Pulmonary shunt in critical care: a practical approach with clinical scenarios

BACKGROUND

Pulmonary shunt refers to the passage of venous blood into the arterial blood system bypassing the alveoli-blood gas exchange. Pulmonary shunt is defined by a drop in the physiologic coupling of lung ventilation and lung perfusion. This may consequently lead to respiratory failure.

MAIN BODY

The pulmonary shunt assessment is often neglected. From a mathematical point of view, pulmonary shunt can be assessed by estimating the degree of mixing between oxygenated and deoxygenated blood. To compute the shunt, three key components are analyzed: the oxygen (O2) content in the central venous blood before gas exchange, the calculated O2 content in the pulmonary capillaries after gas exchange, and the O2 content in the arterial system, after the mixing of shunted and non-shunted blood. Computing the pulmonary shunt becomes of further importance in patients on extracorporeal membrane oxygenation (ECMO), as arterial oxygen levels may not directly reflect the gas exchange of the native lung.

CONCLUSION

In this review, the shunt analysis and its practical clinical applications in different scenarios are discussed by using an online shunt simulator.

Mathematical modelling of oxygenation under veno-venous ECMO configuration using either a femoral or a bicaval drainage

BACKGROUND

The bicaval drainage under veno-venous extracorporeal membrane oxygenation (VV ECMO) was compared in present experimental study to the inferior caval drainage in terms of systemic oxygenation.

METHOD

Two mathematical models were built to simulate the inferior vena cava-to-right atrium (IVC → RA) route and the bicaval drainage-to-right atrium return (IVC + SVC → RA) route using the following parameters: cardiac output (Q_C), IVC flow/Q_C ratio, venous oxygen saturation, extracorporeal pump flow (Q_EC), and pulmonary shunt (PULM-Shunt) to obtain pulmonary artery oxygen saturation (S_PAO₂) and systemic blood oxygen saturation (SaO₂).

RESULTS

With the IVC → RA route, S_PAO₂ and SaO₂ increased linearly with Q_EC/Q_C until the threshold of the IVC flow/Q_C ratio, beyond which the increase in S_PAO₂ reached a plateau. With the IVC + SVC → RA route, S_PAO₂ and SaO₂ increased linearly with Q_EC/Q_C until 100% with Q_EC/Q_C = 1. The difference in required Q_EC/Q_C between the two routes was all the higher as SaO₂ target or PULM-Shunt were high, and occurred all the earlier as PULM-Shunt were high. The required Q_EC between the two routes could differ from 1.0 L/min (Q_C = 5 L/min) to 1.5 L/min (Q_C = 8 L/min) for SaO₂ target = 90%. Corresponding differences of Q_EC for SaO₂ target = 94% were 4.7 L/min and 7.9 L/min, respectively.

CONCLUSION

Bicaval drainage under ECMO via the IVC + SVC → RA route gave a superior systemic oxygenation performance when both Q_EC/Q_C and pulmonary shunt were high. The VV-V ECMO configuration (IVC + SVC → RA route) might be an attractive rescue strategy in case of refractory hypoxaemia under VV ECMO.

Hemodynamic response to positive end-expiratory pressure and prone position in COVID-19 ARDS

Background

Use of high positive end-expiratory pressure (PEEP) and prone positioning is common in patients with COVID-19-induced acute respiratory failure. Few data clarify the hemodynamic effects of these interventions in this specific condition. We performed a physiologic study to assess the hemodynamic effects of PEEP and prone position during COVID-19 respiratory failure.

Methods

Nine adult patients mechanically ventilated due to COVID-19 infection and fulfilling moderate-to-severe ARDS criteria were studied. Respiratory mechanics, gas exchange, cardiac output, oxygen consumption, systemic and pulmonary pressures were recorded through pulmonary arterial catheterization at PEEP of 15 and 5 cmH₂O, and after prone positioning. Recruitability was assessed through the recruitment-to-inflation ratio.

Results

High PEEP improved PaO₂/FiO₂ ratio in all patients (p = 0.004), and significantly decreased pulmonary shunt fraction (p = 0.012), regardless of lung recruitability. PEEP-induced increases in PaO2/FiO2 changes were strictly correlated with shunt fraction reduction (rho=-0.82, p = 0.01). From low to high PEEP, cardiac output decreased by 18 % (p = 0.05) and central venous pressure increased by 17 % (p = 0.015).

As compared to supine position with low PEEP, prone positioning significantly decreased pulmonary shunt fraction (p = 0.03), increased PaO₂/FiO₂ (p = 0.03) and mixed venous oxygen saturation (p = 0.016), without affecting cardiac output. PaO₂/FiO₂ was improved by prone position also when compared to high PEEP (p = 0.03).

Conclusions

In patients with moderate-to-severe ARDS due to COVID-19, PEEP and prone position improve arterial oxygenation. Changes in cardiac output contribute to the effects of PEEP but not of prone position, which appears the most effective intervention to improve oxygenation with no hemodynamic side effects.

Effect of tidal volume on gas exchange during rescue ventilation

Tidal volume V_T required for mouth-to-mouth (MTM) and bag-valve-mask (BVM) rescue ventilation remains debatable owing to differences in physiology and end-point objectives. Analysis of gas transport may clarify minimum necessary V_T and its determinants. Alveolar and arterial O₂ and CO₂ responses to MTM and air BVM ventilation for V_T between 0.4 and 1.2 liters were computed using a model of gas exchange that incorporates inspired gas concentrations, airway dead space, cardiac output, pulmonary shunt, blood gas dissociation curves, tissue compartments, and metabolic rate. Parameters were adjusted to match published human data. Steady state arterial oxygen saturation reached plateaus at V_T above 0.7 liters with MTM and 0.6 liters with air ventilation at 12 breaths per minute. Increasing shunt shifted oxygenation plateaus downward, but larger tidal volumes did not improve oxygen saturation. Carbon dioxide retention occurred at V_T below 2.3 liters for MTM ventilation and 0.6 liters for air ventilation. Results establish a physiological foundation for tidal volume requirements during resuscitation.

Effects of epidural anesthesia combined with inhalation anesthesia or intravenous anesthesia on intrapulmonary shunt and oxygenation in patients undergoing long term single lung ventilation

Objective:

To investigate the effect of epidural anesthesia combined with inhalation or intravenous anesthesia on intrapulmonary shunt and oxygenation in patients undergoing long term single lung ventilation.

Methods:

Eighty patients, aged 35-75, American Society of Anesthesiology (ASA) classification of I-III, undergoing thoracic surgery with one lung ventilation more than three hour, were randomly divided into propofol group (group Pro), propofol combined with epidural anesthesia group (group Pro+Epi), isoflurane group (group Iso) and isoflurane combined with epidural anesthesia group (group Iso+ Epi), 20 patients in each group. Arterial blood and mixed venous blood were taken for blood gas analysis, and hemodynamic data were recorded at following time points: before induction in supine position (T₁), 30min after bilateral lung ventilation (T₂), 15min after one lung ventilation (T₃), 30min after one lung ventilation (T4), 60min after one lung ventilation (T5), 180min after one lung ventilation (T6), intrapulmonary shunt (Qs/Qt) was calculated according to the correlation formula.

Results:

Qs/Qt values at T_2-6 in four groups were significantly higher than that of T1, and Qs/Qt values at T_3-6 was significantly higher than that of T₂ (P< 0.05); PaO2 at T_2-6 were significantly higher than that of T1, with PaO2 at T_3-6 were significantly lower than T₂ (P< 0.05). Between groups, Qs/Qt values in group Iso were significantly higher than that of group Pro, Pro+Epi and Iso+Epi at T_3-5 (P< 0.05). There was no significant difference in PaO₂ between groups (P> 0.05). CI at T3-6 in group Iso and Iso+Epi were significantly higher than that of T₁ (P<0.05), and were significantly higher than that of propofol group (P<0.05). MAP at T_3-6 in group Pro+Epi and Iso+Epi were significantly lower than that at T₁ (P <0.05). Heart rate at T_4-6 in group Iso were significantly higher than T₁, and higher than group Pro and group Iso+Epi (P <0.05).

Conclusion:

One lung ventilation may predispose to increase of intrapulmonary shunt and decrease in arterial partial pressure of oxygen; isoflurane inhalation anesthesia is more likely to cause intrapulmonary shunt, but no changes in arterial partial pressure of oxygen.

Effect of low tidal volume with PEEP on respiratory function in infants undergoing one-lung ventilation

BACKGROUND

An increasing number of studies have shown that low tidal volume (TV) with positive end-expiratory pressure (PEEP) offers lung protection during one-lung ventilation (OLV). Considering the unique physiological characteristics of infants, we aimed to determine the feasibility and effect of low TV with PEEP in infants undergoing OLV during thoracoscopy.

PATIENTS AND METHODS

We randomized 60 infants to a conventional group (group I: TV, 8-10 ml/kg; RR, 23-45 bpm; PEEP, 0 cmH₂O) or a low TV with PEEP group (group II: TV, 5-7 ml/kg; RR, 23-45 bpm; PEEP, 4-6 cmH₂O). Arterial blood gas analyses were performed at four time points: 5 min of two-lung ventilation (TLV, T₀), and 20 min, 40 min, and 60 min of OLV (T₁, T₂, T₃); hemodynamic parameters (heart rate, mean blood pressure), temperature, as well as gas exchange (SpO₂ and P_ETCO₂) and ventilation parameters (FiO₂, PEEP, P_max) were recorded simultaneously. Lung compliance and shunt were also calculated.

RESULT

No significant difference was found between both groups at T₀. Compared with T₀, P_ETCO₂, P_max, PaCO₂, lactic acid, and intrapulmonary shunt volume (Qs/Qt) were increased while PaO₂ and respiratory system compliance (Cdyx) were decreased noticeably in both groups at T₁, T₂, and T₃. At T₁, T₂, and T₃, P_max and Qs/Qt were much lower while P_ETCO₂, PaCO₂, and Cdyx were higher in group II than in group I. There was no significant difference in lactic acid and PaO₂ measurements between the two groups at T₁, T₂, and T₃.

CONCLUSION

Low TV with PEEP could be an effective intraoperative ventilation strategy for infants undergoing OLV during video-assisted thoracoscopic surgery and may reduce the risk of lung injury. However, this strategy, as well as the influence of intraoperative hypercapnia on infants, needs further investigation.

Use of an Amplatz Canine Ductal Occluder (ACDO) device to close an acquired aortopulmonary fistula with a hybrid approach in a dog

A 2-year-old recently spayed female Rottweiler was referred as an emergency with cardiac tamponade and the presence of an anomalous retrograde flow in the pulmonary artery. Echocardiography and angiography demonstrated a left-to-right aortopulmonary fistula. Clinical history and data indicated a possible infectious aetiology. Antibiotics and heart failure medications were administered for 30 days before intervention. Initial attempt at insertion of an Amplatz occluder by means of a percutaneous catheterization technique was tried but a safe release of the device was judged to be not possible due to the angle and the fragile and irregular margins of the window. A decision was made to proceed with a hybrid technique combining thoracotomy and direct pulmonary artery catheterization. This hybrid approach was successful with resolution of congestive heart failure with only residual mild paraprosthetic leakage.

Respiratory complications in the postanesthesia care unit: A review of pathophysiological mechanisms

General anesthesia and mechanical ventilation impair pulmonary function, even in normal individuals, and result in decreased oxygenation in the postanesthesia period. They also cause a reduction in functional residual capacity of up to 50% of the preanesthesia value. It has been shown that pulmonary atelectasis is a common finding in anesthetized individuals because it occurs in 85% to 90% of healthy adults. Furthermore, there is substantial evidence that atelectasis, in combination with alveolar hypoventilation and ventilation-perfusion mismatch, is the core mechanism responsible for postoperative hypoxemic events in the majority of patients in the postanesthesia care unit (PACU). Many concomitant factors also must be considered, such as respiratory depression from the type and anatomical site of surgery altering lung mechanics, the consequences of hemodynamic impairment and the residual effects of anesthetic drugs, most notably residual neuromuscular blockade. The appropriate use of anesthetic and analgesic techniques, when combined with meticulous postoperative care, clearly influences pulmonary outcomes in the PACU. The present review emphasizes the major pathophysiological mechanisms and treatment strategies of critical respiratory events in the PACU to provide health care workers with the knowledge needed to prevent such potentially adverse outcomes from occurring.