High Risk Obstetrics Ventilator Management Case File

High Risk Obstetrics Ventilator Management Case File

Eugene C. Toy, MD, Edward Yeomans, MD, Linda Fonseca, MD, Joseph M. Ernest, MD

Case 33

A 26-year-old female, G3P2002 at 25 weeks’ gestation, presents to the labor and delivery suite with a complaint of 5 days of coughing and flulike symptoms and shortness of breath. She states that one of her children has had similar symptoms. She has had fever and chills but no nausea or vomiting. Her medical history is significant for seasonal allergies. On initial evaluation she is found to have a respiratory rate of 42 breaths/min, temperature 101°F, blood pressure 136/82 mm Hg, pulse 122 bpm. A pulse oximeter is applied and her saturation (SpO2) is 90%. Her physical examination shows a weight of 230 lb, a height of 64 in, a nontender gravid uterus of appropriate size, coarse breath sounds bilaterally with few expiratory wheezes, and diminished sounds at both lung bases. She has no jugular venous distention, and her heart is regular in rhythm, without murmurs, rubs, or gallop. Electronic fetal monitoring reveals irregular low-amplitude contractions and fetal heart rate of 177 beats per minute with minimal variability. Arterial blood gas values are pH: 7.26, CO2: 52 mmHg, PaO2: 65 mmHg, HCO3: 21 mEq/L. Her hematocrit is 34%, with a WBC of 14,000/mm3 and a slight bandemia. Her platelet count is 160,000/mm3. A shielded chest x-ray is obtained and reveals a pattern of diffuse patchy infiltrates in all lung fields and consolidation in the right lower lobe. It is interpreted as possible early acute respiratory distress syndrome (ARDS) or pneumonia.

➤ What are possible initial diagnoses?

➤ What should be your initial steps?

➤ What are potential complications from her disorder?

ANSWERS TO CASE 33:

Ventilator Management

Summary: A 26-year-old woman who is at 25 weeks’ gestation has respiratory insufficiency, and arterial blood gas findings of hypoxemia and hypercarbia.

➤ Initial diagnoses: Viral upper respiratory infection, pneumonia secondary to URI, bronchospastic process like asthma, acute lung injury/early ARDS.

➤ Initial steps: IV access and cautious hydration, oxygenation, obtain arterial blood gases and chest x-ray, ICU/anesthesiologist consultation.

➤ Potential complications: Respiratory failure, fetal compromise from prolonged hypoxemia, sepsis, potential multiple organ failure.

ANALYSIS

Objectives

1. Recognize the differential diagnoses for respiratory failure.
2. Be familiar with the respiratory changes in pregnancy that will impact mechanical ventilation.
3. Learn about the use of ventilator support and potential lung injury and hemodynamic compromise.
4. Recognize complications from ventilatory support (barotrauma, oxygen toxicity).

Considerations

The physiologic changes of pregnancy predispose a pregnant woman to develop severe respiratory compromise from what can appear at times to be a nonsevere insult. Oxygen consumption rises by nearly 20% in pregnancy, therefore, there are many changes in respiratory function to ensure that this increased demand for O2 is met. Maternal minute ventilation is increased by 50% in pregnancy. As there is essentially no increase in respiratory rate in pregnancy, this increase in minute ventilation results from the almost 40% increase in tidal volume. Concomitant with this is a decrease in the functional residual capacity. These changes induce a respiratory alkalosis that is compensated for by an increase in bicarbonate excretion by the kidneys. The maternal end result is a state of compensated respiratory alkalosis with an almost normal pH (7.4-7.44) and a decrease in the PaCO2 to 30 to 31 mm Hg, and a serum bicarbonate level of 18 to 22 mEq/L. The overall effect is to optimize fetal O2 exchange and eliminate fetal CO2. However, these changes also mean that the pregnant woman is more likely to experience rapid declines in oxygenation and be less able to buffer an acidosis, thus making her susceptible to significant compromise by smaller insults. Due to a decrease in plasma colloid osmotic pressure, the pregnant patient is also at increased risk for developing pulmonary edema.

APPROACH TO

Ventilator Management

DEFINITIONS

ACUTE RESPIRATORY DISTRESS SYNDROME (ARDS): Formerly “adult” instead of “acute”, its origin dates back to the 1960s. To diagnose it, there should be no evidence of heart failure. Diffuse infiltrates on chest x-ray are caused by noncardiogenic pulmonary edema and the result is severe hypoxemia that does not usually respond to supplemental oxygen. A key component of the definition of ARDS is a PaO2 to FIO2 ratio of less than 200.

BAROTRAUMA: A term which encompasses complications like pneumothorax and pneumomediastinum. It can accompany the high ventilatory pressures sometimes needed to achieve adequate oxygenation. Any of several pressures— high PEEP, high peak inspiratory pressure or plateau pressure—associated with positive pressure mechanical ventilation may cause barotrauma.

CLINICAL APPROACH

This 26-year-old gravida presents with symptoms and signs of respiratory failure. Respiratory failure in the pregnant woman can result from conditions that may or may not be related to pregnancy such as infection, trauma, drug overdose, cardiogenic pulmonary edema, hypertension, hemorrhage, asthma, aspiration, and pulmonary emboli. Pregnancy-specific conditions such as preeclampsia, HELLP syndrome, pulmonary edema due to tocolytic agents, chorioamnionitis, and amniotic fluid embolism should also be considered in the differential diagnoses. The initial goals for the patient in respiratory failure are stabilization and a thorough investigation for an underlying cause. Immediate respiratory support is often necessary. Options for therapy range from conservative management with face-mask oxygenation, bronchodilators, and diuresis to mechanical ventilatory support. A thorough history and physical examination, combined with laboratory findings such as hematocrit, white blood cell count, arterial blood gases, and electrolytes as well as selected imaging studies (shielded chest x-rays, CT scanning, ultrasound, or echocardiograms) should help to determine the etiology and guide therapy. ARDS typically yields diffuse interstitial infiltrates on the chest x-ray (Figure 33–1). In the pregnant woman at or beyond the stage of viability (24 wk), fetal status

Figure 33–1. A chest radiograph showing the exudative phase of ARDS with diffuse

interstitial and alveolar infiltrates. (Reproduced, with permission, from Fauci AS et al

(eds). Harrison’s Principles of Internal Medicine. 17th ed. New York, NY: McGraw-Hill;

2008:1681.)

will also need to be considered in the initial assessment and a plan outlining timing and route of delivery may be necessary. Fetuses remote from term may best be served by remaining in utero during maternal therapy. Those with a high likelihood of survival may be better managed by delivery when an opportunity arises. In any case, the safety of the mother must be placed first. In the mother with respiratory failure, measures to ensure maternal safety will also be the best initial steps for the fetus. 

Steps to relieve hypoxemia, preserve airway, and improve ventilation must be among the first actions. A patient who presents with hyperventilation in the face of a rising CO2 and diminished PO2 has impending respiratory failure and should be considered a candidate for mechanical ventilation. Several factors have to be weighed simultaneously. The overall goal is to restore oxygenation, maintain a near-normal pH if possible, and eliminate excess CO2. Choices in optimum ventilatory rate, tidal volume, oxygen percentage, and the possible addition of positive end expiratory pressure (PEEP) must be weighed against the potential for barotrauma and worsening hemodynamic performance secondary to increased intrathoracic pressure and lower intravascular volume. Similarly, in choosing the mode of ventilation (pressure vs volumedriven modes), lung compliance and the possibility of inducing bronchospasm must be considered. Table 33–1 presents choices in ventilation with their benefits and drawbacks.

Data from Van Hook JW. Acute respiratory distress syndrome in pregnancy. Semin Perinatol.

1997;21(4):320-327; and Whitty JE. Airway management in critical illness. In: Dildy GA, Belfort MA,

Saade GR, et al. (eds). Critical Care Obstetrics. 4th ed. London: Wiley-Blackwell Publishing; 2003.

Ventilator and Hemodynamic Management

Initial settings for mechanical ventilation should attempt to establish the optimal minute ventilatory rate to improve oxygenation with the lowest risk for barotrauma. For example, with volume-driven modes such as assist control, volume control, or synchronized intermittent mandatory ventilation (SIMV), a tidal volume of approximately 5 to 8 cc/kg, with a rate of 12 breaths per minute would deliver a reasonable 6 to 7 L per minute respiratory volume to a 100 kg woman. Whether or not to maintain this volume should be determined by its impact on peak and mean airway pressures and the risk of lung trauma. If a volume mode appears to generate unacceptably high airway pressures, a pressure-regulated mode may be preferable. In that case, pressures should be established that result in volumes adequate to achieve oxygenation and CO2 elimination. Initially a fractionally inspired oxygen (FIO2) of 100% should be chosen.1 It is a generally acceptable practice to attempt weaning of FIO2 to a level below 50% as soon as it can be done while maintaining oxygen saturation levels above 94%. Weaning must be gradual, due to the fact that when a critical PaO2 to PAO2 ratio is reached, small changes in FIO2 can produce large drops in oxygenation. In order to improve oxygen delivery, and achieve a lowering of the FIO2, addition of positive end expiratory pressure (PEEP) can be considered. This, however, may induce two factors that can have a negative impact on lung dynamics and systemic oxygen delivery. First, the addition of PEEP will often increase peak airway pressures and increase the risk of barotrauma. It is therefore considered prudent to try to maintain peak and mean airway pressures below 35 cm H2O. Second, with a patient who has poor intravascular filling either from prolonged work of breathing and insensible fluid losses, or hemodynamic shifts from third spacing, the addition of PEEP may lead to cardiovascular compromise and a decrease in cardiac output, venous return, and delivery of oxygen to the fetus. Maintaining euvolemia is also an integral part of management. Because of the need for close monitoring of hemodynamic status, an arterial line should be placed, and serious consideration should be given to early placement of a pulmonary arterial catheter for monitoring cardiac output and preload values (CVP, PAWP).

ARDS

A reasonable assumption for the patient presented in the preceding discussion is that her compromise is due to a persistent upper respiratory infection, with a possible underlying opportunistic bacterial pneumonia. Her history and physical examination, as well as laboratory and imaging results, are strongly suggestive of acute respiratory distress syndrome. The presentation and current status of this patient makes her a candidate for mechanical ventilation with possible hemodynamic support. Sedation and/or paralysis of the patient may be necessary if long-term mechanical ventilation is required.

Table 33–2 DEFINITION OF ACUTE LUNG INJURY AND ACUTE RESPIRATORY DISTRESS SYNDROME

Acute lung injury • Acute onset • PaO2/FiO2 ≤ 300 mm Hg (regardless of PEEP) • Bilateral infiltrates on chest x-ray • Pulmonary artery occlusion pressure of ≤ 18 mm Hg ARDS All of the above except: • PaO2/FiO2 ≤ 200 mm Hg (regardless of PEEP)

 

Data from Bernard GR, Artigas A, Brigham KL, et al. Report of the American-European Consensus

conference on acute respiratory distress syndrome: definitions,mechanisms, relevant outcomes, and

clinical trial coordination. Consensus Committee. J Crit Car. 1994;9(1):72-81.

As defined by the American-European Consensus Committee (Table 33–2), acute respiratory distress syndrome (ARDS) is the most severe form of acute lung injury (ALI).2 This injury is a result of an inflammatory process that produces diffuse alveolar damage in both lungs which, in turn, leads to flooding of the alveoli and compromises pulmonary gas exchange and leads to decreased lung compliance and increased pulmonary arterial pressure. As in the case described in the preceding discussion, patients with ARDS will typically present with signs and symptoms of both the underlying/precipitating disease and of the ARDS—which can include chest pain, cough, tachypnea, dyspnea, tachycardia, and hypoxemia. A key feature of ARDS is the development of a large shunt. Patients with this disorder, therefore, will have poor or minimal response to supplemental oxygen therapy. Mechanical ventilation will, therefore, almost always be required in the management of a patient with ARDS. Regardless of treatment modality, ARDS in pregnancy is associated with a high fetal and maternal mortality. Maternal mortality is reported to be between 14% and 44%.3-6 The most common cause of maternal death in pregnant patients with ARDS is multiple organ dysfunction syndrome (MODS). 

The main goal of management in the presence of ARDS is to provide respiratory support while the underlying condition is treated and/or resolved and the acute lung injury healed. Concurrent goals are to minimize barotrauma, provide adequate nutritional support, and prevent complications such as nosocomial infections and deep vein thrombosis (DVT).7 There is not a clearly preferred method of mechanical ventilation in ARDS, but there are some basic principles to bear in mind when choosing a mode of ventilatory support in ARDS. The goal is to “recruit” undamaged alveoli. It is a fine balance to obtain desirable gas exchange using these undamaged alveoli without causing additional damage. High tidal volume ventilation can damage unaffected alveoli, so ventilation strategy should aim to limit overdistension of these unaffected alveoli, therefore, lower tidal volume may be needed. Peak plateau airway pressure of less than 35 cm H2O is preferable.7 The respiratory rate will need to be increased in order to compensate for decreases in tidal volume. Airway pressure release ventilation (APRV) or high-frequency oscillatory ventilation (HFOV) may be needed in patients with persistent respiratory failure in the face of low-tidal ventilation.

One strategy that is employed to reduce the risk of tidal volume-related barotrauma in ARDS is that of “permissive hypercapnia” in which the tidal volume is lower than that conventionally used and the arterial PCO2 is allowed to rise above normal. A theoretical negative effect of this strategy is difficulty removing carbon dioxide from the fetal compartment, and therefore it should be used with caution during pregnancy. An important goal in caring for pregnant women with respiratory failure is to optimize fetal outcomes. Delivery is often necessary, and while some studies have suggested that delivery improves the respiratory status of the critically ill gravida, these studies are limited. There is currently not enough data to clearly guide decisions regarding timing and route of delivery.3,8 When immediate delivery is not necessary or not possible due to maternal status, attempt at a vaginal delivery appears to be a reasonable option as it is associated with smaller fluid shifts and avoids other risks of surgery—all are desirable goals in this population. In one case series that included 43 pregnant women on ventilator support, 86% of the women delivered during their admission (mean EGA [estimated gestational age] was 31.6 wk).3 Of these, 65% underwent cesarean delivery primarily for obstetric complications. Maternal mortality in this study was 14% and perinatal mortality was 11%. The leading cause of perinatal mortality was complications of prematurity.

Summary

Respiratory failure in the pregnant patient presents significant challenges. Early recognition of respiratory failure and prompt initiation of respiratory support are essential. Identification and treatment of underlying causes of respiratory failure/ARDS is also critical. Throughout management of these patients, the physiologic changes of pregnancy that affect respiratory function must be continuously considered. A plan for delivery based on ensuring maternal safety is also important. A coordinated response and management by a team that includes an obstetrician and an intensivist (either obstetric or medical) as well as anesthesiologists, a neonatologist, and skilled nursing care will help to optimize outcomes for these critically ill mothers and their fetuses. When ventilation will be prolonged, supportive care to include DVT prophylaxis with heparin and/or sequential compression devices, adequate nutrition, and medical management to reduce gastrointestinal stress should also be provided.

Comprehension Questions

33.1 A pregnant patient at 32 weeks EGA presents with respiratory failure; her fetal heart rate tracing reveals a baseline of 160 beats per minute, minimal variability, and late decelerations. Which of the following is the best approach regarding delivery?

A. Deliver the patient as soon as possible as this will maximize the chance of fetal survival and will help improve maternal status.

B. Stabilize and intubate the patient and deliver for obstetric indications.

C. Stabilize and intubate the patient and then proceed with a cesarean section.

D. Proceed to delivery only if there is maternal cardiac arrest.

33.2 Which of the following statements regarding ARDS in the obstetric patient is true?

A. Fetal/perinatal mortality is high when there is ARDS in pregnancy, but maternal mortality is low.

B. Most cases of ARDS respond to management with supplemental oxygen.

C. Pregnant women with ARDS will almost always require mechanical ventilation.

D. Sepsis is the leading cause of death among pregnant women with ARDS.

33.3 It is preferable in the management of ARDS to maintain peak and mean airway pressures below which level?

A. 30 cm H2O

B. 35 cm H2O

C. 40 cm H2O

D. 45 cm H2O

ANSWERS

33.1 B. By stabilizing and intubating the patient, oxygenation will be improved, and the fetal heart rate pattern is likely to normalize.

33.2 C. ARDS in pregnant woman often requires mechanical ventilation.

33.3 B. It is optimal to maintain peak and mean airway pressures below 35 cm H2O to minimize the risk of barotrauma.

Clinical Pearls

See US Preventive Services Task Force Study Quality levels of evidence in Case 1

➤ Respiratory adaptations of pregnancy that promote increased oxygen availability and facilitate CO2 elimination also make pregnant women particularly susceptible to respiratory failure (Level III).

➤ Early recognition of respiratory failure, and endotracheal and mechanical ventilation intubation to restore oxygenation and adequate CO2 elimination is crucial in order to optimize maternal and fetal outcomes (Level III).

➤ Avoiding both barotrauma and oxygen toxicity is important in ventilator management.This may involve permissive hypercapnia to limit both peak and plateau inspiratory pressures (Level II-3).

➤ Positive end expiratory pressure (PEEP) is often necessary to obtain adequate oxygenation in ARDS. PEEP, however, increases the risk of cardiovascular compromise and,therefore,hemodynamic monitoring is indicated (Level II-3).

➤ With respiratory failure, fetal status will be optimized when maternal status is optimized (especially in the initial period of stabilization). When determining when and how to deliver the obstetric patient with ARDS, maternal safety must drive decision-making. In almost all cases, emergent cesarean section of the gravida on mechanical ventilation should be undertaken for obstetric and maternal indications (worsening preeclampsia, HELLP,maternal cardiac arrest) and not for fetal indications (Level III).

REFERENCES

1. Parrillo JE. Adult respiratory distress syndrome. In: Current Therapy in Critical Care Medicine. St Louis, MO: Mosby; 1997. 

2. Bernard GR, Artigas A, Brigham KL, et al. Report of the American-European Consensus conference on acute respiratory distress syndrome: definitions, mechanisms, relevant outcomes, and clinical trial coordination. Consensus Committee. J Crit Car. 1994;9(1):72-81. 

3. Jenkins TM, Troiano NH, Graves CR, Baird SM, Boehm FH. Mechanical ventilation in an obstetric population: characteristics and delivery rates. Am J Obstet Gynecol. 2003;188:549-552. 

4. Mabie WC, Barton JR, Sibai BM. Adult respiratory distress syndrome in pregnancy. Am J Obstet Gynecol. 1992;167:950-957. 

5. Chen CY, Chaen CP, Wang KG, Kuo SC, Su TH. Factors implicated in the outcome of pregnancies complicated by acute respiratory failure. J Reprod Med. 2003;48:641-648. 

6. Cole DE, Taylor TL, McCullough DM, Shoff CT, Derdak S. Acute respiratory distress syndrome in pregnancy. Crit Care Med. 2005;33:269S-278S. 

7. Van Hook JW. Acute respiratory distress syndrome in pregnancy. Semin Perinatol. 1997;21(4):320-327. 

8. Tomlison MW, Caruthers TJ, Whitty JE, Gonik B. Does delivery improve maternal condition in the respiratory-compromised gravida? Obstet Gynecol. 1998;91:108-111.

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