blood as part of the arterial sample, thereby underestimating the actual SaO2 [13]. Skin pigmentation — Data regarding the impact of skin pigmentation on pulse oximetry are conflicting [15,55]: In theory, skin pigmentation should have no effect, since it should absorb at a constant level and be subtracted out as part of the background in the SaO2 calculation. This includes altered pigmentation due to hyperbilirubinemia [11].However, an increased incidence of both signal detection errors and readings erroneously elevated by 4 percent or more have been described in African-American patients [13,56]. In addition, erroneously low pulse oximetry readings were reported in a child with bronze baby syndrome [57]. (See "Benign skin and scalp lesions in the newborn and young infant", section on 'Bronze baby syndrome'.) Nail polish — The use of nail polish can potentially affect pulse oximeter readings if the polish absorbs light at 660 nm and/or 940 nm [1]. A small study of volunteers wearing black, green, and blue nail polish revealed a drop in SaO2 of 3 percent, 5 percent, and 6 percent, respectively [15]. This problem can be avoided by mounting the probe on the finger sideways, rather than in a dorsal-ventral orientation [13]. Red nail polish does not appear to have an effect on pulse oximetry readings. In addition, later generation oximeters appear to be less susceptible to interference from nail polish than earlier models [58]. Artificial acrylic nails may also affect the accuracy of pulse oximetry readings, depending on the device used. It is recommended that the probe be mounted on an alternative site or at least one of the acrylic nails be removed [59]. Vital dyes — Vital dyes, such as methylene blue, indocyanine green, fluorescein, indigo carmine, and isosulfan blue (which is used intraoperatively to mark breast and melanoma tumors), can cause erroneously low pulse oximetry readings [15,60-65]. Methylene blue has the greatest impact, as it absorbs significantly at 670 nm. However, these effects tend to be transient, and resolve rapidly as the dyes are diluted and metabolized [7,15]. Errores relacionados Con el Paciente Pulse oximetry

Hipoxemia perioperatoria Medidas Oxigenacion Hipoxemia perioperatoria Tension arterial de O2 PaO2 Gradiente Alveolo-arterial O2 Radio PaO2/FiO2 Radio Oxigeno Alveolo-arterial Indice de Oxigenacion Fraccion Shunt Oxygenation and mechanisms of hypoxemia

Hipoxemia perioperatoria Gradiente A-a Hipoxemia perioperatoria Diferencia cantidad oxigeno en el alveolo PAO2 y cantidad de O2 disuelto en el plasma PaO2 Varia con la edad Gradiente A-a= (edad/4) + 4 10 mmHg jovenes, 30mmHg 70 años PAO2=(FiO2x[Patm-PH2O])-(PaCO2/R) Oxygenation and mechanisms of hypoxemia

Gradiente A-a http://www.mdcalc.com/a-a-o2-gradient

Hipoxemia perioperatoria Gradiente A-a Hipoxemia perioperatoria Hipoxemia Causa ↓PIO2 Hipoventilacion Mismach V/Q Shunt D-I Ttno Difusion P(A-a)O2 Normal ↑↑ Rta O2 100% No Mejora PaCO2 Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Hipoxemia perioperatoria Gradiente A-a Hipoxemia perioperatoria ↑FiO2 = ↑PAO2 y ↑PaO2 La PAO2 ↑desproporcionadamente ˂40 años FiO2 100% 8 a 82 mmHg ˃40 años FiO2 100% 3 a 120 mmHg the normal A-a PO2 gradient increases 5 to 7 mm Hg for every 10% increase in FiO2. Oxygenation and mechanisms of hypoxemia

Qs/Qt=(CcO2-CaO2)/(CcO2-CvO2) CaO2=(1.34xHgbxHgbO2)+(0.003xPO2) Fraccion Shunt D-I Gold standar de oxigenacion eficiente en los pulmones Shunt 50%= Falla respiratoria severa Shunt 5%= Normal Qs/Qt=(CcO2-CaO2)/(CcO2-CvO2) where C–VO2 = pulmonary capillary oxygen content (mL/dL), CaO2 = arterial oxygen content, and C–VO2 = mixed venous oxygen content. Oxygen content is calculated as follows: CaO2=(1.34xHgbxHgbO2)+(0.003xPO2) Monitoring Respiratory Function

Hipoxemia perioperatoria Medidas Oxigenacion Hipoxemia perioperatoria 33 pctes con SDRA Relacionaron shunt con radio O2 A/a, PaO2/FiO2, gradiente A-a, IR, radio O2 a/A PaO2/FiO2 variable mas facil que predice con exactitud el grado de shunt en falla respiratoria The clinical course of 33 patients with acute respiratory distress syndrome (ARDS) was monitored by noninvasive oxygen derived variables and compared to data obtained by invasive monitoring. A total of 350 data points were used to compare the physiologic shunt fraction (Qsp/Qt) with the ratio of arterial oxygen to inspired oxygen concentration (PaO2/FIO2), the alveolar-arterial oxygen pressure difference [P(A-a)O2], the respiratory index (RI)-[P(A-a)O2/PaO2], and the ratio of arterial oxygen to alveolar oxygen (a/A). The PaO2/FIO2 ratio, the RI and the aA ratio correlated well with Qsp/Qt (r = 0.87 to 0.94). The P(A-a)O2 correlated less well (r = 0.68). Changes in the cardiac index (CI) and the arteriovenous oxygen content difference C(a-v)O2 had only a minimal effect on the correlation of the oxygen derived variables with Qsp/Qt, although a higher correlation resulted when these extrapulmonary factors were within normal range. We conclude that a number of oxygen derived variables may accurately reflect the degree of Qsp/Qt. The PaO2/FIO2 ratio is the easiest of these variables to calculate, yet accurately predicts the degree of Qsp/Qt throughout a course of acute respiratory failure. Oxygen derived variables in acute respiratory failure Crit Care Med. 1983 Aug;11(8):646-9

Hipoxemia perioperatoria Radio PaO2/FiO2 Hipoxemia perioperatoria Normal= 300 a 500 mmHg ˂300 mmHg= Intercambio gaseoso anormal ˂200 mmHg= Hipoxemia severa se correlaciona con Shunt ˃20% Oxygenation and mechanisms of hypoxemia

Shunt Derecha-Izquierda Causas . ↓Oxígeno inspirado Hipoventilación Alteración V/Q Difusión limitada Hipoxemica Anemica Circulatoria Afinidad Histotoxica Shunt Derecha-Izquierda Hipoxemia Hipoxia Monitoring Respiratory Function Oxygenation and mechanisms of hypoxemia

Bajo PIO2 Hipoxemia perioperatoria PERIOPERATIVE HYPOXIA The Clinical Spectrum and Current Oxygen Monitoring Methodology Barometric pressure (PB) and inspired oxygen (O2 ) tension or partial pressure in millimeters of mercury (mm Hg) related to altitude in thousands of feet and in meters. The PB at Denver, the Mile High City, is 640 mm Hg, and the PIO2 there is 134 mm Hg in dry air. Point 1 is at 5500 meters in the Peruvian Andes, the highest point of continuous human habitation, where the PB is 380 mm Hg, and the PIO2 in dry air is 80 mm Hg. The air is not dry at the alveolus. By the time the inhaled air reaches the alveolus, it is saturated with H2 O vapor (47 mm Hg at 37°C irrespective of the elevation or PB). At 5500 meters, H2 O vapor decreases the PIO2 from 80 to 70 mm Hg). Supplemental oxygen is usually required at point 2, although Mount Everest has been climbed without O2 . Point 3 represents the highest ascent with O2 but without superatmospheric pressure. Reduction of the PiO2 will decrease the PAO2. This impairs oxygen diffusion by decreasing the oxygen gradient from the alveolus to the artery. The net effect is hypoxemia. A reduced PiO2 is most commonly associated with high altitude. Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

PIO2 = FiO2 X (PB – PH20) Ecuación 1 ↓PIO2 = ↓PAO2 Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Flujometros Guarda Hipóxica Flujo Metabolico O2: Peso¾ x 10 Mecánica, neumática o electrónica Garantiza FiO2 25% Flujo Metabolico O2: Peso¾ x 10 Cuando la valvula de control del oxido nitrosose gira dos vueltas (28 dientes), la valvula de control de flujo de oxigeno da una vuelta por la relacionde engranaje 2-1. la proporcion final de flujo es de 3-1 Morgan, Edward: Anestesiologia clinica – Seccion I capitulo 3 y 4. 2007

Hipoxemia perioperatoria Hipoventilación Hipoxemia perioperatoria Ventilación minuto inadecuada (↑CO2) FR x VC Normal Hypoventilation is defined as an inadequate minute ventilation ability (respiratory rate × tidal volume) to remove the carbondioxide produced ( CO2 ) by the patient under his or her current condition (irrespective of deadspace, fresh gas flow, and soforth) Whenever alveolar ventilation is inadequate, carbon dioxide increases in the blood and alveolus. As the amount of carbon dioxide increases in the alveolus, there is correspondingly less room remaining for other gases (including oxygen); when hypoventilation occurs in a patient breathing room air, hypoxia inevitably occurs. O2 CO2 CO2 O2 CO2 O2 CO2 CO2 CO2 O2 O2 O2 O2 CO2 Hipoventilado Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

↓Elasticidad Pared Torácica Hipoventilación Corrige al ↑FiO2 Gradiente A-a normal Hipoventilación Pura Depresion SNC Obesidad Alt Conducción Neural Debilidad Muscular ↓Elasticidad Pared Torácica Sobredosis Lesión estructural Lesíón isquemica Sx Pickwickian ELA Guillant Barre Lx Cervical alta Paralisis N Frenico B. Aminoglicosido Miastenia Gravis Paralisis diafragmatica Idx Polimiositis Distrofia muscular Hipotiroidismo Torax Inestable Cifoescoliosis PAO2=0.21x(760-47)-PaCO2/0.8 100=0.21x(760-47)-40/0.8 Ventilacion Normal PaCO2 40 50=0.21x(760-47)-80/0.8 Hipoventilacion PaCO2 80 FiO2 21% 114=0.3x(760-47)-80/0.8 FiO2 30% Hypoxemia due to pure hypoventilation can be identified by two characteristics. First, it readily corrects with a small increase if the fraction of inspired oxygen (FiO2). Second, the A-a gradient is usually normal. An exception exists when the hypoventilation is prolonged because atelectasis can occur, which will increase the A-a gradient Obesity hypoventilation syndrome (OHS) is defined as obesity (body mass index [BMI] ≥30 kg/m2) and chronic alveolar hypoventilation (arterial carbon dioxide tension [PaCO2] >45 mmHg) during wakefulness, which occur in the absence of other conditions that cause hypoventilation [1]. OHS is also known as Pickwickian syndrome [2]. This alternative name is based upon a character in Charles Dickens' book, The Posthumous Papers of the Pickwick Club. The character, Joe, was a "wonderfully fat boy, standing upright with his eyes closed“  Alveolar hypoventilation in patients with obesity hypoventilation syndrome (OHS) is probably a consequence of several obesity-related physiologic abnormalities and conditions [1]. They include obstructive sleep apnea, increased work of breathing, respiratory muscle impairment, a depressed central ventilatory drive, and diminished effects of neurohumoral modulators (eg, leptin) due to decreased levels or resistance [3]. The importance of obesity in the pathogenesis of OHS is supported by the observation that weight loss alone decreases PaCO2 during wakefulness in patients with OHS Oxygenation and mechanisms of hypoxemia Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Imbalance V/Q El balance V/Q es complejo Volumen ventilatorio Presion alveolar Compliance pulmon y caja toracica Resistencia de la via aerea Gravedad Posicion del paciente Flujo sanguineo pulmonar Modo ventilatorio V sin Q Espacio Muerto Q sin V Shunt Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Imbalance V/Q Enfermedad pulmonar obstructiva Enfermedad vascular pulmonar Enfermedad intersticial Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Falla Respiratoria Hipoxemica Aguda Hipoperfusion Shock Tipo I Hipoxemica Aguda Tipo IV Hipoperfusion Shock Hipoventilacion Alveolar The aforementioned factors work synergistically to decrease functional residual capacity (FRC) below increased closing volumes. The result is collapse of dependent alveolar units. • Surgery is not required to precipitate “perioperative” respiratory failure, which can also be seen in medical patients with severe obesity or ascites. Tipo II Critical Care. Just the Facts 2007 Tipo III Perioperatoria Aguda Atelectasias, dolor incisional analgesia inadecuada, alteracion tos, uso de tabaco 6 semanas precx sobrehidratación

Pulmonary Atelectasis Atelectasias Pulmonary Atelectasis A Pathogenic Perioperative Entity In contrast, with atelectasis (B), alveolar inflation and deflation may be heterogeneous, and the resulting airway stress causes epithelial injury. Because the blood vessels are compressed, perfusion may be traumatic because of flowinduced disruption of the microvascular endothelium. Both epithelial and endothelial injury may initiate or ropagate lung injury. This figure depicts the advanced stage of lung injury caused by atelectasis. The initial injury is simple collapse of alveoli. However, with time, this leads to an inflammatory reaction. As the derecruited lungs cause epithelial injury and loss of epithelial integrity, both type I and type II alveolar cells are damaged. Injury to type II cells disrupts normal epithelial fluid transport, impairing the removal of edema fluid from the alveolar space. In ddition to collapse, derecruited lungs also become fluid filled. Neutrophils adhere to the injured capillary endothelium and migrate through the interstitium into the alveolar airspace.In the airspace, alveolar macrophages secrete cytokines, interleukin (IL)-1, -6, -8, and -10, and tumor necrosis factor (TNF)-, which act locally to stimulate chemotaxis and activate neutrophils. IL-1 can also stimulate the production of extracellular matrix by fibroblasts. Neutrophils can release oxidants, proteases, leukotrienes, and other proinflammatory molecules, such as platelet-activating factor (PAF). MIF macrophage inhibitory factor. In normal lungs (A), the alveolar inflation and vascular prfusion are associated with low stress and are not injurious. Two separate barriers form the alveolar– capillary barrier, the microvascular endothelium, and the alveolar epithelium. e Anesthesiology, V 102, No 4, Apr 2005 Anesthesiology, V 102, No 4, Apr 2005

Shunt (Qs/Qt) Derecha a izquierda Anatomico Transpulmonar Intracardiaco, MAV, Venas bronquiales Transpulmonar Area Q no V Atelectasias Neumonia Edema pulmonar Aspiracion Unico mecanismo de hipoxemia que no mejora con FiO2 al 100% Anesthesiology Clinics of North America - Volume 19, Issue 4 (December 2001)

Etiologia y Patogenesis Atelectasias Etiologia y Patogenesis Tres sets de mecanismos causan o contribuyen a la formacion de atelectasias Anesthesiology, V 102, No 4, Apr 2005 Compresion Pulmonar Alteracion Surfactante Reabsorcion Gas Anesthesiology, V 102, No 4, Apr 2005

Hagberg: Benumof's Airway Management, 2nd ed Compresion Pulmonar Anesthesia and surgery are usually performed with the patient in the supine position. In changing from the upright to the supine position, FRC decreases by 0.5 to 1.0 L [27] [38] [45] because of a 4-cm cephalad displacement of the diaphragm by the abdominal viscera ( Fig. 4-33 ). Pulmonary vascular congestion may also contribute to the decrease in FRC in the supine position, particularly in patients who experienced orthopnea preoperatively. These FRC changes are magnified in obese patients, with the decrement directly related to BMI.[ Figure 4-33  Anesthesia and surgery may cause a progressive cephalad displacement of the diaphragm. The sequence of events involves assumption of the supine position, induction of anesthesia, establishment of paralysis, assumption of several surgical positions, and displacement by retractors and packs. Cephalad displacement of the diaphragm results in decreased functional residual capacity (↓ FRC). Pab, pressure of abdominal contents.  (Redrawn with modification from Benumof JL: Anesthesia for Thoracic Surgery, 2nd ed. Philadelphia, WB Saunders, 1995, Chapter 8.) Ventilatory effects of regional anesthesia depend on the type and extension of motor blockade.4 Neuroaxial blockade that has significant cephalad extension reduces inspiratory capacity by up to 20%, and expiratory reserve volume approaches zero51; less extensive blockade affects pulmonary gas exchange only minimally, and arterial oxygenation and carbon dioxide elimination are well maintained during most spinal and epidural anesthesia. 52,53 Closing capacity and FRC remain unchanged.54 Hagberg: Benumof's Airway Management, 2nd ed

Anesthesiology, V 102, No 4, Apr 2005 Reabsorcion Gas Oclusion completa de la via aerea ↑FiO2 Va/Q Bajo When the FIO2 is increased, PAO2 increases, causing the rate at which oxygen moves from the alveolar gas to the capillary blood to increase greatly. The oxygen flux may increase so much that the net flow of gas into the blood exceeds the inspired flow of gas, and the lung unit becomes progressively smaller. Collapse is most likely to occur when the FIO2 (and duration of exposure) is high or where the VA/Q ratio (and mixed venous oxygen content) is low.15,16 Anesthesiology, V 102, No 4, Apr 2005

Anesthesiology, V 102, No 4, Apr 2005 Atelectasias Anesthesiology, V 102, No 4, Apr 2005

Anesthesiology, V 102, No 4, Apr 2005 Atelectasias Complicaciones Hipoxemia VC bajo, hiperoxia (microatelectasias) Reversada por hiperinflacion Alt Compliance Pulmonar Reduccion volumen pulmonar, macroatelectasias. Empeora oxigenacion sistemica ↑RVP Vasoconstriccion hipoxica pulmonar ↓ Tension Oxigeno alveolar y venoso mixto Lesion pulmonar Atelectasias + VC ↑ Prevenida con PEEP Anesthesiology, V 102, No 4, Apr 2005

Prevencion - Reversion Atelectasias Prevencion - Reversion Depende del Pulmon! Pulmon sano Hiperinsuflación 3 sucesivas -20 cm H2O x 10 seg -30 cm H2O x 15 seg -40 cm H2O x 15 seg Nunn et al -40 cm H2O x 40 seg Tusman Reclutamiento alveolar Anesthesiology, V 102, No 4, Apr 2005 Anesthesiology, V 102, No 4, Apr 2005

Reclutamiento Alveolar Schematic representation of the “Alveolar Recruitment Strategy”: PEEP is incremented in 3 steps of 5 cmH2O each. The vertical rectangles represent tidal breathing with a tidal volume of 7-9 mg/kg BW at a respiratory rate of 8 bpm. At a PEEP of 15 cmH2O tidal volumes are increased until a maximum tidal volume of 18 ml/kg or a peak airway pressure of 40 cmH2O is reached. These settings are applied for 10 breaths. Thereafter, tidal volumes are reduced to the previous values. Finally, PEEP is set to a level of 5 cmH2O in two steps. Pulmón Sano Tusman G et al. Alveolar Recruitment Strategy normalizes arterial oxygenation

Prevencion - Reversion Atelectasias Prevencion - Reversion Depende del Pulmon! Pulmon sano Hiperinflación pasiva 3 sucesivas -20 cm H2O x 10 seg -30 cm H2O x 15 seg -40 cm H2O x 15 seg Nunn et al -40 cm H2O x 40 seg Tusman Reclutamiento alveolar Pulmon Lesionado -Evitar ↑VC, presiones pico elevadas, atelectrauma -Usar PEEP Anesthesiology, V 102, No 4, Apr 2005 Anesthesiology, V 102, No 4, Apr 2005

Ventilacion Mecanica Sobredistension VENTANA SEGURA Atelectasias Baro - Volotrauma - LAD VENTANA SEGURA Atelectasias Estres por deslizamiento, alteracion por surfactante Barboza, Miguel Fisiologia de la ventilacion Unipulmonar

Gracias! Por la Paciencia

Management of the Difficult Airway Incidencia Management of the Difficult Airway A Closed Claims Analysis * Percent of row resulting in death or brain damage (death/BD). † Bonferroni P 0.04, 1993–1999 vs. 1985–1992. The finding that 70% of hypoxemia occurs during only one-fifth of the time spent in the operating room is consistent with our clinical experience, which suggests that the time close to induction of anesthesia and emergence are particularly high-risk periods for hypoxemia. This intuition is supported by data from the ASA Closed Claims database, which has shown that difficult airway claims continue to comprise the highest percentage of claims.24 The majority of these claims are related to incidents that occurred during induction (67%), with similar numbers occurring during surgery (15%) and extubation (12%).25 Anesthesiology, V 103, No 1, Jul 2005